Free TEAS Science Study Guide | Module 3 2024

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Physiology and general anatomy 

We move into the TEAS Study Guide science module now, and we start this module with physiology and general anatomy. 

Standard anatomical position

The standard anatomical position when describing the various terms used for orientation and direction on the body. 

In that position, the person will be standing.

Their feet will be forward, while the width between them is the same as their hip width.

Their arms will be at their sides, and their torso and legs will be kept in an upright position. 

The palms of their hands must face forward. 

In the body, there are three primary planes. 

• The sagittal plane can divide body parts, or the body itself, into left and right sections. It runs parallel to the midline of the body that the sagittal plane runs. 
• The coronal plane, also known as the frontal plane, can divide body structures or the body itself into front and back sections. These are known as anterior or posterior sections. It runs at right angles to the midline, vertically through the body. 
• The transverse plane, also known as the horizontal plane, divides the body into two halves, the imaginary upper (superior) and lower (inferior or caudal).

Terms of direction

You will need to understand these fully.

• Medial: Near the body’s midline. 
• Lateral: This pertains to things that are further away from the midline of the body and is the opposite of the medial.
• Proximal: This pertains to the things that are closer to the body’s center. 
• Distal: Further away from the body’s center
• Anterior: Structures in the front 
• Posterior: Structures at the back 
• Cephalad/Cephalic: Near the head
• Cranial: Near the skull
• Caudad: Near the posterior
• Superior: Closer to the head, this term means above. 
• Inferior: Closer to the feet, this term means below.

Abdominal areas and underlying organs

These include:

• A: Right hypochondriac – This encompasses the small intestine, gallbladder, liver, and right kidney.
• B: Epigastric – This encompasses the small intestine, spleen, pancreas, adrenal glands, liver, and stomach. 
• C: Left hypochondriac – This encompasses the colon, pancreas, spleen, and left kidney.
• D: Right lumbar – This encompasses the gallbladder, liver, and ascending colon.
• E: Umbilical –  This encompasses the umbilicus, duodenum, and small intestine.
• F: Left lumbar – This encompasses the left kidney and descending colon. 
• G: Right iliac  – This encompasses the appendix, and the cecum. 
• H: Hypogastric – This encompasses the sigmoid colon, female reproductive organs, and the bladder
• I: Left iliac – This encompasses the sigmoid and descending colon.

Let’s also quickly look at quadrants within the abdominal region. 

The navel is the center of these four quadrants, which are:

• RUQ or right upper quadrant: This encompasses the right adrenal gland, right kidney, pancreas (head), gallbladder, liver, hepatic flexure, part of the ascending colon, part of the transverse colon, and the duodenum.
• RLQ or right lower quadrant: This encompasses the right ovary, right fallopian tube, right ureter, appendix, and cecum.
• LUQ or left upper quadrant: This encompasses part of the descending colon, part of the spleen, splenic flexure, left adrenal gland, left kidney, pancreas (body), liver (left lobe), and stomach. 
• LLQ or left lower quadrant: This encompasses the sigmoid colon, part of the descending colon, the left ovary, the left fallopian tube, and the left ureter. 

Ventral cavity

This is found on the trunk’s anterior aspect and is made up of both the abdominopelvic and thoracic cavities.

These two cavities, however, are separated by the diaphragm. 

The organs found here are viscera, while the cavity walls comprise bone, skeletal muscle, and skin. 

• Thoracic cavity: The mediastinum and lungs are found in this cavity, which is situated above the diaphragm. The cavity itself is divided into a left and a right compartment. In the mediastinum, you will find the thymus gland, esophagus, trachea, and heart. 
• Abdominopelvic cavity: This contains the lower pelvic cavity and the upper abdominal cavity (which are not divided), and it is found below the diaphragm. In this cavity, you will find numerous organs such as the small intestines, large intestines, pancreas, kidneys, spleen, gallbladder, liver, and stomach. Closed in by the pelvic bones, the pelvic cavity includes the distal part of the colon, the bladder, as well as internal reproductive organs. 

Dorsal cavity

It’s in the posterior (dorsal) aspect of the body that this is located.

Here you will find the brain as well as the spinal cord. 

• Cranial cavity: On top, this has the skull cap, while on the bottom, you will find the cranial bones and both of these encase this cavity. Located within this cavity are the pituitary gland, 12 cranial nerves as well as the brain. The cavity itself is lined by the meninges, which themselves are made up of the pia mater, the dura mater, and the arachnoid mater. This also covers the spinal cord and the brain, and here, between the pia mater and the arachnoid mater, you will find cerebrospinal fluid. The task of this fluid is to provide protection for the dorsal cavity and to cushion it as well. The pia mater is connected to both the surface of the brain as well as the spinal cord, while the arachnoid mater (the middle layer), includes connective tissue but these don’t include any blood vessels or nerves. The outer layer that is next to the bones and helps separate the brain into various compartments is the dura mater. It is made up of cranial bones and the endosteal layer. The vertebral cavity is lined by the meningeal layer, which then lines the endosteal layer. This is found in the cranium.
• Vertebral cavity: The spinal cord and the vertebrae are found in this cavity. It’s here from the cranial cavity that the meninges extend and cover the vertebral cavity. 

Respiratory system

General function of the respiratory system

A number of body parts and organs make up the respiratory system, including the lungs, bronchial tree, larynx, pharynx, sinuses, nasal cavity, mouth, and nose. 

All of these play a role in getting oxygen moving through the body to cells where it is needed for cellular respiration. 

Inhaled air is brought into the body and into the respiratory zone via the conductive zone, while this is where the gas exchange happens. 

In this process, carbon dioxide is removed from the blood and replaced by oxygen. 

Because they help to enlarge the chest cavity as we breathe, the intercostal muscles and the diaphragm play a critical role in this process.

When we breathe out, there is a gas exchange taking place between our blood and lungs, but when we breathe in, this exchange takes place between our blood and various tissues throughout our body. 

The respiratory system also has various secondary functions that it carries out.

These include regulating blood pH levels, odor detection, helping to produce speech, and thermoregulation. 

Let’s look at thermoregulation and gas exchange in a bit more detail. 

When oxygen is placed in pulmonary blood, the gas exchange process takes place and carbon dioxide is removed. 

The air that’s inhaled comes in through the mouth/nose with the alveoli as its final destination.

To get there, it travels through the pharynx, larynx, trachea, right and left bronchi, and bronchioles.

Here in the respiratory membrane, between the capillaries and the alveoli, the gasses diffuse down partial pressure gradients. 

In other words, it’s into the blood that oxygen diffuses to be carried throughout the body to tissues, and carbon dioxide is diffused out of the blood to be expelled through cellular respiration as a waste product. 

Thermoregulation, or helping to regulate body temperature, is part of the process carried out by the respiratory system.

In the respiratory tract, as a way to conserve heat, capillaries can retract.

In order to release heat, they will dilate.

The body is also cooled when we breathe because we are releasing warm, moistened air. 

Next, let’s look at particulate matter filtration and how the respiratory system provides protection against diseases. 

Both of these are secondary functions of the respiratory system. 

To start, when we breathe in through our noses, our nose hairs will filter some but not all of the particles that we breathe in. 

Those particles that aren’t filtered out will move into the mucus, where they are broken down by lysozymes. 

Once that occurs, it’s removed from the respiratory tract by the cilia that line it. 

Pathogens that enter the body are neutralized by antibodies that are found in the mucosal lining. 

The immune system is alerted to any threat thanks to mast cells. 

These are also found in the respiratory tract, and an alert takes place when they produce inflammatory chemicals. 

Macrophages, which are large phagocytic cells, enclose particulates and small cells, and by doing so, provide protection for the lungs. 

Lung and alveoli structure

Almost all of the thoracic cavity is occupied by the lungs. 

The cavity itself is lined by a serous membrane, and this is called the pleura lines. 

You will also find this on the surface of the lungs.

The mediastinum separates the two-lobed left lung from the three-lobed right lung, while the trachea merges into the primary bronchi there. 

Entry for the bronchi is at both lungs, in an area called the hilum. 

They enter along with lymphatic and blood vessels.

Secondary bronchi split off from the primary ones, and from there into tertiary bronchi and then bronchioles.

This formation is called the bronchial tree. 

The bronchioles themselves split even further into respiratory bronchioles, which have some alveoli.

They, in turn, move into alveolar ducts, and it is in alveolar sacs that these end. 

Clusters of alveoli are found inside these alveolar sacs, and gas exchanges, which we described earlier, take place in these tiny pouches, of which there are hundreds of millions. 

A single layer of epithelial (type I) cells is what each wall of the alveolus is made up of, and they are part of the exchange of gasses.  

To stop the alveoli from collapsing, cuboidal (type II) cells will secrete surfactant. 

Adjacent alveoli are connected to each other because their walls are perforated by pores that carry out this process.

This helps air travel through different passages if ducts become blocked. 

The respiratory membrane (which includes alveolar epithelial and capillary cells) is formed by the fusion between the alveolar basement membrane and the basement membrane of a capillary. 

Breathing mechanisms 

The thoracic cavity is separated from the abdominal cavity by a dome-shaped, thin muscle called the diaphragm.

This works in conjunction with the intercostal muscles (both internal and external), and it’s within the lungs that these help to change pressure. 

Boyle’s law is followed by the breathing process, so, in other words, there is an inverse relationship between the gasses’ volume and pressure. 

This is only true, however, if the temperature remains the same. 

The volume of the thoracic cavity will go up when the external intercostals and the diaphragm contract. 

This results in elevation of both the rib cage and the sternum, which then expands outward. 

Because of this, the volume will increase and the intrapleural pressure will decrease as, through a process called inspiration, air enters the lungs. 

As a result of the pressure in the lungs being lower than atmospheric pressure, this process is known as “negative pressure breathing.” 

There is a more passive process when it comes to expiration. 

Here, the muscles described above during inhalation will now simply relax. 

The volume of the thoracic cavities then decreases as there is an increase in intrapleural pressure.

This occurs as the lungs expel air because both the abdominal and internal intercostal muscles contract at this point.

Cardiovascular system

Cardiovascular system: Heart functions

Cardiovascular system functions

The transport of fluids, ions, hormones, nutrients, and oxygen, as well as the removal of metabolic wastes, is the primary concern of the circulatory system. 

The following equation is used to work out cellular respiration, where glucose is either combined with or burned together with oxygen: C6H12O6 + 6O2 → 6CO2 + 6H2O.

Most of the oxygen will bind with the hemoglobin molecules that are found in red blood cells, and this happens when it travels down from the air into the blood of the alveolar capillaries. 

Some of it, however, will be dissolved in the blood.

This oxygen is necessary so that, during cellular respiration, cells can transfer glucose’s energy to ATP. 

Diffusion out of the alveolar capillaries occurs with carbon dioxide that’s formed during cellular respiration as it is moved away from the tissue. 

Almost all of it will travel as bicarbonate ions, but some will either combine with hemoglobin or dissolve in the blood. 

The kidneys will filter other metabolic waste products, such as urea. 

Fluid and iron levels in the blood are also regulated by these organs.

Targeted cells will receive digested nutrients that are circulated to them.

These nutrients include fats, amino acids, and glucose.

In the same way, these targeted cells also receive hormones that the endocrine system releases. 

A carrier protein is necessary to transport any lipid-soluble molecules in the blood. 

Let’s discuss the role of the cardiovascular system in thermoregulation.

To begin, the role that it plays is a critical one. 

Because it is necessary for defense against various pathogens and optimal metabolic processes in the body, a temperature of around 98.6 Fahrenheit is what the body needs to maintain. 

In response to signals that they receive from the brain, our blood vessels will dilate or constrict as necessary to ensure the correct heat exchange is carried out at the surface of our skin. 

The hypothalamus receives impulses from thermoreceptors, which are sensory neurons that notice temperature changes. 

The hypothalamus will act on the impulses it receives by sending signals to the smooth muscles that are found around the arterioles.

These are called effectors, and they will relax when our body temperature is too high, allowing the arterioles to dilate, while they contract if the temperature is too low, constricting the arterioles. 

This in turn will allow more blood to flow (when arterioles dilate) or less blood to flow (when arterioles contract) into the capillary beds that are near the skin’s surface. 

This helps regulate temperature, which is something that sweating and shivering also contribute to. 

The heart

There are three layers of tissue in the wall of the heart.

These are:

• Epicardium (outer layer): This releases a lubricating serous fluid while at the same time, protecting the heart.
• Myocardium (middle layer): This pumps blood by contracting.
• Endocardium (inner layer): The chambers and valves are lined by this layer.

There are four chambers within the heart – two receiving chambers (atria) and two discharging chambers (ventricles). 

The first of the atria receives blood into the heart from the vena cava and is called the right atrium, while the left atrium, the second chamber, receives blood from the pulmonary veins.

The first of the ventricles is the right ventricle, which is tasked with expelling blood into the pulmonary trunk. 

The second, the left ventricle, will expel blood into the aorta. 

When the ventricle contracts, backflow into it is prevented by the tricuspid valve (also called the right AV valve or the right atrioventricular valve).

To stop blood from returning to the right ventricle, the pulmonary semilunar valve plays the same role. 

When the ventricle contracts in the left atrium, the bicuspid valve (also called the mitral or left AV valve) stops blood from entering, and backflow of blood into the left ventricle is controlled by the aortic semilunar valve when it leaves the aorta. 

While talking about the heart, we must mention diastolic and systolic pressure. 

Blood exerts pressure on the walls of vessels, and that pressure, which we know as blood pressure, is its force per unit. 

Usually, this is for the major arteries, unless mentioned otherwise. 

Why the arteries only?

Well, it’s because while there is blood pressure in arterioles, capillaries, venules, and veins, it is far less than what the arteries will experience. 

Expressed in units of millimeters of mercury (mmHg), blood pressure is usually shown as two numbers.

Systolic pressure is denoted by the first number, and during systole, this is the maximum pressure exerted. 

Blood is forced into the aorta and pulmonary trunk when ventricles contract during this time. 

As arteries have blood entering them, they will accommodate it by having their walls stretch due to the increase in volume.

During diastole, these walls will then move back to their regular diameter, so this is when blood pressure is at its lowest due to the relaxation of the ventricles. 

120/90 mmHg is the normal resting blood pressure for most adults. 

If a patient has high blood pressure, they are at increased risk of strokes, heart failure, and heart disease, while those with low blood pressure can suffer from fainting or lightheadedness, from time to time. 

Let’s move on to pulmonary and systemic circulation. 

Blood is carried via the systemic circuit from the heart’s left ventricle to the aorta, from which the arteries rise and move out into arterioles, where it travels further into tissues through capillary beds. 

Here the tissues receive nutrients and oxygen and offload waste such as carbon dioxide to be carried away by the deoxygenated blood.

This moves away via venules, which, in turn, converge into larger veins.  

The blood pressure in this circuit, which is longer than the pulmonary circuit, is much higher. 

More oxygen is carried by arteries than by blood traveling through veins, which is unlike the pulmonary circuit. 

Vessels will dilate to encourage blood to flow to tissues that need it when oxygen levels are low. 

The pulmonary circuit is a component of the system of circulation that sees blood travel from the heart to the lungs and back. 

It’s through the pulmonary trunk, which then splits into the right and left pulmonary arteries, that deoxygenated blood travels once it is expelled from the right ventricle.

Oxygenated blood is returned to the left atrium from the four pulmonary veins. 

They are merged with veins that carry this blood from the capillaries through the venules. 

One thing to note, however, is that the arteries contain fewer oxygen molecules than the veins do. 

Vasoconstriction, which redirects blood to the parts of the lungs that are better ventilated, results from low blood oxygen in the pulmonary circuit. 

Last, in this section, we look at the electrical conduction system within the heart. 

The electrical impulses that help maintain the heart’s rhythmic electrical contractions are emitted and maintained by the electrical conduction system. 

This system is made up of the following:

• Sinoatrial (SA) nodes
• Atrioventricular (AV) nodes
• Internodal pathways
• Bundle of His
• Right and left bundle branches
• Anterior fascicles
• Posterior fascicles

The process starts with a spontaneous electrical impulse generated by the SA node.

This helps encourage atrial contraction, which will show up on an ECG as a P wave.

The AV node is next in line for this impulse, and it will slow in speed at this point, which on an ECG will correspond to the PR segment. 

The Bundle of His is the next destination in the journey of the impulse, but it moves through this as well as the bundle branches onto the Purkinje fibers.

The impulse is carried by these fibers, and the ventricles are stimulated by it, causing them to contract and depolarize, as seen on a QRS complex. 

Cardiovascular system: Venous and arterial systems, electrocardiograms, cardiac arrhythmias

Electrocardiograms

We start this section by looking at the cardiac cycle and ECG relationship. 

On an ECG, each beat of the heart reflects as three major waves (sometimes referred to as complexes). 

It is in the sinoatrial node where the heart rhythm starts, and because of this, atrial depolarization occurs. 

On an ECG, this depolarization is shown by the P wave. 

The QRS interval follows that, and that shows ventricular depolarization.

The ST segment and T wave then follow, and this shows ventricular repolarization.

In some cases, further repolarization of the ventricle might be evident if a small U wave follows the T wave.

Generally, a P wave, which has a smooth and round appearance, lasts for around 0.08 to 0.1 seconds.

The contraction (activation) of the right atrium is represented by the first segment of the P wave, and then the activation of the left atrium follows. 

The middle of the P wave shows the atrioventricular node contracting, and arrhythmias are a possible reason why P waves might be absent when carrying out an ECG.

On the ECG, where it marks the start of the QRS complex, it’s as a negative wave that the Q wave is shown. 

The lead used to measure this wave defines just how long the duration of this wave is, but note that it is small, typically less than 0.03 seconds. 

A myocardial infarction or ventricles that conduct abnormally are possibilities if an ECG presents with an enlarged Q wave. 

As the predominant portion of the QRS complex, the R wave shows as a positive wave on an ECG.

In some cases, the R wave might be followed by another.

Following the R wave in the QRS complex, the S wave shows as a negative peak.

Ventricular depolarization is represented by the combination of the Q, R, and S waves. 

The final two waves on an ECG are the T wave and the U wave.

Ventricular repolarization is represented by the T wave, which has a positive peak and follows the S wave. 

Because it is longer than the QRS complex, this will show that in the cardiac cycle, repolarization is longer than depolarization. 

It is shaped like the P wave, with a round and smooth appearance, and if abnormal, could show that a patient has an electrolyte imbalance, or even heart disease. 

The T wave is the cardiac cycle’s final major wave, but not the last.

The final repolarization of the ventricles is shown by the U wave, which is relatively small when presenting on an ECG. 

On an ECG, the period from the start of atrial depolarization to the start of ventricular polarization is known as the PR interval.

It’s from the start of the P wave that this interval is measured, and that measurement ends at the start of the QRS complex.

0.12 to 0.20 seconds is the typical length of this interval. 

A heart will have accelerated conduction if the length is shorter than 0.12 seconds.

The interval may be longer than 0.20 seconds if there is a heart block or AV block present. 

Note that as heart rate increases, PR level can decrease, while age, short stature in childhood, and lengthening in adulthood can also cause an increase.

Let’s move onto the QRS complex. 

This is indicated by the Q,R, and S waves, which show when the left and right ventricles are depolarized (or activated). 

Typically, this is between 0.06 and 1 second, and should it be longer, this could indicate that the ventricles have impaired conduction. 

Next is the ST segment. 

This shows when there is complete depolarization of the ventricle, and it follows the QRS interval. 

The measurement of this segment is from when the QRS complex ends to when the T wave starts. 

Where the QRS complex and ST segment meet, you will find the J point, and to the S-wave, this will be a 90 degree angle. 

The ST segment can help diagnose hypoxia as well as ventricular ischemia, and it can present as either elevated or depressed. 

The whole action potential – the time of ventricular depolarization and repolarization – is shown by the QT interval, and usually, this is between 0.2 and 0.4 seconds. 

This timespan will shorten if heart rate increases, while arrhythmias are an indication of longer QT intervals.  

Measuring heart rates is simple enough from an ECG, but only if the patient has a regular heartbeat.

To do this, you will check the waves between the P wave and QRS complexes, and between the recurring waves, count the number of horizontal squares on the graph paper. 

You determine the heart rate by counting these cardiac cycles and then multiplying by 10. 

Cardiac arrhythmias

An abnormal heartbeat is known as cardiac arrhythmia, and this will occur in adult patients. 

This could either be a result of myocardial infarction or if, during surgery, the conduction system was damaged. 

When pulse rates are very slow, this is known as bradyarrhythmias and could result from:

• Complete atrioventricular (AV) block: This could be as a consequence of surgical trauma, or it could be congenital. 
• Sinus bradycardia: This could be due to an oxygenation decrease, a consequence of hypotension or triggered by the autonomic nervous system. 
• Junctional/nodal rhythms: This often happens when patients have had surgery and it’s characterized by output and heart rate that’s stable, but a missing P wave on an ECG. For the most part, treatment won’t be necessary unless the patient is compromised in some way.

When pulse rates are unusually fast, this is known as a tachyarrhythmia and could result from:

• Sinus tachycardia: Infection or fever can cause this
• Supraventricular tachycardia:  200-300 beats per minute when heart rate is measured, which could lead to congestive heart failure. 

Some irregular pulses can happen following operations, and these are termed conduction irregularities.

They aren’t something that’s usually necessary to worry about, however. 

Venous and arterial systems

There are three layers in the walls of all blood vessels (except capillaries).

These are:

• Tunica intima: Which is the innermost wall.
• Tunica media: Made up of elastic fibers and smooth muscle cells
• Tunica adventitia: Which are the outermost wall

Let’s look at the various vessels in more detail.

• Elastic arteries: When blood is forced out of the heart, these will stretch. When pressure lowers, however, they recoil. The aorta and major branches are included in this category, and within the arterial system, they are the biggest of all vessels.
• Muscular arteries: Through vasoconstriction/vasodilation, these arteries will help to control blood flow. These are found moving off elastic arteries. When compared to them, they have fewer fibers that are elastic in nature and include more smooth muscle cells. 
• Arterioles: When it comes to vasoconstriction/vasodilation, these are the main vessels involved. The blood flowing to the capillaries is controlled by the arterioles. 
• Venules: These help to move blood into the larger veins. It’s from the capillary beds that these tiny vessels exit. They don’t have many elastic fibers and have thin, porous walls.
• Veins: They are tasked with ensuring that blood returns to the heart. Valves within veins ensure that blood doesn’t backflow. 

Cardiovascular system: Blood and capillary beds

Capillary beds

Let’s start this section by looking at gas and solute exchange mechanisms. 

Resting on a basement membrane, all capillaries have only one layer of endothelial cells. 

To help with the exchange of gasses and solutes between interstitial fluid and blood, interconnected capillaries (capillary beds) are necessary. 

Here, capillary blood sees waste products, as well as carbon dioxide, enter it, and it’s in the interstitial fluid that nutrients and oxygen are found. 

Because of diffusion, moving across the endothelial cell membranes is easy for lipid-soluble substances and gasses.

That’s not the case with larger particles and ions.

They can only be transported via vesicular transport and transport proteins.

The most prevalent types of capillaries in the body are continuous capillaries, those with a nonporous continuous endothelium, which are mostly porous. 

In the small intestine and kidneys, you find capillaries that are more porous, and these are called fenestrated capillaries.

The most permeable of all capillaries are called sinusoidal capillaries, and because they have a discontinuous endothelium, larger particles, as well as blood cells, can pass through them.

Next, we move on to the source of peripheral resistance. 

Blood vessel resistance, because blood flow causes friction, is known as peripheral resistance. 

The rate of blood flow therefore drops as this resistance rises. 

Peripheral resistance is influenced by several factors, including the length of the vessel and its diameter, but also blood volume and overall viscosity. 

It’s mostly changes in the blood vessel’s diameter that affect resistance the most. 

There’s an inverse relationship between the radius and resistance, and as the first decreases,  the other increases in a proportionate manner. 

So as to keep the resistance and flow at the right level, vasodilation and vasoconstriction play a critical role. 

Due to an increased surface area, as the vessel lengthens, so does the resistance, which increases proportionately too. 

Blood composition

Blood cells, chemicals, and plasma make up the blood that flows through our bodies. 

55% of our blood is made up of a clear, straw-colored liquid that we know as plasma. 

The remaining 45% comprises platelets as well as red and white blood cells.

Plasma itself is made up of many things, including the following:

• Carbohydrates
• Amino acids
• Hormones
• Gasses
• Salts
• Vitamins
• Lipids
• Albumin
• Antibodies
• Clotting proteins
• Waste products

On the whole, however, water makes up around 92% of all plasma.  

Red blood cells comprise most of the cells found in blood, and they are often referred to as erythrocytes or RBCs. 

A protein known as hemoglobin is found in them, and this is necessary for the carbon dioxide and oxygen that the blood carries to bind to. 

Hematocrit is the name given to the percentage of red blood cells by volume.

It’s different in men (46%) than in women (42%). 

Platelets and white blood cells make up just under 1% of blood.

White blood cells, also called WBCs or leukocytes, include a nuclei, and are the only type of blood cells to have this.

They have another property, however, that makes them unique, and this is because, unlike other blood cells, they are able to travel in and out of vessels.

WBCs come in various forms, each with their own unique ability to fight pathogens that enter the body. 

Outnumbering WBCs by around 40:1, platelets, also called thrombocytes, help initiate clotting when bleeding occurs and are essentially cell fragments. 

The production of erythrocytes is a process known as erythropoiesis. 

Red bone marrow is responsible for this, and this whole process starts when oxygen levels drop significantly.

That results in a hormone produced by the kidneys and liver, known as erythropoietin, springing into action. 

Marrow is targeted by this hormone, where myeloid stem cells are triggered. causing them to transform into erythroblasts. 

Newly synthesized hemoglobin is then filled with immature RBCs that are dividing continuously. 

Some endoplasmic reticulum will remain, but as the nuclei condense, they, along with other organelles, are ejected.

The endoplasmic reticulum cells are called reticulocytes, and following the above process, they move into the blood, where mature erythrocytes (which cannot be divided) will form after one to two days. 

Phagocytes that recognize old and damaged RBCs that are concentrated in the liver, spleen, and bones will begin to absorb them after around 120 days. 

The globin polypeptide chains and heme groups that makeup hemoglobin are separated, and from these chains, the resulting amino acids are released. 

Any remaining iron from the heme groups will either move to the marrow, where it produces more hemoglobin, or it will move to the liver to be stored as ferritin. 

What about WBCs (leukocytes)?

Well, we know these are a critical part of the immune system.

They are also divided into various different types, such as T cells and B cells. 

There are others, however, for example, monocytes and granulocytes.

With a well -defined nucleus, a monocyte, which is made in the bone marrow, is tasked with helping our body’s immune response when pathogens enter. 

Granulocytes include cytoplasm granules, and when certain allergens or infections enter the body, they will be released. 

These are the most prevalent types of WBC.

In fact, they make up around 75% of the total amount of WBC. 

There are also various types found in our blood, but you can expand on that in your coursework.

Blood types

Our blood types form part of the ABO group classification.

In total, there are four types of blood:

• A
• B
• O
• AB

Further classification takes place within each group depending on the presence of A and B red cell antigens.

So, A antigens are prevalent in individuals with the A blood type, B antigens in those with the B blood type, and both A and B antigens in those with the AB blood type.

A or B antigens are not found in those who have O blood. 

In our blood, antibodies to the various antigens are found, and this allows for further classification when it comes to blood types. 

So those with AB blood don’t produce antibodies, those with A type blood will produce anti-B antibodies, and those with B type blood produce anti-A antibodies, while there are no antibodies produced by those with AB blood. 

Anti-A and Anti-b antibodies are produced by those who have O group blood. 

Furthermore, a person is either Rh factor positive or negative in the Rhesus group of blood types. 

Clotting mechanisms and coagulation

Smooth muscle will begin to constrict as soon as a blood vessel is damaged.

This allows platelets to start their critical job, which takes place at the vessel wall where damage has occurred and collagen is exposed.

The platelets begin to cling to the collagen while simultaneously attracting further platelets through the release of chemicals. 

Vasoconstriction is further enhanced by this as well. 

As a result of the aggregation of platelets, a plug will form, but this isn’t all that’s needed to seal up a damaged vessel.

While it’s described in far greater detail in your coursework, which you should check, we are going to focus on a brief description of the coagulation process that’s essential too.

Essentially, the clotting occurs when various clotting mechanisms combine to help form a network with mesh-like qualities.

This, in turn, helps to trap red blood cells and platelets, which eventually will seal the blood vessel in the area where the injury has occurred. 

Blood and the role it plays in the transport of oxygen

Hemoglobin molecules are tasked with helping to transport oxygen around the body in erythrocytes. 

Earlier, we learned that hemoglobin is made up of globin, which is composed of polypeptide chains broken down into two alpha chains and two beta chains. 

In turn, the heme group is found in these chains, and the red color of our blood comes from this. 

Importantly, a single iron atom, which a protoporphyrin (an organic ring) surrounds, will help to pick up oxygen as it passes through the lung’s capillaries. 

Oxyhemoglobin is the name given to the oxygenated form of hemoglobin. 

Note that hemoglobin doesn’t only carry oxygen but carbon dioxide as well, with close to a quarter of all carbon dioxide moving through the body being carried out in this manner. 

Digestive system: Liver, stomach, and ingestion

In this section, we look at how the digestive system works and the various parts that contribute to it.

The process of digestion is one that starts from the moment we begin to chew our food. 

This is called mechanical digestion, and the process of chewing is known as mastication.

From there, we swallow our food in a process known as deglutition, which is all aided by our mouth and the pharynx’s skeletal muscles. 

There are three phases to this, but you can read up on them in your coursework, as we won’t expand further here. 

Peristalsis is the involuntary process by which food is moved from our pharynx via the esophagus on the path to our stomach. 

In the intestines, a further process takes place, and this is known as segmentation. 

Here, chyme, which is partially digested food, is moved back and forth through the contraction and relaxation of certain portions of the digestive tract. 

Up to four times a day, chyme will be moved towards the rectum via mass peristalsis. 

The gastroesophageal sphincter, the pyloric sphincter, and the anal sphincter also control movement along the digestive tract. 

Ingestion

Saliva immediately moistens food as soon as it is ingested. 

This saliva comes from three major glands along with smaller minor salivary glands.

The major glands are:

• Sublingual gland
• Submandibular gland
• Parotid

A host of enzymes are found in salvia, that act as solutes.

Simple sugars result from the breakdown of polysaccharides, which begins with salivary amylase, while the breakdown of fats starts thanks to the lingual lipase. 

That’s not the only task of saliva, however, as it also carries antimicrobial agents, for example, lysozyme. 

Along with immunoglobulin A, it helps ensure that the bacteria’s cell walls start to get broken down. 

To help keep the proper pH levels for all the enzymes in saliva, it also includes bicarbonate ions. 

Let’s move on to the esophagus. 

Essentially a passageway for food, the esophagus is a tube around 25 cm long that runs to the stomach from the pharynx.

The only role of the esophagus is in the transport of food; it doesn’t help with nutrient absorption or digestion at all.

Through the secretion of mucus, the esophagus is lubricated, so food can travel easily to the stomach.  

There are four layers making up the esophagus:

• Mucosa
• Submucosa
• Muscularis externa
• Adventitia

While the top third is made up of skeletal muscle, most of the esophagus is comprised of muscularis externa.

This is essentially smooth muscle tissue.

Some of the middle section includes skeletal and smooth muscle, while the latter part of the esophagus is only made up of smooth muscle. 

Again, peristalsis plays a role in how food is moved down the esophagus and into the stomach. 

Stomach

To start, let’s look at how the stomach churns and stores food. 

When it receives a high volume of food, the stomach is able to stretch to accommodate it.

This is because it is a muscular organ. 

The main role that the stomach plays in digestion is that of a mechanical breakdown of food.

While there is some chemical digestion, it’s not that much. 

Also, the stomach is tasked with storing food.

Expansion of the stomach occurs as food comes in and the mucosa, or inner surface of the stomach, which is folded into ridges called rugae, fills out. 

After a regular meal, the human stomach holds around 1 liter.

If necessary, however, it’s able to cope with around four times that amount. 

Unique to the stomach and found in the muscularis externa is a third muscle layer that helps to churn the food.

Typically, this process will take around four hours. 

As a result of mixing with gastric juices, food is turned into chyme, which we spoke about earlier, and it is then moved into the small intestine. 

What about the production of digestive enzymes?

Gastric glands that open into many gastric pits are found in the mucosa of the stomach. 

Within these glands, four cell types reside:

• Mucous cells
• Endocrine cells (G cells)
• Chief cells
• Parietal cells

Of the three, only endocrine cells do not play a part in aiding gastric juices in digestion.

Instead, along with gastrin, they release various hormones into the blood. 

Let’s look at some of the other cells and their functions, starting with parietal cells. 

So that the small intestine can absorb vitamin B12 effectively, these cells are tasked with excreting intrinsic factors. 

Additionally, they exude hydrochloric acid.

This helps to ensure that a pH range of 1 to 3 is maintained in gastric juices. 

An acidic environment is necessary, as in this way, pepsinogen, which is released from chief cells, is stimulated. 

These cells help with some of the digestion of fat by releasing gastric lipase, although most of this process is the task of the small intestine, along with protein digestion. 

A digestive enzyme called pepsin, the active form of pepsinogen, helps split proteins into smaller peptide chains. 

Lastly, bicarbonate-containing mucus helps provide protection for the stomach for digestive enzymes, and acidity is released by the mucous cells.

Let’s talk about the stomach’s structure. 

Found on the abdomen’s left superior side, the stomach is a muscular organ. 

To stop acid reflux, between the esophagus and stomach, you will find the gastroesophageal sphincter. 

There are four main parts of the stomach itself:

• Cardiac region: This area of the stomach is where food will be emptied into
• Fundus: The stomach’s most superior area
• Body: The central region of the stomach. It’s also the largest piece
• Pylorus: The narrowest part of the stomach, it’s here that the body reduces food. Chyme is released into the small intestine via the pyloric sphincter, but just small amounts at a time. 

Liver

What role does the liver play in the gastrointestinal system?

While not a component of the alimentary canal, the liver still plays a massive role in the gastrointestinal system.

It’s tasked with many functions, including:

• Detoxification
• Nutrient metabolism
• Bile production

In terms of digestive function, the main role that the liver plays is through bile production. 

This is made up of electrolytes, cholesterol, pigments of bilirubin (as a result of hemoglobin breakdown), and bile salts. 

Of all of these, in aiding digestion, only bile salts will play a role. 

This is a mechanical, not a chemical process, where fats are emulsified into micelles.

The lipases of the small intestine can then act on these to break them down.

While bile salts are synthesized from cholesterol by liver cells, the absorption of fat-soluble vitamins (A, D, E, and K) is bile-induced as well. 

The gallbladder is where bile is stored.

This organ contracts when the small intestine starts filling with food.

This signal to do so comes from the CCK (cholecystokinin) hormone. 

Once this happens, bile travels to the common bile duct, which is connected to the pancreatic duct. 

From there, bile moves into the duodenum via the hepatopancreatic ampulla and duodenal papilla. 

Note, however, that the duodenum can also receive bile straight from the liver. 

What about the role of the liver in detoxification and the regulation of blood glucose?

One of the major metabolic functions that the liver performs is ensuring that blood glucose concentration is regulated (on average, this is around 100 mg/dl). 

Via the hepatic portal vein, blood flows from the digestive tract into the liver. 

The liver will form glycogen by polymerizing glucose if the blood sugar level is too high.

This is known as glycogenesis. 

Stored glycogen is broken down by the liver, which releases glucose monomers if the blood sugar level is too low.

This is known as glycogenolysis. 

It’s from proteins and fats (non-carbohydrate sources) that the liver will produce glucose when long fasting is in effect.

This is known as gluconeogenesis. 

Lastly, the liver is a critical component when it comes to detoxification. 

The metabolism of amino acids produces ammonia, which, in the liver, is converted to urea before being expelled by the kidneys.

The kidneys also expel any liver-inactivated hormones that are found in the blood. 

Drugs, alcohol, and other exogenous compounds are also broken down by the liver. 

Be sure to read more about the digestive system in our coursework, particularly with regard to the pancreas, intestines, and elimination, as we are not covering it here. 

Nervous system

Nervous system: Organization and function

Vertebrate nervous system organization

The central nervous system (CNS) and the peripheral nervous system (PNS) are the two main parts of our nervous systems.

The CNS is made up of the spinal cord and brain, while the PNS includes nervous tissues found outside of the CNS, such as ganglia and nerves.

Information is sent from the PNS to the CNS, which in turn will integrate the information that it receives.

This allows the CNS to communicate with all parts of the body. 

In this process, the PNS’s afferent neurons transmit pulses to the CNS, while impulses are transmitted to the effectors via efferent neurons. 

The autonomic system (ANS) and somatic nervous system (SNS) are further divisions of the PNS. 

Voluntary movements, for example, skeletal muscle contractions, are controlled by the SNS, while contractions of cardiac and smooth muscle and other involuntary movements are carried out by the ANS. 

The ANS is further subdivided.

Here, you find the sympathetic division and the parasympathetic division.

When we talk about “fight or flight,” this is controlled by the sympathetic division, while the parasympathetic division controls “rest and digest.”

So let’s look at the above-mentioned in a bit more detail.

When “fight or flight” is triggered, the preganglionic neurons within the SNS spring into action.

Acetylcholine (ACH) is released, and in turn, this leads to postganglionic neurons releasing norepinephrine, which ensures a coordinated and swift response from target tissues.

That response translates into a rise in the rate of respiration and heart rate, more blood flowing to the heart, an increase in skeletal muscle, dilation of the pupils, and the degradation of glycogen. 

The “rest and digest” response is totally different.   

Acetylcholine is released by parasympathetic pre- and postganglionic neurons in this case.

Because events are less essential for immediate survival and move at a slower pace, the parasympathetic nervous system will slow the heart rate, lower the respiration rate, make the pupils constrict, and cause the synthesis of glycogen.

Peristalsis is encouraged from a digestive standpoint, and the blood flow to the digestive organs is increased.  

Impulses are sent to the CNS via sensory neurons, and on occasion, it depends on the stimulus type to which they respond as to how they are classified. 

For example, if there are pressure or tension changes, then mechanoreceptors will respond. 

These receptors include the following:

• Meissner’s corpuscles
• Pacinian corpuscles
• Merkel’s disks
• Ruffini endings

Other examples of receptors are photoreceptors (for light), chemoreceptors (for taste and olfactory), thermoreceptors (for temperature), and nociceptors (for pain).

Location is also used as a way to group sensory neurons. 

Information about the outside environment is transmitted via exteroceptors close to the body’s surface.

Information regarding equilibrium,  movement, and position is provided by proprioceptors found in our joints, skeletal muscles, and the inner ear, while information regarding internal stimuli within the body is the domain of blood vessels and visceral organ-linked interoceptors. 

Effector neurons (motor neurons) send the impulse from the CNS to trigger glands and muscles.

It’s only after the sensory information has been interpreted that this will occur, however. 

Without synapsing with another neuron, all somatic division motor neurons go straight from the effector into the CNS.  

Note that two-neurons make up the pathways used by the autonomic division.

CNS major functions

Made up of a complex network of neurons and supported by neuroglial cells, the responsibility of controlling and coordinating the many activities taking place in our bodies is the task of our nervous systems.  

The sensory, motor, and integrative processes of this system are all executed by these  neurons. 

Pain, light, temperature, pressure, and other changes in both our internal and external environment are detected by various sensory receptors. 

All the information that these receptors collect is analyzed and determined via the CNS.

The maintenance of memory, language, emotion, sensations, movement, temperature regulation, breathing, and heart rate are all determined by the information received by the receptors and how the CNS responds.

The CNS is also responsible for how body systems integrate with each other. 

All body systems are integrated via our nervous systems.

The brain and spinal cord, which make up our CNS, are an essential part of this. 

Because it combines the sensory information it receives, the CNS is the center for not only integration but control too, and via the PNS, communication with the rest of the body takes place when acting on this stimulus. 

Very close cooperation exists between the nervous and endocrine systems. 

Here, the hypothalamus plays a critical role as it regulates how the pituitary gland releases hormones when it receives information about the condition of the body from nerve impulses. 

All other glands within the endocrine system are controlled by the pituitary gland, hence the fact that it is often called the master gland. 

So that they remain in homeostasis, every system in the body is directed by the nervous system. 

Let’s look at the adaptive capability of our nervous system to outside influences. 

Within our bodies, the first system to respond to any environmental changes is the nervous system. 

These changes are picked up by various sensory receptors found across the body, and usually, it’s via their location, morphology, rate of adoption, and the type of stimuli they detect that they are classified. 

Although adaptation rates differ significantly between the various receptor types, we divide them into two main categories. 

• Phasic receptors (most chemo and tactile receptors are in this group): When these receive constant stimulus, they adapt quickly. So over time, the action potential diminishes and ultimately ceases. An example of this is the way something we smell quickly disappears, even though the source of the odor remains. 
• Tonic receptors: As they slowly adjust to stimulus, tonic receptors frequently send action potentials to the CNS. 
• Proprioceptors: Providing feedback regarding movement and body position is the job of this receptor type. 
• Photoreceptors: Detect light
• Nociceptors: Detect pain

Nervous system: Reflexes and the CNS

The brain

This critical organ is made up of the forebrain (cerebrum, thalamus, and hypothalamus), midbrain (which integrates and responds to sensory signals), and hindbrain (pons, cerebellum, and medulla oblongata). 

The brain is split into two hemispheres, each of which performs a variety of tasks. 

Further, there is a division into four lobes as well.

These are the temporal lobe, the occipital lobe, the parietal lobe, and the frontal lobe.

Processing information, judgment, planning, decision making, short-term memory, and working memory are the responsibilities of the frontal lobe. 

The body’s spatial positioning, and sensory input are the responsibility of the parietal lobe.

Visual input, processing, as well as output are the responsibility of the occipital lobe, while the temporal lobe is tasked with input and output of an auditory nature, as well as the processing thereof. 

When it comes to how we process and store memories (those learned during classical conditioning), the cerebellum plays a major role. 

The brain stem is connected to the spinal cord, and it is in the posterior area of the brain. 

Through this, information from the body travels to the brain, and information from the brain travels to the necessary parts of the body. 

The brainstem is a key part of breathing, digestion, and blood circulation.

Situated above the medulla oblongata and the pons, you will find the midbrain, which plays a critical role in hearing and vision. 

The tectum, the tegmentum, and the ventral tegmentum are all components of this part of our brain. 

This part of the brain sees the pons sending information from the cerebrum to the cerebellum and the medulla oblongata, which connects to the spinal cord.  

It’s a critical part of the autonomic nervous system. 

The cranial cavity comprises the brain, the 12 cranial nerves, and the pituitary gland. 

It is made up of the cranial bones at the bottom and the skull cap at the top. 

The dura mater, arachnoid mater, and pia mater make up the meninges. 

They line the cavity, cover the brain and spinal cord, and hold cerebrospinal fluid between the arachnoid and pia mater.

This fluid is held in the subarachnoid space, where, along with the meninges, it cushions and protects the dorsal cavity.  

Connected to the brain’s surface as well as the spinal cord, you will find the vascularized pia mater. 

Connective tissue is found in the arachnoid mater (middle layer), but there are no blood vessels or nerves found here. 

Next to the bones, and folding inward is the dura mater, which helps separate the brain into compartments. 

It’s made up of two layers.

Lining the cranial bones is the endosteal layer, while beneath it and lining the vertebral cavity is the endosteal layer.  

Let’s move on to cranial nerves. 

There are 12 pairs of these and control our muscles, senses, and organs.

Here they are:

• Olfactory: Controls smell (sensory)
• Optic: Controls sight (sensory)
• Oculomotor: Controls aspects of eye movement, and adjustment of the pupil (motor)
• Trochlear: Controls aspects of eye movement (motor)
• Trigeminal: Controls sensation in the face, as well as chewing. The largest nerve in the face (sensory and motor)
• Abducens: Controls aspects of eye movement (motor)
• Facial: Controls facial movement and parts of the tongue (sensory and motor)
• Vestibulocochlear: Controls sound (sensory)
• Glossopharyngeal: Controls taste, saliva, and certain aspects of swallowing (sensory and motor)
• Vagus: The peripheral nervous system is controlled via this nerve (sensory and motor)
• Accessory: Head and neck movement, certain aspects of swallowing (motor)
• Hypoglossal: Speech and certain aspects of swallowing (motor)

Let’s focus on the spinal cord. 

This is a column of nerve fibers surrounded by vertebrae, both of which are contained in the vertebral cavity that protects and supports it.

It connects our brain to our body.

The nervous tissue within it mostly controls how its limbs move and how its organs work. 

All the major nerve tracts move up from our spinal cord to our brain. 

The meninges surround the vertebral cavity and extend from the cranial cavity. 

There are five regions of the spinal cord (from the bottom up): coccyx, sacral, lumbar, thoracic, and cervical. 

What role does the spinal cord play?

The spinal cord links the afferent and efferent pathways, and is a significant reflex center. 

While the core interior is made of gray matter, it’s surrounded by an exterior layer of white matter. 

Glial cells and myelinated bundles of axons that form tracts to and from the brain comprise the latter.

White matter lacks cell bodies and dendrites, however. 

Aside from motor neurons and glial cells found there, gray matter is mainly made up of interneurons. 

It is in the dorsal root ganglia, which are located outside the spinal cord, that afferent neurons’ cell bodies reside.  

Through the anterior root, afferent fibers enter the posterior/dorsal aspect of the spinal cord (also known as the posterior gray horn).

Efferent fibers subsequently leave the posterior root on the anterior/ventral aspect (called the anterior gray horn). 

Typically, spinal neurons innervate neck-inferior structures.

Reflexes

When receiving a stimulus, a reflex is an unconscious, involuntary response that’s almost instantaneous. 

Picked up by afferent neuron receptors, the stimulus is sent to the spinal cord by interneurons, where it then travels to the effectors (muscles, for example) via motor neurons, and then a reaction takes place.

For example, placing your hand on a hot plate accidentally will lead to you withdrawing it without even thinking, and before you can get seriously burned. 

In some cases, reflex actions can be overridden by the brain, while in others, like when your knee is tapped (patellar reflex), there is a link between the sensory and motor neurons.

This is known as a monosynaptic reflex. 

You can read more about reflexes in your coursework, specifically, reflex arcs.

Feedback control and the nervous system’s integration with the endocrine system

The endocrine system and the nervous system are closely intertwined and work with each other all the time to control the body.

The endocrine system does this using hormones, which are slower to act, but longer lasting, while the nervous system can act quickly via electrical impulses.   

The hypothalamus provides a link between the endocrine system and the nervous system because it not only controls the autonomic nervous system (impulses) but also the pituitary gland (hormones). 

Also, the connection extends even further because, within the hypothalamus, there are axons from the neurons found there that travel through the infundibulum and end up in the pituitary gland.  

Antidiuretic hormone (ADH) and oxytocin are produced in the hypothalamus, but it’s the job of the pituitary gland to secrete them.

The hypothalamus also controls the regulation of other hormones secreted by the pituitary gland. 

It’s in controlling various body functions as well as endocrine glands that these hormones play a critical role in homeostasis maintenance, metabolism, development, and growth. 

How milk is released by a mother during nursing is an illustration of how the two systems interact. 

The stimulus of a nursing baby causes the hypothalamus to receive an impulse, which sees the pituitary emit oxytocin into the blood.

The hormone stimulates the production of milk by the mammary gland.

Nervous system: Nerve cells

The body of the nerve cell: Nucleus and organelles

A neuron’s cell body is known as a soma.

In it, you will find organelles.

The metabolic activities of the neuron are the responsibility of these cells.  

A nucleus with a noticeable nucleolus is found inside the cell body. 

The nucleus contains DNA, and the proteins necessary for a functioning neuron get their encoded information from it. 

Almost all of the organelles are located in the neuronal cytoplasm.

These include mitochondria, peroxisomes, lysosomes, Golgi bodies, smooth and rough endoplasmic reticulum, and the cytoskeleton.

In order to accommodate the high metabolic needs of the neuron, mitochondria are found in extremely high numbers. 

Proteins for use within the cell are synthesized by granular nissl bodies. 

These comprise free ribosome clusters as well as rough ER.

Because differentiated neurons can no longer divide, you won’t find centrioles in mature neurons. 

Dendrites

The cell body has branched extensions known as dendrites. 

They are tasked with dealing with input neurotransmitters (incoming chemical signals), and they receive these from other neurons. 

With a tree-like appearance, they have branches that taper as they move outward.

This is important because the surface area for synaptic inputs is maximized in this manner. 

The same organelles as the cell body are found in the cytoplasm that the dendrites carry. 

This, however, is not true of the nucleus. 

When the presynaptic cells’ axon terminal releases a neurotransmitter, it is to the dendrite receptor sites on postsynaptic cells that it will bind. 

These signals are either inhibitory or excitatory. 

Depending on the signal, neurons are either prepared to fire or they are inhibitory. 

If they are prepared to fire, it’s down the axon that a chemical message, converted into an electrical impulse, will travel. 

You can learn more about the structure and functions of the axon in your coursework. 

Myelin sheath

Neurons have a lipid-based coating called myelin that covers their axons. 

Similar to the protective coating on an electrical wire, myelin ensures the axon is insulated, while helping to improve the traveling rate of impulses. 

The nodes of Ranvier, which are sporadic gaps in the sheath, allow the impulse to move from one node to the next as quickly as possible. 

Peripheral nervous system neurons have Schwann cells, which myelinate them. 

The axon has these glial cells curled around it, which in turn sees the formation of multiple lipid-rich layers as plasma membranes wrap around. 

While it’s outside the myelin sheath that the nucleus and cytoplasm remain, they are still covered by the Schwann cell’s neurilemmal sheath. 

Schwann cells can support very tiny axons, but they do not myelinate them, and these are known as non-myelinating Schwann cells.

Sheathing the neurons in the CNS is the task of oligodendrocytes. 

They differ from Schwann cells because numerous axons can be myelinated by just one oligodendrocyte.

By wrapping its membrane around the axons and extending it in various directions, it is able to  achieve this. 

A significant number of the axons in the CNS’s white matter are myelinated, whereas those in the gray matter are not.

Myelin damage is the cause of multiple sclerosis and several other illnesses. 

Here, neurons cannot effectively conduct an impulse due to the fact that they are not insulated properly. 

Nodes of Ranvier

These increase the rate of conduction and take the form of uninsulated gaps between myelinated parts of the axon.

There is a significant density of voltage-gated sodium and potassium channels found in these 1 m-long exposed regions. 

The membrane becomes depolarized as a result of these ions passing through the channels. 

Ions cannot diffuse through the myelin, so the action potential must move on to the next node in a process known as saltatory propagation.  

Compared to continuous conduction, this is much more efficient as well as faster, and it’s along the whole length of the unsheathed axion that this occurs. 

When compared to thin, unmyelinated axons, large-diameter one’s conduct impulses much faster. 

Their rate is around 80-120 m/s while the latter is only 0.5-10 m/s.

Unmyelinated axons, however, have less neuroplasticity than myelinated ones. 

This means that forming new connections with other neurons is more limited. 

Glial Cells

These are also known as neuroglia.

Within the peripheral and central nervous systems, they are tasked with supporting and protecting neurons.

Within the nervous tissue, when compared to the number of neurons, you will find many more glial cells, and this is regardless of their capacity to conduct impulses. 

They can divide as well, and because of this, most brain tumors come from Glial cells.

Let’s look at the types of Glial cells found in the PNS and CNS as well as their functions.

We start with those within the CNS. 

• Astrocytes: Within the neural tissue, these are the most abundant cells found. Their functions include neuron anchoring, helping neurons and capillaries exchange materials, as well as taking up excess ions and neurotransmitters.
• Microglia: These don’t have many extensions. Their function includes digesting debris and dead neurons as well as immune defense. 
• Oligodendrocytes: The axons of CNS neurons have these extensions wrapped around them. Neurotransmission is sped up and insulation of CNS neurons occurs by the myelin sheaths that oligodendrocytes produce.
• Ependyma: The epithelial lining of the spinal cord’s ventricles and central canal is formed by ependyma. Their function includes ensuring materials are interchanged between the brain and spinal cord’s interstitial fluid and the cerebrospinal fluid as well as circulating it. 

Next are those found in the PNS.

• Schwann cells: Form layers when they wrap around the PNS axon with their lipid membranes. Their function is to insulate PNS neutrons by insulating them with the myelin sheaths that they produce. Neurotransmission is also sped up through Schwann cells. 
• Satellite cells: Within the PNS ganglia, these will surround the neuron’s soma. Their function is only to cushion and protect neurons in the PNS.

Types of neuroglia

The communication junction between two neurons or an effector (a muscle or gland) and a neuron is called a synapse. 

They are made up of:

• Postsynaptic element
• Synaptic cleft
• Presynaptic element

Through the actions of neurotransmitters, it’s across the synaptic cleft that impulses will be transmitted.

It’s based on the nature of their postsynaptic elements that we classify synapses:

• Axodendritic: It’s on the dendrites of a postsynaptic neuron that these synapses terminate. 
• Axosomatic: It’s on the postsynaptic soma that these synapses terminate.
• Axoaxonic: It’s on the postsynaptic axon that these synapses, which are rare, terminate. 

The mode in which the impulse is transmitted can also be used to classify synapses. 

It’s as unidirectional chemical junctions that most synapses operate. 

Here, messages are sent to the postsynaptic cell via neurotransmitters. 

As the axon terminals receive the impulse, a fusion occurs between the plasma membrane and  the vesicles that store the neurotransmitters.

In turn, it’s within the synaptic cleft that signals are released, which will facilitate their attachment to receptors on the postsynaptic target. 

It’s either a depolarized (excited) or hyperpolarized (inhibited) position that the postsynaptic membrane takes on at this point. 

There is no neurotransmitter use when it comes to bidirectional synaptic junctions. 

Instead, it is through gap junctions that they are linked, and through this, between the cells, ions will flow. 

While we won’t cover it here, you can read up more on transmitter molecules and synaptic activity in your coursework. 

Muscular system: Major muscles, movement, and function

Important functions of muscular systems

We start this section by looking at the important functions that muscular systems carry out within our bodies.

The first thing we will look at is how they support mobility. 

Skeletal, smooth, and cardiac muscles are what make up our muscle systems.

These muscles contract, and in doing so, they produce almost all the movements our bodies make. 

Any voluntary action, such as walking, eye movements, or smiling, is produced by our skeletal muscles, which are controlled by our somatic nervous system. 

So how do they work?

Well, these muscles are connected via tendons to our bones, and they will get shorter when the bone moves.

Other than tendons, these muscles will help stabilize joints by also working together with bones and ligaments.  

Some muscles are under autonomic control, and when they contract, it’s an involuntary action on our part. 

An example of this would be peristalsis, which we covered earlier, where food is rhythmically moved along the gastrointestinal tract through smooth muscle contractions. 

Another example of this involuntary contraction is when smooth muscles on vessel walls contract and relax to help regulate our blood pressure.  

Vasoconstriction causes blood pressure to rise and blood flow to fall, whereas vasodilation causes blood pressure to fall and blood flow to rise. 

What about our hearts?

Well, yes, that’s a muscle too, and it’s a critical one that ensures blood is pumped through our bodies.

Next, muscles help with peripheral circulatory assistance. 

The skeletal muscle pump then helps the blood get back to the heart. 

Vascular valves in the large peripheral veins of the arms and legs stop blood from flowing backward. 

When the skeletal muscles around these deep veins contract, the vessel is compressed, and blood is forced through the valves in the direction of the heart. 

The rate of blood flow is increased when these muscle groups are exercised.

Venous return is also helped by the thoracic pump. 

When we draw in breath, the thoracic cavity will expand as the intercostal muscles expand and the diaphragm contracts. 

There is a decrease in overall pressure as the volume increases, and this is passed on to the right atrium.

Blood is aided on its journey back to the heart by this drop in pressure. 

There’s another consequence of this pressure drop as well, because in the abdominal cavity, pressure will then rise, and through this, blood is squeezed towards the heart from the inferior vena cava.

Thermoregulation is also something that muscles and muscular systems play a role in. 

As soon as a drop in temperature is picked up by thermoreceptors, the posterior hypothalamus will receive impulses, and in turn, it is to the effectors that it sends signals of its own.

The blood flow near the surface of the skin is slowed down when, within the cutaneous artery walls, smooth muscles begin to contract, and in doing so, heat loss is minimized. 

More muscles also play a role in trying to raise our body temperature; for example, our hairs will stand on end to try to trap warm air, and this is caused by the contraction of the arrector pili muscles. 

The posterior hypothalamus will trigger the shivering reflex if core body temperatures drop even further. 

This involuntary action occurs when skeletal muscles are rapidly contracted and relaxed.

Energy is then released in the form of heat, which is generated in two ways.

Firstly, the muscles create friction through their sliding filaments.

Secondly, hydrolysis of the ATP is necessary for muscle contraction.

As soon as an upward spike in temperature is noted by the thermoreceptors, there is a relaxation of the smooth muscle surrounding the cutaneous arterioles, as instructed by the anterior hypothalamus. 

To prevent overheating, more blood is near the skin because of vasodilation. 

Movement

Muscles help with various forms of movement. 

Let’s start with the sagittal plane of motion. 

It is through the front and back of the body that this plane passes, and it splits it into a left and right side.

Here are some of the movements associated with this plane:

• Flexion: The angle between two body parts is decreased, for example, an elbow bending.
• Extension: The angle between two body parts is increased, for example, an elbow straightening.
• Dorsiflexion: This is a flexion of the ankle, for example, toes moving upwards towards the shin.
• Plantar flexion: This is an extension of the ankle, for example, toes moving downward toward the ground. 

Next, we have frontal plane motion.

It is through the left and right of the body that this plane passes, and it splits it into an anterior and a posterior.

Here are some of the movements associated with this plane:

• Adduction: It’s towards the midline that movement occurs, for example, moving an arm next to the body.
• Abduction: It’s away from the midline that movement occurs, for example, moving an arm away from the body.
• Elevation: This is a superior movement of the scapula, for example, when your shoulders go up during a shoulder shrug.
• Depression: This is an inferior movement of the scapula, for example, when your shoulders come down during a shoulder shrug.
• Eversion: Here, the lateral border of the foot is lifted, and the sole is faced outwards.

Then there is the transverse plane of motion. 

Through the body, this plane passes in a line parallel to the floor, so the body is divided into a top and bottom section.  

Here are some of the movements associated with this plane:

• Pronation: Wrist and hand rotation medially from the bone, for example, lying on your back, but placing your hand, palm first, on the floor.
• Supination: Wrist and hand rotation laterally from the bone, for example, lying on your back, with the wrist and hand pointed at the ceiling.
• Horizontal adduction: On the horizontal plane, the angle between two joints is decreased.
• Horizontal abduction: On the vertical plane, the angle between two joints is increased. 
• Rotation: This is when it’s on an axis that pivoting or twisting occurs, and a classic example is your head turning left and right to look before you cross the road.

While we won’t cover them here, make sure you know the various major muscles found within the body. 

Structure of striated, smooth, and cardiac muscles

Because it is under voluntary (or somatic) control, skeletal muscle needs external stimulation to contract.

This means that it does not show myogenic activity.

We’ve seen how this type of muscle is necessary for a range of things, including thermoregulation, helping blood return to the heart, bone movement, and support.

We call these striated muscles, where there are light and dark bands that alternate in their muscle fibers. 

As for their shape, well, they take on a cylindrical form and are filled with nuclei.

What about smooth muscle?

We have no control over these; the body does, and this muscle is found in vessels and organ walls.

It is tasked with the movement of food, blood, and other substances throughout our body. 

Unlike striated muscle, non-striation occurs in this muscle type, which is spindle-shaped and uninucleated. 

Cardiac muscle, too, shows myogenic activity and is under autonomic control. 

As you can guess from its name, this muscle is found in the heart walls and is tasked with pumping blood. 

Cells within this muscle are uninucleate (but can have two nuclei), striated, and branched.

Intercalated disks help them connect to each other, with communication between cells occurring because of the presence of gap junctions. 

Make sure you read up more on the muscular system, particularly with regard to structure, contraction, and nervous control, in your coursework.

The reproductive system

Male and female structural differences

We start this section by looking at how the male and female reproductive systems are different from a structural perspective.

Because they share a common developmental pathway, a range of male and female structures found in the reproduction systems of both are homologous. 

They do, however, become specialized for the various roles that they play in reproduction. 

The production of sperm and delivery of it to the female for fertilization is what the male reproductive system is designed to do.

As a result, a large percentage of reproductive structures found in this system are external.

Keeping the sperm at the correct temperature is one of the reasons for this. 

Also, when compared to female gonads, the testes have a far higher level of testosterone and, following puberty, are able to make gametes in their millions each day. 

The female reproductive system is very different, and the main concern here is embryo development, while it is internally that all reproductive structures are housed. 

Compared to the male gonads, the ovaries in females have significantly higher levels of estrogen. 

One oocyte is released during ovulation per month.

Interestingly, while the male urethra is connected to the reproductive system – because it releases sperm – the female urethra isn’t. 

Let’s look at the genitalia of a male. 

The following make up the internal male genitalia:

• Epididymis
• Vas deferens
• Accessory glands (semi vesicles, prostate gland, Cowper’s gland)

The task of the epididymis is to help nourish sperm when they finish their overall maturity, and they are also stored here till ejaculation occurs.

It’s a tube that’s connected to the outside of the testicle. 

Sperm will travel through the sperm duct (vas deferens), ejaculatory duct, and urethra before leaving the penis during ejaculation.

Around 60% of semen’s volume is made up of fluid secreted by the seminal vesicles. 

This fluid is made up of proteins, prostaglandins, and fructose. 

Around 30% of the semen’s volume is a result of secretions from the prostate gland, and this helps to improve their motility while also nourishing them. 

Lubricating fluid, which makes up about 2% to 5% of the semen volume, is released by the Cowper’s gland. 

The penis and scrotum make up the external male genitalia. 

While the penis delivers sperm to the female reproductive organs, the scrotum is tasked with keeping the right temperature for the testes as well as providing protection for them.

What about female genitalia?

Well, we know that this is internal and is made up of the following:

• Ovaries
• Fallopian tubes
• Uterus
• Vagina

While oocytes are produced in the ovaries, they also serve another function, and that’s the secretion of sex hormones.

The fallopian tubes (sometimes called the oviduct or uterine tube) will grab an oocyte that’s released during ovulation. 

There is no direct connection between the ovaries and the fallopian tubes. 

It’s in these tubes that fertilization will take place.

Usually, it is in the endometrium of the uterus that the fertilized egg is implanted.

The developing egg is then protected, and nourished by the uterus, a pear-shaped, muscular organ. 

The cervix is the neck of the uterus that opens into the vagina.

It’s in the vagina, a muscular canal, that the penis will deposit semen during intercourse.

The vagina also serves as the passageway through which a baby will pass during childbirth. 

The external parts of the female genitalia include the vulva, made up of the mons pubis, labia minora, and labia majora. 

The mons pubis lies over the public bone, and this is a mound of fatty tissue. 

Protecting the delicate tissues that it covers is the job of the labia, which is made up of folded skin. 

The vagina is lubricated via Bartholin’s glands, while erectile tissue, the clitoris, contributes to sexual arousal and pleasure. 

Gonads

While we’ve briefly mentioned the gonads, it’s time to look at them in more detail. 

Found in the reproductive system, the gonads have several tasks that they fulfill.

For example, they will secrete hormones, but they also produce sex cells (called gametes). 

In men, the testes are the male gonads, while in women, it’s the ovaries.

It’s in the scrotum that you will find the testes, and it’s here that the tunica albuginea, a fibrous layer of connective tissue, encloses them. 

Around 250 to 300 compartments are known as lobules, and they are found within the testes.

This division is because it is from the tunica albuginea that thin layers of tissue extend.

There are four seminiferous tubules within each lobule, and these are the sites of spermatogenesis.  

The cells that produce sperm, the spermatogenic cells, are what the epithelial lining of these tubes consists of. 

You will also find Sertoli (or sustentacular) cells present here, and these provide nourishment. 

Cells that help stimulate the production of sperm, called Leydig (or interstitial) cells, produce testosterone, and they are found around the seminiferous tubules. 

The rete testis is a network of channels formed when the seminiferous tubules join.

When sperm mature, they are brought via this tube to the efferent ducts.

When the point comes, they will enter the epididymis from the testes. 

Let’s look at the ovaries, which, as we said earlier, are the female gonads.

They sit in the ovarian fossa, a slight depression found on either side of the uterus where peritoneal ligaments hold them in position. 

The germinal epithelium, a layer of simple cuboidal epithelium, as well as an underlying tunica albuginea, are the two types of tissue that cover the ovaries. 

Each ovary is subdivided into two sections.

These are the inner medulla and the outer cortex. 

There are thousands of nourishing follicles in it, which gives it a granular appearance. 

Oocytes are found within each of these follicles.

While these are eventually surrounded by granulosa cells that produce estrogen, in the beginning, they are surrounded by follicular cells. 

The corpus luteum gland forms following ovulation. 

It will release small amounts of estrogen as well as progesterone, and unless pregnancy happens, this gland will dissipate. 

Loose areolar connective tissue makes up the medulla, or interior, of the ovary. 

Here you will find lymphatic and blood vessels.

It also contains nerves, and these come and go via the hilum. 

Female reproductive cycle

Both the uterine lining (endometrium) and the ovaries will change during the female reproductive cycle. 

There are three phases in the ovarian cycle, and these are:

• Follicular phase
• Ovulation
• Luteal phase

Let’s start with the follicular phase. 

Here, the maturation of the follicle is stimulated by FSH.

Estrogen will start to be released once this has occurred, and by doing so, the uterine lining lost during menstruation is regenerated. 

During a surge in LH, the secondary oocyte from the ovary is released, and this happens during the ovulation stage. 

From the remnants of the follicle, the corpus luteum will form, and that indicates the beginning of the luteal phase. 

FSH and LH are then inhibited by the release of both estrogen and progesterone from the corpus luteum. 

The thickness of the endometrium is also maintained by progesterone. 

Regression of the corpus luteum will start without the implantation of an egg that is fertilized. 

Because of this, progesterone and estrogen levels will drop. 

The cycle renews as the levels of LH and FSH are no longer impeded. 

You will find three phases in the uterine cycle as well. 

These are:

• Proliferative phase: Here the uterine lining is regenerated.
• Secretory phase: Nutrients are released, preparing for implantation, and the endometrium becomes more and more vascular.
• Menstruation: Where the endometrium will be shed should implantation not have occurred. 

We won’t cover pregnancy, parturition, and lactation here; you can read through them in your coursework. 

Integumentary system

Structure

The outermost layer of our skin is called the epidermis. 

The epidermis’ stem cells are the keratinocytes of the stratum basale, and as they move towards the surface, they will give rise to differentiating cells. 

The epidermis’s bottom layer, or stratum basale, is its deepest.  

The basement membrane is typically covered by a single layer of cuboidal or columnar cells. 

Due to their proximity to the dermal capillaries, these cells are the most nourished. 

Between eight and ten layers of spiny cells, connected by desmosome-like structures, make up the stratum spinosum. 

In this layer’s deeper section, there is only sporadic mitotic activity. 

Between two and five layers of cells with slight flattening and keratohyalin granules make up the stratum granulosum. 

The nuclei of the cells in this layer’s topmost section are lost. 

Only the palms and soles of the feet have the stratum lucidum, which is made up of two to five layers of dead, flattened keratinocytes. 

Eleidin, a protein derived from keratohyalin that is translucent and water-resistant, is present in these cells. 

The stratum corneum, which is the topmost layer, is made up of 15 to 30 layers of squamous cells that are dead but filled with keratin.

This layer aids in halting the body’s water loss.

Let’s look at the types of cells that you will find in the epidermis.

• Keratinocytes: The epidermis’ most prevalent type of cell, they develop from stem cells in the stratum basale. As they move to the skin’s surface, they will begin to flatten and eventually die. They create keratin, a fibrous protein that hardens the cell and aids in the production of the skin’s water resistance.
• Melanocytes: Melanin, the pigment responsible for skin color and UV protection, is produced by these cells.
• Langerhans cells: In the immune system, these are antigen-presenting cells. While they are found in other layers of the epidermis, they are found more frequently in the stratum spinosum. 
• Merkel cells: Found in the stratum basale, it is light that these cutaneous receptors are tasked with detecting. 

Here are the types of cells that are found in the dermis:

• Fibroblasts: Components of the extracellular matrix are released by these cells. This includes glycosaminoglycans, elastin, and collagen. 
• Adipocytes: Fat cells.
• Macrophages: Any potential pathogens are engulfed by these phagocytic cells. 
• Mast cells: Tasked with releasing histamine, these play a part in the body’s reply to an inflammatory response. They are antigen-presenting cells. 

Integumentary system functions

Let’s look at three critical functions that this system carries out. 

They are thermoregulation, osmoregulation, and homeostasis.

Pain, temperature, pressure, and direct touch are picked up by a variety of sensors found within our skin. 

This allows the body to respond in an immediate manner to any changes that occur. 

Harmful substances cannot enter the body easily either because the skin acts as a physical barrier. 

If they do, macrophages within the skin will provide a secondary line of defense. 

Ultraviolet radiation is another thing that our skin protects us against, thanks to the fact that melanin production increases as melanocytes respond when our bodies are exposed to the sun. 

What about thermoregulation?

Well, both vasodilation and vasoconstriction help to control this. 

In terms of osmoregulation, or the loss of water, the skin isn’t the main organ that deals with this, but it does play a role. 

This is because it prevents excessive water loss while also ensuring that the body does not absorb too much water.

Urea and ammonia are ways in which the skin helps the body get rid of metabolic wastes and salt. 

You can read more about the location of sweat glands in the dermis in your coursework as well as vasoconstriction and vasodilation in surface capillaries. 

Physical protection 

Let’s focus a little more on the physical protection that the skin offers.

This is provided by our hair, calluses, and nails. 

Hardened keratinocytes that form into dense plates are the best way to describe our nails. 

They are tasked with providing protection for the ends of our toes and fingers as their primary job, but they can also help us when picking up objects.

Growing from the nail matrix, our nails comprise epidermal tissue that’s being significantly modified. 

Pressure can be detected by the nail, but it doesn’t have any sensory receptors. 

Calluses are protective pads that form when dead cells build up. 

This is a result of mechanical abrasion that a region of skin might continually experience. 

In this case, the stratum basale reacts by speeding up the rate of mitosis, which makes the stratum corneum grow exponentially. 

What about hair?

Well, its role is protective as well.

For example, hair prevents the scalp from receiving too much ultraviolet light.

It can also cushion our skulls if we fall, for example.

It helps regulate body temperature by insulating our skulls. 

Hairs also act as sensory receptors and will stop foreign particles from entering our eyes (eyelashes and eyebrows) and nose (hairs found in our nostrils). 

Our skin will also protect the body from minor abrasions. 

Endocrine system

Hormones, the chemical messengers found within the body, are secreted by the glands and tissues that are found within the endocrine system. 

We’ve already seen in our section on the nervous system that, together with the endocrine system, coordination and regulation of critical processes within the body are carried out.  

This includes water and electrolyte balance, stress response, reproduction, immune function, metabolism, development, and growth. 

In terms of ensuring the body stays in homeostasis, the endocrine system will play a massive role. 

Within the endocrine system, there are two major types of glands at work.

Without using a duct, an endocrine gland will release the hormones it produces right into our blood via interstitial fluid. 

The target organ then receives the hormones via the circulatory system. 

Sweat, bile, tears, oil, and other non-hormone products are released to their target areas by exocrine glands.

This is done via an epithelial surface or cavity.

These can be both inside and outside the body.

These non-hormone products differ from regular hormones because they will not connect to receptors. 

Essentially, hormones are just various molecules that will bind to the receptors that they are aimed at, and for the most part, they take the form of steroids, although not all of them.

We’ve talked earlier about the hypothalamus, pituitary gland, and others, but make sure that you work through your coursework to see exactly which hormone types they produce and what the hormone is specifically used for.

It’s easiest to learn this by drawing up a table for each of these specific glands, listing the hormones and what they do.

You can also read through your coursework to brush up on your knowledge of the major types of hormones. 

Urinary system

The structure of the kidneys

Found in the lumbar region of the body, these bean-shaped organs play a critical role. 

Not only do they help the body excrete waste products, but they will also filter our blood. 

Three layers of connective tissue surround each kidney, and these act as protection.

They are:

• Renal fascia
• Adipose capsule
• Renal capsule

The renal cortex is the capsule that the outer region of the kidney is surrounded by. 

Here, you will find nephrons, which are filtration units.

They reach the interior region of the kidney via tubules in the medulla.

Here, they run parallel to each other.

This forms tissue known as a medullary pyramid.

Urine is funneled into the ureter in a cavity known as the renal sinus. 

This occurs specifically in the renal pelvis.

Nerves and blood vessels enter and leave the kidney via a concave region known as the hilum. 

It’s through the renal artery that blood will enter our kidneys. 

From here, it splits into small arteries until it comes to the glomerulus, which is a tuft of capillaries. 

It’s at this point that all blood entering the kidneys will be filtered, and once that’s done, it leaves the kidneys through a vein network that flows into the renal vein. 

Let’s elaborate a little more on the structures found within the kidneys.

We start with the cortex.

The outside section of the kidney is called the renal cortex.

Here, ultrafiltration, where blood is filtered under high pressure, takes place, while the majority of the reabsorption of water also occurs in the renal cortex. 

Because of nephrons, the cortex is granular in appearance and very vascular.  

The loops of Henle extend into the renal medulla, a region next to the cortex that is home to the renal corpuscles and convoluted tubules of the nephrons (forming the cortical labyrinth). 

Medullary rays, which begin in the cortex and run perpendicular to the capsule, are made up of the thick, straight sections of the proximal and distal tubules and also collecting ducts. 

Henle loops that only marginally extend into the medulla are present in about 85% of nephrons. 

Deeper loops are present in the remaining 15%. 

Between the medulla’s renal pyramids are extensions of the cortex known as renal columns.

Let’s move on to the medulla. 

This is the inner part of the kidney.

It continues the process of reabsorbing water and salts that the cortex started.

These substances get into the peritubular capillaries that are connected to the nephrons. 

If the circulatory system doesn’t use the filtrate, it will leave the body as urine.

There are cone-shaped areas of tissue in the medulla called renal pyramids. 

Renal columns separate the renal pyramids from each other. 

The tops of the pyramids face the kidney’s pelvis, and the bottoms face the cortex. 

Renal filtrate is carried along tubules from the renal cortex to the renal pyramids’ tips. 

The processed filtrate (now called urine) exits the medulla through ducts in the renal papilla and enters calyces, which are collecting chambers. 

Urine then travels down the ureter and into the bladder from the renal pelvis.

Nephron structure

It is the nephron that performs the kidney’s essential functions, you will find corpuscles and tubules.

• Renal corpuscle: Tasked with filtering blood
• Renal tubule: Tasked with collecting the filtrate and then concentrating it

A structure known as Bowman’s capsule partially encloses the glomerulus, which is a collection of capillaries. 

Within this capsule, you will find a looping continuous tube, which is called the renal tube. 

It is made up of various regions, each of which has its own distinct structure and purpose. 

The loop of Henle is a U-shaped structure that is formed when the proximal convoluted tubule begins at Bowman’s capsule and then continues into the medulla. 

After that, it transforms into the distal convoluted tubule, which remains connected to the collecting duct throughout its entirety. 

The collecting duct is regarded as an independent structure that is not a component of the nephron.

Be sure to look through your coursework, where you can find out more detail about the glomerulus, Bowman’s capsule, proximal tubule, loop of Henle, collecting duct, and distal tubule. 

Urine formation

Glomerular filtration is the starting point of this whole process, and it’s the hydrostatic pressure of the blood that drives it.

As a result of the efferent arterioles having a smaller diameter where they exit the glomerulus compared to where they enter, compared to other capillaries, this pressure is far greater. 

Large particles are left behind when water and small solutes are pressed through capillary fenestrations. 

Before it can move into the Bowman’s space as renal filtrate, all fluid is first passed through a 3-layered filtration membrane. 

The layer breakdown is as follows:

• 1st layer: Endothelial lining of the capillaries
• 2nd layer; Basement membrane
• 3rd layer: Visceral lining of the Bowman’s capsule

Between the podocytes, you will find very thin filtration slits, and only very small particles can pass through these. 

The concentration of the blood and the solute found in the glomerular filtrate are very similar.

This means that around one-fifth of the blood is filtered on average, but this is pressure dependent. 

The next stage is the secretion and reabsorption of solutes. 

Solutes are removed from the blood through secretion, and then the filtrate will have these solutes added to it.

Reabsorption is the opposite, and here, the filtrate will have the solute removed and returned to the blood. 

The medulla’s high solute concentration will influence the overall concentration of urine. 

There’s another factor at play here too, and that is the hormones that control the distal convoluted tubules’ and collecting ducts’ permeability. 

You can read more about this in your coursework.

Ureter, bladder, and urethra: Storage and elimination

Urine is transported from the kidney to the bladder via a tubular organ called the ureter. 

Urine is then stored in the bladder. 

It is expelled from the collecting ducts, which serve as the body’s final sites of reabsorption, into the ureter. 

From there, urine is moved into the bladder by gravity and peristalsis. 

The bladder is an organ that resembles a bag and has the capacity to store up to 600 milliliters of urine.

The urge to urinate, however, will begin at 150 millimeters and upwards.

Transitional epithelial tissue lines the ureters, bladder, and upper portion of the urethra.

This makes expansion possible. 

When an organ swells, the stretched epithelium looks like it has fewer cell layers.

Urine is held in the bladder until it can be expelled from the body through the urethra when the detrusor muscle contracts. 

This is a smooth muscle found within the wall of the bladder. 

The parasympathetic nervous system controls the contraction of the bladder muscle, and it does so when the sacral region of the spinal cord receives impulses from stretch receptors in the bladder.

The bladder is then instructed to contract by impulses moving along efferent neurons.

Relaxation takes place in a circular smooth muscle, the internal urethral sphincter.

Sometimes it also takes place in the voluntary external urethral sphincter, if the timing is correct. 

Micturition is the process by which urine is expelled from the body. 

Here, urine travels from the bladder to the urethra and is then expelled.

Sphincter muscle

The process of the bladder being emptied is slowed down by two sphincters found in the urethra and there are the internal urethral sphincter (IUS) and the external urethral sphincter (EUS).

Made up of smooth muscle, it is between the bladder and the urethra that the IUS is found.

Until the micturition reflex is triggered, this muscle is kept contracted by the sympathetic nervous system.

As a consequence of sympathetic inhibition, the IUS relaxes, which makes it possible for urine to move through. 

The somatic nervous system governs the EUS, which is composed of skeletal muscle. 

Under the right conditions, it is possible to consciously decide to relax the EUS.  

The detrusor, which contracts involuntarily, pushes urine out of the body, and the abdominal muscles, which contract voluntarily, can speed up the flow by compressing the bladder.

Urinary system and the role it plays in homeostasis

Let’s begin with the role played in blood pressure. 

The pituitary gland releases antidiuretic hormone (ADH) in response to an increase in blood osmolality sensed by osmoreceptors or a decrease in blood pressure sensed by baroreceptors. 

Because ADH encourages kidney reabsorption of water, less fluid is lost in the urine. 

The blood’s volume and pressure both rise as a result of this.

One more way that blood pressure is kept in check is by the renin-angiotensin-aldosterone system (RAAS). 

In response to a decrease in blood pressure, the afferent arterioles’ granular juxtaglomerular cells secrete the enzyme renin. 

Angiotensin I is the product of the interaction between renin and angiotensinogen, a protein found in the plasma. 

When angiotensin I reaches the pulmonary capillaries, another enzyme acts on it, transforming it into angiotensin II. 

By increasing vasoconstriction and stimulating the adrenal cortex to produce aldosterone, this hormone increases blood pressure. 

Blood volume and pressure both increase because of aldosterone, which increases sodium reabsorption.

What about osmoregulation?

The process of controlling the amounts of water and solutes in the body’s fluids is known as “osmoregulation,” with the kidney being the main organ involved in this.  

The osmolality of the blood will increase due to dehydration or an overabundance of salt taken in.

The pituitary gland secretes ADH in response to signals from the hypothalamus’s osmoreceptors if they notice an increase in osmolality.  

When ADH is present, the collecting ducts in the kidneys become more water-permeable, allowing water to pass from the urine into the interstitium.

From there, it returns to the blood.   

In turn, this causes an erosion of blood osmolality and an elevation of urine osmolality.

Osmoregulation is influenced by aldosterone as well. 

This hormone is released by the adrenal cortex in response to hypotension. 

A higher blood osmolality can be attributed to aldosterone because it stimulates sodium reabsorption, which in turn increases water loss from the collecting tubule. 

It controls the levels of potassium and chloride ions, among others, as well. 

We also need to look at the balance of acids and bases. 

Blood pH must be tightly controlled between 7.35 and 7.45, and the kidneys play a crucial role in this process. 

The proportion of hydrogen ions to bicarbonate ions is controlled to be able to do this. 

Keep in mind that the more H+ ions there are, the lower the pH will be.

The body has built-in defenses against shifts in H+ concentration, including buffer systems like the phosphate, protein, and bicarbonate systems. 

Also remember that the bicarbonate buffer system, which is part of the respiratory system, helps balance pH.

In the presence of water, carbon dioxide forms carbonic acid (H2CO3), which then dissociates into bicarbonate ions (HCO3) and H. 

Here is the reaction for that: CO2 + H2O ↔ H2CO3 ↔ HCO3 + H+.

As respiration rates rise, H+ concentrations fall, and, as a result, pH rises, while if you slow down your breathing, the opposite holds true. 

A kidney response is more gradual but more long-lasting. 

Urine becomes more acidic as H+ ions are excreted by the renal tubules, while bicarbonate ions are retained because of the lowered blood pH.  

New bicarbonate ions can also be synthesized by both the intercalated cells in the late distal tubule and the collecting duct. 

The kidneys function to reduce blood acidity by reabsorbing H+ ions and excreting bicarbonate ions.

Finally, we look at the removal of soluble nitrogenous waste. 

Urine contains a variety of nitrogenous wastes, including ammonia, urea, uric acid, and creatinine. 

The breakdown of amino acids produces ammonia, a toxic base. 

The liver’s enzymes alter it into the safer urea form. 

Since the collecting ducts are urea-permeable, it accumulates at a high concentration in the medulla. 

The interstitium is responsible for the reabsorption of much of the urea that is secreted into the descending loop of Henle. 

Although urea makes up the majority of nitrogen waste in urine, only a small amount actually leaves the body via urination.

That’s because a large percentage is simply recycled. 

An advantage of the interstitial fluid’s high urea concentration is that it facilitates water reabsorption. 

Another nitrogenous waste product that makes its way out into one’s urine is uric acid. 

It is a waste product of purine nucleotide catabolism and is actively transported back into the body in the proximal tubule. 

Similar to urea, only a fraction of it is actually eliminated from the body.

Muscles generate creatinine as a waste product during the breakdown of creatine phosphate, after which the kidneys filter it and then it is excreted. 

Creatinine is not tubule-reabsorbed like urea and uric acid.

Immune system

Innate vs adaptive immunity

Let’s start with innate immunity. 

Innate immunity is the body’s natural, preexisting resistance to infectious diseases, and this starts from birth.  

This defense mechanism cannot learn from experience with different types of pathogens or modify itself to face changing dangers. 

Mechanical barriers, such as the skin and mucous membranes, are the first lines of defense against infection. 

Low pH in gastric juice, interferons that block viral replication, tear lysozyme, defensins, collectins, complements, and other antimicrobial proteins are some of the mechanical defenses.

Additionally, pathogens can be eliminated through phagocytosis or natural killer cells. 

Non-specific defense can also be provided by a high body temperature (fever) and inflammation.

Then what is adaptive immunity?

Over time, your immune system learns to adapt. 

Though it takes some time to get going, the immune system eventually “learns” how to respond to a specific antigen, making subsequent reactions to that antigen much quicker. 

Antigens are foreign proteins that are recognized by the immune system and then associated with specific types of cells (usually lymphocytes) that will fight off the associated pathogen. 

Lymphocytes are immune cells that can produce antibodies, kill infected cells, or orchestrate other types of immune responses in response to infections.

System cells related to adaptive immunity

T-lymphocytes (T cells) and B-lymphocytes (B cells) are the two main lymphocytes involved in adaptive immune responses. 

The thymus is responsible for the development of T cells, which play a role in cell-mediated immunity. 

When T cells recognize their antigen on the surface of an antigen-presenting cell (APC), they become activated. 

T cells grow and divide into different subsets when they bind to APCs. 

To eliminate infected or abnormal cells, the body produces cytotoxic T cells. 

In response to future infections, certain cytotoxic T cells can generate memory T cells. 

As well as alerting other types of WBCs, helper T cells produce cytokines that promote the proliferation of T and B cells. 

In order to halt the immune response, regulatory or suppressor T cells suppress both T and B cells.

Bone marrow is where B cells develop, and these cells play a role in humoral immunity. 

B cells are initially activated when they come into contact with circulating antigens. 

Many B cells, however, need a helper T-cell acting as co-stimulation.

B cells undergo terminal differentiation into plasma cells and memory B cells after they bind to their target antigens. 

Antibody binding antigens are secreted by plasma cells. 

Antibodies can be made by memory B cells, but only in response to a future infection.

System cells related to innate immunity

Many different types of cells, including phagocytes, contribute to innate immunity. 

The vast majority of circulating white blood cells are neutrophils. 

These phagocytes are the first to arrive at the scene of an infection, and they use chemotaxis to track down and devour the microbial invaders. 

Eosinophils control the immune system’s inflammatory response and secrete chemicals that are lethal to parasitic worms and other invaders.  

Histamine is released by mast cells and basophils to promote inflammation, as well as heparin to prevent blood clots. 

Macrophages, which are large WBCs that surround pathogenic microorganisms and debris and function as antigen presenters to effector T cells.

They come from monocytes, the largest leukocytes. 

Dendritic cells perform a similar role, but they stimulate naive T cells. 

Infected cells are eliminated by natural killer cells, which are not phagocytes but instead bind to them and release granzymes that cause apoptosis.

Lymphatic system

Let’s look at the various structures found in this system. 

The thymus, the marrow in the bone, the tonsils, the spleen, the lymphatic vessels, the lymph nodes, and the lymph are all components of the lymphatic system. 

Lymph is a colorless liquid that resembles plasma in its chemical makeup. 

The subclavian veins are the destination of this flow, which only occurs in one direction (toward the neck). 

Leaking blood capillary fluid, and white blood cells make up lymphatic fluids. 

Like veins, lymphatic vessels have thin walls and valves to prevent blood from flowing backwards. 

However, because their walls are more permeable, the lymph can flow into them and be circulated. 

Muscle contractions, both smooth and skeletal, are responsible for the movement of lymph. 

The only tissues in which lymphatic vessels are absent are the central nervous system and the avascular tissues, otherwise, they are found throughout the body.  

Lymph nodes, which are oval-shaped masses of tissue that contain lymphocytes and filter out foreign substances, interrupt the vessels. 

Mature lymphocytes are created in the lymphatic system’s two primary organs, the bone marrow and the thymus.  

Lymphocytes can also be found in auxiliary organs like the spleen and tonsils. 

Disease-causing microorganisms are no match for these specialized white blood cells.

Antigens and antibodies

Antigens are substances that cause the body’s immune system to react. 

Antigens are large biomolecules (typically proteins) that are recognized as foreign by the immune system. 

They are located on the outer layers of antigenic substances like bacteria, fungi, viruses, and pollen. 

The origin of foreign antigens lies outside the human body.

However, the immune system rarely reacts to self-antigens because they are naturally occurring in the body. 

They cause reactions in other people, such as the rejection of donated organs and tissues.

Antibodies, also known as immunoglobulins, are produced by B cells and are able to recognize and bind to a variety of antigens. 

Antibody-antigen binding can neutralize pathogens in several ways. 

A common method of killing pathogens is to cause them to agglutinate (clump together) before clearing the area of them. 

The antigen’s ability to attach to cells can be neutralized by antibodies, and the antigen itself can become insoluble.

Occasionally, they trigger the protein system known as complement, which boosts the efficiency of the immune response. 

Through opsonization, additional immune cells can be activated, and phagocytosis can be improved. 

Antibodies aid in stopping the spread of infection by triggering an inflammatory response.

Transport of large glycerides and proteins and fluid distribution equalization

The maintenance of fluid balance between the blood and tissues is an important immune system function. 

Leakage of fluid from blood vessels into surrounding tissues occurs due to the higher hydrostatic pressure within the blood vessels compared to the interstitial fluid. 

Excess interstitial fluid (now called lymph) is drained through porous lymphatic capillaries into the right and left subclavian veins. 

Lymph is filtered and cleansed by lymph nodes along its path through the lymphatic system. 

At some point, it returns fluid to the bloodstream via the thoracic duct (which drains into the left subclavian vein) or the right lymphatic duct (which drains into the right subclavian vein). 

Edema results from fluid leaking back into tissues when lymphatic vessel pressure is too high.

A number of biomolecules can also be moved around with the help of the lymphatic system. 

Specific lymph capillaries called lacteals are located in the villi of the small intestine and are responsible for fat absorption. 

Chylomicrons, which carry these fats, give the lymph (called chyle) its characteristic white color. 

Plasma proteins or cells that have escaped from their blood vessel confines can be transported back into the circulatory system via the lymphatic system. 

Skeletal system

Structure

Different types of bones include long, short, flat, irregular, and sesamoid bones. 

The primary roles of the body’s long bones are in movement and in bearing the body’s weight. 

Their length exceeds their width, giving them the appearance of rods. 

Long bones have wider ends (epiphyses) than their middle section (diaphysis), which is covered in articular cartilage. 

The majority of the bones in your arms, legs, and even your collarbones are classified as “long bones.” 

Short bones can be either spherical or cube-shaped. 

They serve more of a stabilizing and supporting role than a mobility one. 

The carpals in the hand and the tarsals in the foot are two examples of short bones. 

As their name suggests, flat bones are thin, flattened, and can sometimes include a curve.

Their wide design is optimal for both providing protection and muscle attachment. 

Flat bones include the scapulae, sternum, ribs, pelvic ilia, and some cranial bones. 

Bones that don’t fit the above categories are called “irregular,” and their unusual appearance serves a specific purpose. 

The vertebrae and many of the bones in the face are examples of irregular bones. 

The kneecap is an example of a sesamoid bone.

There is a great deal of mechanical stress exerted on these bones, and because of this, they are embedded in tendons.

While we’ve talked about structure in a more general sense, let’s move on to the actual makeup 

of the bone itself. 

Hard, dense, compact bone (also called cortical bone) makes up the skeleton’s articular surfaces and the shafts of long bones. 

Osteons, also known as Haversian systems, are the spherical building blocks of this structure. 

Perforating canals (also known as Volkmann’s canals) connect the Haversian canals at the center of each osteon, which house nerve fibers and blood vessels. 

Calcified lamellae encircle the Haversian canal, and in each of the tiny spaces between them called lacunae, an osteocyte can be found. 

The lacunae are linked by canaliculi, which carry oxygen and nutrients to the osteocytes and remove waste products.

Porous, spongy bone resides at the ends of long bones as well as within the vertebrae and flat bones.  

It lacks the strength and density of compact bone and also lacks osteons. 

Instead, it is made up of squarish plates that are linked together to form trabeculae. 

Blood cells are made in the red bone marrow, which is located in the spaces between the trabeculae. 

Osteocytes are not located in central canals but rather in lacunae that are linked by canaliculi.

What about bone and its cellular composition?

The extracellular matrix in bone serves a role similar to that of reinforced concrete by surrounding bone cells. 

Calcium phosphate (hydroxyapatite) and calcium carbonate (and other minerals) make up the majority of the inorganic material in the matrix, around two-thirds. 

About a third of the matrix is made up of organic material. 

Collagen and other ground substance proteins like glycosaminoglycans (GAGs) give the matrix strength and pliability.

Bone cells can be divided into three categories. 

Osteoblasts, which themselves originate from osteoprogenitor cells, absorb calcium from the blood and then use it to synthesize the matrix of bone, which is composed largely of collagen fibers. 

The osteoblast becomes the osteocyte, the mature bone cell, once it is completely encased in the matrix. 

The most numerous bone cells are osteocytes, which keep the matrix healthy by calcium salt recycling. 

Large, multinucleated cells called osteoclasts are produced when monocytes (large white blood cells) fuse together. 

They release digestive enzymes and acids that help break down bones, with the calcium then returning into the blood and can be found on the surface of bones.  

Skeletal system functions

Structural support and rigidity

The bones, ligaments, tendons, cartilage, and other tissues that make up the skeletal system serve as a support system for the rest of the body. 

This structure is crucial for maintaining the body’s physical integrity. 

Bones serve many functions, including bearing the body’s weight, shaping various body parts, and securing the organs of the body. 

Muscles are able to move the body because bones provide anchors for them to attach to. 

Tendons connect skeletal muscles to bones, and ligaments connect bones to each other. 

Cartilage can flex more than bone and provides structural support for a wide range of body parts. 

The skeleton provides additional safety for the internal organs. 

The pelvic girdle shields the lower digestive tract, the bladder, and the internal reproductive organs from injury, just as the skull protects the brain and spinal cord from injury, for example.  

Bone marrow, a soft tissue, is likewise shielded within the solid structures of certain bones.

Bones play a role in calcium storage as well.

The cells of bone create an inorganic acellular matrix that is roughly 65% mineral and 35% collagen. 

Hydroxyapatite, a calcium phosphate, accounts for the bulk of the inorganic material. 

Muscle contraction, nerve impulse transmission, and blood clotting are all processes that require calcium. 

About 99% of the calcium the body absorbs from food is stored in our teeth and bones. 

Osteoblasts are bone-forming cells that actively remove calcium from the blood and insert it into the bone matrix when blood calcium levels are high. 

Over time, these cells become encased in the dense, calcium-rich secretion and mature into osteocytes.  

Osteoclasts are specialized cells that dissolve bones to replenish blood calcium levels. 

The rate of calcium deposition is similar to the rate of calcium loss in a healthy person. 

When calcium homeostasis is disrupted, hypercalcemia or hypocalcemia can develop.

Bone fracture types

When the force exerted on a bone is greater than the bone’s ability to withstand it, the bone fractures or breaks, and these can be classified in a multitude of ways.

It is called a closed fracture when the shattered bone does not protrude through the skin. 

When bone cracks and pokes through the skin, exposing the broken edge, this is called an open fracture.

When the bone breaks in several places, it is called a comminuted fracture. 

This is typical following a traumatic event like a car wreck or during intense athletic competition. 

When a section of a bone bends but does not completely break, it is called a greenstick fracture, and it is seen more in young children because their bones are not fully developed yet. 

A bone that has been spiraled or rotated like a corkscrew is known as a spiral fracture. 

When a tendon or ligament is overstressed and pulled, it can tear away from the bone and cause a fracture known as an avulsion fracture. 

An oblique fracture occurs when the outside force that caused the break was perpendicular to the bone. 

When a bone breaks perpendicular to its long axis, it is called a transverse fracture.

When disease weakens a bone to the point where it breaks easily under normal pressure, the result is a pathological fracture.

Structure and formation of cartilage

Cartilage is an elastic connective tissue that is also resistant to being stretched out of shape.  

Except for the perichondrium that makes up the surfaces of nearly all cartilage, cartilage lacks innervation and a blood supply. 

Chondroblasts are the immature cells of cartilage that are responsible for matrix secretion. 

Maturated cells, known as chondrocytes, can be found in lacunae and are produced by chondroblasts.

Hyaline, elastic, and fibrocartilage are the three types of cartilage that you will find throughout the body, with hyaline being the most common.  

It has a glassy appearance because of the collagen fibrils that make up its composition. 

Ribs, nose, trachea, and articular surfaces are all examples of places you’ll find cartilage because of the need for both rigidity and mobility. 

Similar to hyaline cartilage but with more give due to elastic fibers, is elastic cartilage. 

It is found in the epiglottis and the external ear. 

Collagen in fibrocartilage is organized into dense fibers, giving it strength against compression and tension, and this is found between vertebrae as well as in the knee and jaw. 

Next, let’s look at tendons and ligaments. 

Bones are held together by ligaments, which also aid in joint stability. 

Muscles are able to move the body because tendons attach them to the skeleton or other structures like the eyeballs, for example. 

Dense regular connective tissue, made up of bundles of collagen fibers and elastic fibers, is a common component in both. 

This makes them robust and capable of withstanding further stretching. 

Tendons, in contrast to ligaments, have more closely packed collagen fibers. 

And unlike the fibers of many ligaments, they are bundled together in parallel fashion. 

Ligaments are more flexible than tendons, but tendons are stronger. 

Because of the protein elastin, some ligaments appear yellow.

It plays a part in endocrine control too.

Bone resorption and being broken down are processes controlled by hormones in the endocrine system that affect calcium balance. 

1,25-hydroxyvitamin D, also known as calcitriol, is synthesized by the kidneys. 

By encouraging intestinal calcium absorption, which raises blood calcium levels, this hormone serves to maintain healthy physiologic ranges for calcium. 

Additionally, calcitriol stimulates osteoclasts, which release calcium from bones into the bloodstream. 

Thyroid parafollicular cells release the peptide hormone calcitonin in response to elevated blood calcium levels. 

Inhibiting bone-resorbing osteoclasts and encouraging bone-building osteoblasts is what calcitonin does. 

Parathyroid glands release the peptide hormone parathyroid hormone (PTH) in response to a deficiency of calcium in the blood. 

Because of this, both the number and efficiency of osteoclasts increase.

Biology

Biology: Cell support and organization

Cells

The cell is the fundamental unit of organization for all known forms of life. 

There is a purpose for every part of a cell, and together they allow an organism to thrive. 

Cells come in a wide variety, and each kind of organism has its own special set of cells. 

Cells may differ in many ways, but they all share something essential: a membrane, which is semi-permeable and made up of phospholipids.

In addition, there are transport proteins, also known as transport holes, that aid in helping specific molecules and ions move in and out of the cell.

A substance known as cytoplasm or cytosol fills the cellular interior. 

Different organelles, or clusters of complex molecules, contribute to a cell’s continued existence and are surrounded by membranes that are chemically distinct from the cell membrane. 

An increasing number of organelles is required for the survival of a larger cell.

Let’s look at the structural organization of cells. 

Plants, animals, fungi, protists, and bacteria all show evidence of structural organization at the cellular and organismal levels. 

DNA and RNA, the building blocks for proteins, are present in every type of cell, and the cellular structure of all known organisms is extremely well-organized. 

The components of a cell are the nucleus, cytoplasm, and membrane. 

Within the cell, organelles like the mitochondria and chloroplasts perform very specific tasks. 

All the elements required for life can be found within a single cell of a unicellular organism. 

Specialization of cells is possible in multicellular organisms, and various cell types can perform various tasks. 

Whether it’s through sexual reproduction or asexual reproduction, life starts in a single cell. 

Tissues are collections of cells that are then grouped together into organs, which, in turn, work together in various systems within the body. 

We also need to touch on the defining characteristics of eukaryotic cells.

The presence or absence of a nucleus is the primary dividing line between the two broad categories of cells. 

In fact, “true kernel” and “before the kernel” are the literal translations of the terms “eukaryote” and “prokaryote,” respectively. 

Almost all of the DNA in a eukaryotic cell is housed in a membrane-bound structure called the nucleus. 

Linear chromosomes are formed when DNA molecules in eukaryotes coil around their associated proteins, and the genes within them are controlled by molecules in the nucleoplasm. 

The nucleus is the cell’s regulatory hub because of this.  

Other membrane-bound organelles, such as mitochondria, endoplasmic reticulum, Golgi bodies, peroxisomes, and lysosomes, are also characteristic of eukaryotic cells. 

Both prokaryotic and eukaryotic cells have ribosomes and a cytoskeleton, components that are not covered by membranes. 

These types of cells also differ in the way that they divide. 

Eukaryotic reproduction, known as mitosis, is more complex than the process used by prokaryotes to reproduce, known as binary fission. 

Mitosis is the process by which a cell’s duplicated chromosomes are aligned along the cell’s equator and then separated into two identical nuclei at the centromere.

Cell structure

Ribosomes: Protein synthesis from amino acids requires ribosomes. In fact, they account for roughly 25% of the total cell population. There can be thousands of ribosomes in a single cell. Some are free-floating, while others are fixed in place in the rough endoplasmic reticulum.

Golgi complex (Golgi apparatus): This is a crucial step in the synthesis of molecules like proteins that will be secreted from the cell. It’s a collection of membranes that sit close to the nucleus.

Vacuoles: Storage, digestion, and elimination all take place inside these sacs. In plant cells, only one large vacuole exists. Vacuoles found in animal cells and can be quite small and numerous.

Vesicle: A tiny organelle found inside a cell. It’s surrounded by a membrane and helps the cell in many ways, including transporting substances around inside the cell.

Cytoskeleton: The microtubules found in the cytoskeleton aid in cellular structure and support.

Microtubules: These are structural components of the cell’s cytoskeleton and consist of protein.

Cytosol: This is the intracellular fluid. It is made up of water mostly but there are floating molecules in it as well.  

Cytoplasm: The cytosol and the organelles contained within the plasma membrane are meant by this umbrella term; however, the nucleus is not included.

Cell membrane (plasma membrane): This serves as a boundary, defining the cellular space. External substances are blocked from entering the cell while cytoplasm remains inside. 

Endoplasmic reticulum (ER or RER): Rough endoplasmic reticulum, which contains ribosomes, and smooth endoplasmic reticulum, which does not, are the two types of endoplasmic reticulum. It is the tubular network that makes up a cell’s transport system. It starts at the nuclear membrane and continues all the way to the cytoplasmic membrane.

Mitochondrion (pl. mitochondria): Size and number of cells in these structures can range widely. Mitochondrion counts can range from one in some cells to the thousands in others. This structure has multiple roles, including ATP production and participation in cell proliferation and death. The mitochondria have their own DNA, which is separate from nuclear DNA.

Let’s look specifically at the structure of the Golgi apparatus and the role it plays in packaging and secretion. 

The Golgi apparatus is made up of a network of cisternae, which are rounded, flattened sacs. 

Vesicles containing immature proteins are sent from the RER to the cis face of the stack (the side closest to the ER). 

Vesicles deliver proteins to the Golgi by fusing with the membrane there. 

Proteins propagate from one stack to the next, each time budding off a new vesicle that fuses with the subsequent cisterna layer.

A variety of Golgi enzymes make adjustments to the proteins en route. 

Some or all of the sugar residues attached to proteins in the ER during glycosylation may be removed or replaced, while the addition of phosphate and sulfate groups could also occur.

These identifiers, or “tags,” not only affect the protein’s structure and function but also aid in its sorting and delivery to its proper locations. 

It is from the trans (exit) face of the Golgi that these proteins are packaged into vesicles. 

Through the process of exocytosis, some of these proteins are released from the cell, while others are incorporated into the cell membrane. 

Other proteins in lysosomes act as hydrolyzing enzymes.

Let’s also go a little deeper into lysosomes. 

The lysosome is an organelle involved in the degradation of compounds.

They form as extensions of the Golgi apparatus and encapsulate hydrolytic enzymes that would be harmful to the cell if not.

Since these enzymes function best at a pH of about 5, hydrogen ions are constantly being pumped into the lysosome to keep it at that acidic level. 

Through a process called autophagy, lysosomes disassemble and recycle various substrates and nonfunctional intracellular components, contributing significantly to cell homeostasis. 

Autophagosomes, which are double-membrane vesicles, contain many of the degradation-bound substances. 

These (and other endocytosed vesicles) can fuse with lysosomes to trigger the release of digestive enzymes. 

The lysosome membrane can be crossed to allow the entry of other substances. 

When a cell’s lysosomes are compromised, the cell dies through apoptosis or, in extreme cases, necrosis. 

Tay Sachs disease is one of several lysosomal storage disorders that have been linked to mutations in lysosomal hydrolases.

Cytoskeleton

First, we look at the role it plays in movement and cell support.

The cytoskeleton is a network of protein filaments that supports all eukaryotic cells but lacks a membrane. 

The cytoskeleton is made up of microfilaments, intermediate fibers, and microtubules in eukaryotic cells. 

In addition to maintaining the shape and structure of the cell, the cytoskeleton also plays a role in the transport of materials within the cell and the movement of the cell itself. 

Cells are able to keep their shape or adapt to new environments thanks to the dynamic cytoskeleton, which can extend and retract to meet these demands.  

Most organelles are held in place by the network of protein fibers, which also serves as a “railway” for motor proteins to guide vesicles to their correct destinations. 

The cytoskeleton provides structural support for the cell and, in some cases, helps the cell move by forming appendages like cilia and flagella. 

Because of its role in separating sister chromatids and pinching the cell into daughter cells during cytokinesis, the cytoskeleton is essential for cell division.

Let’s move on to microfilaments. 

Microfilaments typically have a diameter of 6-8 nm.

They are the smallest components of the cytoskeleton. 

Made up of actin, which is two-rod-like polymers joined together and made out of protein molecules, these then form flexible tension-bearing filaments by twisting around each other. 

Tasked with keeping cell shape as well as muscle contraction, cell movement, and cytokinesis, they are organized into networks.  

By contracting, microfilaments establish a cleavage furrow during the cytokinesis process.

These microfilaments will decrease in size as they contract.

Their shape is that of a ring.  

When the cytoplasm of a cell is squeezed enough, the parent cell divides in half.

These cells are called “daughter cells.”  

Microfilaments play a role in muscle contraction too.

Myofibrils form when the protein myosin attaches to actin filaments. 

The contraction of the muscle fibers leads to these two parts sliding past one another. 

During actin polymerization, the plus end of a cell is elongated, while the minus end is shortened during acting depolymerization, and this helps with gross movement. 

Next are microtubules.

In the cytoskeleton, these are the thickest components you will find, and they have a 25 nm diameter.

They are composed of tubline, a globular protein that is a dimer of both A-tubulin and B-tubulin. 

Protofilaments are formed when these dimers are stacked in rows.

A ring of 13 protofilaments then creates a hollow tube. 

When tubulin dimers polymerize or depolymerize, the microtubules can lengthen and shorten.

It’s right throughout cells that microtubules are found.

They not only provide a path of travel for proteins (kinesins and dyneins),  but also help the cell fight compressional forces. 

A variety of the above-mentioned motor proteins transport vesicles to their final locations.  

The mitotic spindle, essential for separating the two sets of sister chromatids during mitosis, is mostly made up of microtubules. 

They also help form cilia and flagella, and this occurs when nine pairs that then surround a central pair group together.  

Then there is the part that intermediate filaments play. 

The intermediate fibers found in the cytoskeleton have a diameter of around 10 nm.

This means they are thinner than microtubules but thicker than microfilaments. 

More than fifty individual protein types make up these cells, with each serving a unique function.

The microfilaments that line the interior of the nuclear envelope are created by proteins called lamins. 

They are not polar, like their cytoskeletal counterparts, and they do not play a direct role in cell motility as they are only there to provide support.

They do this through desmosomes, which are cell junctions that help cells stick to one another. 

The nucleus and other organelles are better secured thanks to them as well. 

Because of their enhanced resistance to tensile forces, intermediate filaments play an important role in keeping cells from becoming deformed when subjected to mechanical stress. 

In contrast to microtubules and microfilaments, they do not undergo polymerization and depolymerization.  

Lastly, in this section, what part do flagella and cilia play?

Some cell types have these protruding structures, both of which are made of microtubules, as we learned briefly earlier. 

Doubled up into pairs in eukaryotic cells, you will see a central pair with nine doublets forming a ring around it. 

This is called a 9+ 2 arrangement. 

Despite their similar diameter of 0.25 m, flagella are longer than cilia.

In terms of numbers, however, there are far fewer flagella than cilia in cells. 

Both structures are able to wave back and forth through the action of motor proteins called dyneins.

Cells that are stationary within a tissue may use cilia to move materials along the surface, while cells that move do so using cilia. 

Mucus is expelled from the lungs by ciliated cells, while the egg is transported by ciliated cells in the female reproductive system. 

Some types of cilia are capable of signal detection and intracellular transmission.

Unlike cilia, locomotion is the only task of the flagella cell; for example, they are used by human sperm for motility.  

Biology | Membrane and its channels

Plasma Membrane

This membrane plays a general role in cell containment.

While this is its most basic function, it still does play a role in many others. 

This includes cell to cell recognition, cell signaling, as well as the transport of materials. 

Basically, all types of cells have a membrane formed of a double layer of phospholipids that encompasses the cytoplasm. 

Phospholipids, helped by cholesterol and protein molecules, create a fluid-like barrier within the membrane where the cell’s interior holds structures and molecules.

The proper concentration of various substances is also kept on both sides of the membrane through this. 

The interior of the membrane is hydrophobic.

This is because it is with their fatty acid chains pointing inward that phospholipids orient themselves.

Despite being somewhat impermeable to water soluble substances, this trait allows the membrane to remain intact in its aqueous environment. 

Next, let’s look at phospholipids and phosphatides.

Two nonpolar fatty acid chains are joined to a glycerol-based polar head, a phosphate group, and an organic R-group to form a phospholipid. 

Because they have a hydrophilic (polar head) as well as a hydrophobic (nonpolar tail), phospholipids are said to be amphipathic, and due to this, it is into micelles (bilayers) that they arrange themselves. 

One layer of phospholipids with the tails pointing inward to form a hydrophobic core makes up a micelle, a tiny spherical structure, and they are tasked with moving lipid soluble materials within a cell. 

They are used to transport lipid soluble materials. 

The phospholipids move into parallel layers with the heads pointing out and the tails pointing in, and when they do so, they form a bilayer. 

Phospholipid bilayers that enclose the organelles in eukaryotic cells also do so for liposomes. 

The cell membranes that are created by these bilayers also control how materials enter and exit various cells.

What about the protein component?

The majority of the membrane’s functions are made possible by the proteins within it.

The movement of various ions and molecules across the membrane, catalyzing chemical reactions, fusing together neighboring cells, cell signaling, cellular support and stability, and cell recognition are a few of these. 

Integral proteins are those that pierce the membrane’s hydrophobic interior.

The classification of cells is facilitated by glycoproteins, integral proteins with a connected sugar chain. 

Transmembrane proteins, which are integral proteins that extend entirely through the membrane, are frequently utilized as cell signaling receptors. 

The receptor will bind to a signal molecule (such as a hormone) from the extracellular side, which will then transmit a message to the cytoplasmic side. 

Proteins that cross the membrane are carried by transmembrane proteins. 

Transport proteins, also called channel proteins, are arranged in a tunnel-like fashion. 

In this way, materials can move passively.

Carrier proteins, however, use either passive or active transport to move materials and do this by changing their arrangement.   

Peripheral proteins frequently function as enzymes or receptor proteins and are loosely connected to the membrane on either side.

Osmosis is our next topic in this section, and it is the process of water diffusion taking place across a membrane that is semipermeable.  

Water moves from a higher concentration to a lower concentration, or from a lower solute concentration to a higher solute concentration, according to the principle of net concentration gradient movement. 

When the solute cannot cross the membrane (or cannot cross quickly enough to maintain homeostasis), osmosis can assist in reestablishing equilibrium. 

Water moves equally into and out of cells when the extracellular fluid is isotonic, which occurs when the cytoplasm and extracellular fluid have the same solute concentration.

We’ve mentioned it, but let’s discuss passive transport in more detail, which occurs when, without energy input, substances move across the cell membrane.

In an unplanned process that increases entropy, random particle motion results in a net movement of substances down their concentration gradients. 

This type of transport in cells includes osmosis, simple diffusion, and facilitated diffusion. 

Without a transport protein, substances move across the membrane directly through simple diffusion. 

The hydrophobic interior of the membrane does not hinder small, nonpolar molecules.

These include oxygen, carbon dioxide, as well as uncharged lipids.

Water crosses the membrane passively through osmosis.

While the majority of polar molecules are incapable of using simple diffusion, water molecules are sufficiently small to slowly slip past phospholipids. 

In order to speed up osmosis, water can also channel aquaporin, which is a protein. 

Facilitated diffusion is the process of using proteins to move substances down concentration gradients. 

Carrier or channel proteins might be needed by large, polar, or charged substances to shield them from the membrane’s interior.  

These processes are all ATP-free and are triggered by the different solute concentrations.

What about active transport, which also sees solutes moving either into or out of a cell? 

From low to high concentration areas, substances are pumped here against concentration gradients when this occurs. 

Active transport is essential for functions like maintaining membrane potential and intestinal cells’ absorption of glucose. 

When a carrier protein pumps solutes during primary active transport, ATP hydrolysis occurs at the same time.

The protein undergoes a conformational change as a result of the binding of a phosphate group.

This makes it possible for it to move solutes across the membrane. 

The electrochemical gradient created by secondary active transport, directly drives the active transport of a different solute, with one solute moving down, and another pumped up.

This is dependent on ATP creating the gradient, however.

A symport is when it’s in the same direction that both solutes move, while an antiport is if they move in opposite directions.   

Vesicles are used in the active transport processes known as endocytosis and exocytosis to import and export various substances. 

Let’s look at membrane channels, a transport protein subclass.

It is tasked with ensuring that small, charged particles pass through, and it does so by creating pores. 

They function as a tunnel for particles of a certain size and charge and are associated with the solutes they transport. 

All channels use facilitated diffusion to move materials down their concentration gradient.

Channels move quickly across the membrane because they interact with the solutes they transport in a far weaker manner when compared to carrier proteins.

Aquaporins are always open channel proteins that allow water to pass through, and they are essential to osmosis.

In contrast, ion channels are typically gated.

Depending on various stimuli, they will open and close.  

Changes in membrane potential trigger a response from voltage-gated channels.  

Electrical impulses are produced by these ion channel types in both nerve and cardiac cells. 

When a ligand, such as a hormone or neurotransmitter, binds to the cell, ligand-gated ion channels open. 

Sensory tissues benefit from mechanically-gated ion channels because they react to a physical stimulus, such as membrane stretching.

Last in this section, we cover exocytosis and endocytosis.

For the transportation of very large particles or huge amounts of smaller particles, endocytosis and exocytosis are types of vesicular transport used.

Because both the movement of vesicles and their pinching off require energy, both are active transport examples

Exocytosis is the process by which cellular waste and products are transported via vesicles to the cell membrane.

Here, the extracellular content of the vesicle is released once it fuses. 

Exocytosis is also the integration of particular membrane substances into the cell membrane. 

These can include glycoproteins and glycolipids, for example.

The intake of liquid, substantial particles, or target molecules are all aspects of endocytosis. 

The material is engulfed and pinched off into a vesicle as a result of the cell membrane folding inward when this process occurs.

Pinocytosis, the process of ingesting fluids, is non-specific. 

This means that the nutrients and enzymes are available at the time they are ingested.

Particles and occasionally even whole cells are engulfed during phagocytosis. 

By using this process, immune system cells take in harmful bacteria before destroying them. 

Low-density lipoproteins, or LDLs, are one type of molecule that receptor-mediated endocytosis is specifically designed to target. 

These molecules attach to receptors on the cell membrane, which causes the membrane to enlarge and form a vesicle. 

Biology | Nuclear parts of a cell, mitochondria, and endoplasmic reticulum

Endoplasmic reticulum

Let’s start this section by looking at its smooth and rough components. 

Cisternae are the continuous membranes that make up both the rough and smooth endoplasmic reticulum (ER), but their structure and function are very different. 

The ribosome-studded cisternae of the rough ER look like flattened sacs, and they form a continuous membrane with the nuclear envelope.

After being synthesized by the ribosomes, polypeptides are directed into the lumen of the rough ER, where they undergo modifications before being packaged into a vesicle and transported to other parts of the cell, but mostly the Golgi apparatus.  

When the proteins reach the Golgi, they can be modified further and sorted according to their final destinations. 

The process of exocytosis releases many of them from the cell.

When we compare the cisternae of the smooth ER, for one, they don’t have ribosomes, and their shape is far more tubular.  

It is in the nucleus and the rough ER that their membranes continue. 

One of the many functions of smooth ER is the production of lipids.

This includes cholesterol and phospholipids. 

In the muscles, the smooth ER is responsible for calcium ion regulation and storage, and in the liver, for drug detoxification. 

Next, let’s discuss the rough endoplasmic reticulum site of ribosomes.

Rough endoplasmic reticulum in the cytoplasm is where secretory (proteins destined for export from the cell) and plasma membrane-associated proteins are synthesized. 

These ribosomes will bind to translocons, as they are not fixed in place. 

Free ribosomes in the cytosol are structurally similar to their bound counterparts, but the proteins they produce remain in the cytosol rather than being exported.  

After a polypeptide chain has been synthesized from an attached ribosome, it is transported to the lumen through a tiny pore.

Here it undergoes final folding, with those that fail to take on the correct shape being recycled.  

Lumen-resident enzymes may convert a protein into a glycoprotein by covalently bonding a carbohydrate to it. 

Transport vesicles are used to encase proteins destined for other parts of the cell, and once they reach their destination, they fuse with it.

As for membrane structure, make sure you check out your official coursework, as we won’t cover that small section here. 

Mitochondria

When it comes to a cell’s ATP, it’s the mitochondria that produce most of it.  

These ATPs are made in the inner membrane, while a selective barrier is formed by an outer membrane.

The cristae are folds in the inner membrane where the electron transport chain of aerobic respiration is found.

ATPs are produced in the intramembranous space between membranes and provide the proton motive force that drives chemiosmosis (the process of ATP production). 

During oxidative phosphorylation, protons are moved across the intermembrane space and back into the mitochondrial matrix (the inside of the mitochondrion) via the inner membrane protein ATP synthase. 

ATP synthase receives its energy to phosphorylate ADP from the movement of the proton. 

DNA from mitochondria and ribosomes are housed in the matrix. 

In humans, the 37 genes contained in this DNA are essential for proper mitochondrial function. 

Apoptosis, or programmed cell death, is another process in which mitochondria play a role. 

Under oxidative stress, proteins linked to the inner mitochondrial membrane migrate into the cytoplasm and activate other proteins that kick off the cell’s degeneration.

Nuclear parts of a cell

Nucleus (pl. nuclei): A cell’s nucleus is a compact structure that houses the chromosomes and controls the cell’s DNA. All eukaryotic cells contain a nucleus, it’s the structure that defines them. Generational genetic traits are passed on by the nucleus. It is made up of a nuclear envelope, nucleoplasm, nucleolus, nuclear pores, chromatin, and ribosomes.

Chromosomes: These DNA strands are extremely compact and rod-shaped. Deoxyribonucleic acid (DNA) refers to the genetic material in plants and animals that stores hereditary information.

Chromatin: The genetic material and proteins that together form a chromosome are found here.

Nucleolus: This protein-based structure is housed within the nucleus. It synthesizes and stores RNA (ribonucleic acid), is small and spherical, and lacks a membrane.

Nuclear envelope: This encases the nucleus’s structural components. Lipids make up the outer and inner membranes they are made of. 

Nuclear pores: The transfer of materials between the nucleus and the cytoplasm depends on these.

Nucleoplasm: Similar to cytoplasm, this is the fluid that fills the nucleus.

Storage of genetic information, and compartmentalization 

The majority of a cell’s genetic information is kept in its nucleus.  

The DNA in the nucleus is protected by a double membrane called the nuclear envelope. 

To control the flow of substances like RNA, ribosomal subunits, proteins, ions, and signaling molecules, these pores are constructed from large protein complexes. 

The ribosomal subunits are synthesized in a non-membranous nucleolus that is encased in the nucleoplasm (a semifluid) along with chromatin. 

The nuclear lamina is a network of protein filaments that cover the inner nuclear membrane and serve to stabilize the nucleus while also controlling processes like DNA replication and cell division. 

The outer membrane connects directly to the ER. 

DNA is stored in the nucleus, and replication and transcription (the synthesis of RNA) take place there as well. 

The nucleus is crucial to cell function coordination due to its role in transcriptional regulation of gene expression.

Location and function of the nucleolus 

Producing ribosomal subunits, the nucleolus is the largest structure within the nucleus. 

It is composed of three distinct areas, including a granular region and two regions made up of fibrillar threads. 

Transcriptions of ribosomal RNA genes take place at the FC, or fibrillar center. 

The pre-rRNA is processed in the dense fibrillar center (DFC), and the GC is responsible for the assembly of the ribosomal subunits. 

The nucleolus is the site of rRNA synthesis, with the exception of 5S-rRNA synthesis, which occurs in the nucleoplasm. 

It is the nuclear pores that allow the subunits to leave the nucleus.

During mitosis, the nucleolus disappears during prophase and then reappears during telophase. 

It does not initially appear as a unified structure, but rather as ten smaller units at different locations on the chromosome called nucleolus organizer regions (NORs).

Nuclear pores and envelope 

The nucleoplasm and the cytoplasm are separated by a double membrane called the nuclear envelope, which completely encloses the nucleus. 

The nuclear pores join the two membranes, and the perinuclear space, which is 20-40 nm in width, separates the two phospholipid bilayers. 

Octagonal aqueous channels called nucleoporins are the building blocks of each pore. 

In order to transport large molecules like RNA and specific proteins between the nucleus and the cytoplasm, these proteins communicate with a class of transporter proteins called karyopherins. 

There is no need for a transporter to facilitate the passage of smaller molecules and ions through the pore complex, however. 

The pores are necessary for the export of messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal subunits, which are needed for translation, and the import of enzymes and nucleotides, which are needed for DNA synthesis and transcription.

The lumen (inner space) of the endoplasmic reticulum (ER) is permeable to the perinuclear space, and the nuclear envelope’s outer membrane is integrated with the ER. 

As a result, information and materials can move freely between the two organelles.

A network of protein filaments called the nuclear lamina lines the nucleoplasmic side of the inner membrane, providing structural support for the nucleus and facilitating chromatin organization.

Make sure you read through the section in our coursework about mitosis, the stage in which cells divide. 

Biology | Fertilization and gametes

Gametes production and development  

Diploid germ cells undergo a process called gametogenesis in which they differentiate into haploid gametes (sex cells). 

Beginning in the primitive streak, germ cells eventually make their way to the gonads, where they go through meiosis. 

The ability to undergo both mitosis and meiosis sets germ cells apart from somatic cells. 

No other cell type can undergo meiosis and, therefore, cannot make gametes. 

When cells undergo mitosis, they divide in half once and produce two daughter cells that are genetically identical to the parent cell. 

Cell division occurs twice in meiosis (meiosis I and meiosis II) in a germ cell. 

The first stage of meiosis involves the exchange of DNA between homologous chromosome pairs before they are separated and sent to their respective daughter cells. 

The chromosome number is halved, and the process ensures that each daughter cell is genetically distinct. 

The process of meiosis II, which results in four haploid cells, is very similar to that of regular mitosis. 

Differentiation within these cells results in the formation of mature gametes.

During fertilization, they will fuse to restore a normal diploid number. 

Oogenesis and spermatogenesis, respectively, refer to the processes by which ova and sperm are created.

The average human sperm cell is only about 0.05 mm in length, making it the tiniest type of human cell. 

It is made up of a head, a body, and a tail, and its streamlined design is perfect for its intended purpose. 

The nucleus and centrioles are located in the cell’s head, which also contains tightly wound DNA. 

The acrosome, located on the anterior surface of the head, is a Golgi-derived structure loaded with enzymes that aid in the penetration of the zona pellucida.

This plays a critical role in ensuring fertilization. 

Mitochondria can be found only in the sperm’s midpiece, where they form a spiral of 50 to 100 cells. 

The undulating motion of a sperm’s tail, called the flagellum, is powered by ATP made in the mitochondria.

The microtubules that form the axoneme, the structural heart of the flagellum, are arranged in nine doublets around a central pair.

Next generation: Relative contribution

After completing meiosis and cytokinesis, an oocyte will have one large, viable ovum.  

This makes sure that the zygote has access to nearly everything it needs to grow and develop. 

Nutrients for the zygote and later the daughter cells (blastomeres) created by mitosis are abundant in the egg’s ooplasm. 

Protein synthesis also requires the presence of all the molecules (enzymes, RNA, etc.). 

The organelles are all there too, minus the centrioles that are discarded during the oogenesis process; rather, the sperm cell contributes these components. 

Although mitochondria are present in sperm cells, they are lost during fertilization along with the sperm’s midpiece and tail. 

The maternal mitochondria quickly wipes out any paternal mitochondria that make it into the egg. 

The zygote receives 23 chromosomes, 22 autosomes (non-sex chromosomes), and one sex chromosome. 

An X chromosome is passed on from the egg every time, while either an X or Y chromosome may be passed on from the sperm.

Within twenty-four hours of ovulation, fertilization typically takes place in the fallopian tube. 

On average, only 200 sperm out of the hundreds of millions that are ejaculated make it to the secondary oocyte. 

When a sperm reaches an oocyte, it penetrates the corona radiata and reaches the zona pellucida, where it binds to receptor proteins. 

In order for sperm to reach the oocyte’s membrane, the acrosome secretes enzymes that allow it to pass the zona pellucida. 

Here, the sperm’s pronucleus travels through a tube formed by actin filaments, known as the acrosomal apparatus. 

The cortical reaction is triggered when the pronucleus enters the oocyte; enzymes from cortical granules located just below the oocyte membrane diffuse into the zona pellucida, hardening it.

This means it cannot be fertilized by other sperm. 

The depolarization of the oocyte membrane caused by the release of calcium ions when the sperm reaches the membrane is another barrier to polyspermy. 

After undergoing meiosis II, the oocyte divides unequally, yielding an ovum and a polar body that’s not viable.

A zygote (fertilized egg) is formed when the pronucleus of a sperm fuses with that of an ovum. 

The zygote divides into several cells to form a cluster called a morula after fertilization. 

Peristalsis and the wavelike motions of the cilia propel the morula from the fallopian tube into the uterine cavity. 

It is able to survive on uterine secretions while floating freely for about three days. 

The fluid-filled blastocyst that develops from the morula’s differentiated cells has two distinct cell types. 

The placenta forms from the outer trophoblasts, while the embryo originates from the inner cell mass. 

About six days after fertilization, the zona pellucida degenerates, followed by zona hatching, in preparation for implantation. 

During this process, the blastocyst produces human chorionic gonadotropin (hCG), a hormone that triggers the release of additional hormones. 

The corpus luteum is kept healthy (and menstruation is avoided) by these hormones, and the endometrium is primed for implantation. 

After ovulation, the blastocyst (which now has over 200 cells) connects to the endometrium.

The trophoblast outer cells fuse to make large multinucleated syncytiotrophoblasts that expand like fingers into the endometrium, and these are called chorionic villi.

Inside these villi, fetal blood vessels will develop. 

The decidua (the maternal contribution to the placenta) forms about two weeks after fertilization, when the blastocyst is fully implanted in the endometrium.

The zygote divides mitotically to create a morula during the early stages of development, before the formation of an embryo. 

The fluid-filled blastocyst forms as the morula keeps dividing and differentiating. 

The embryonic stage of development begins when the blastocyst implants in the uterine wall.  

The embryonic cells are reorganized during gastrulation into the three germ layers.

These are ectoderm, mesoderm, and endoderm, and they become the embryo’s tissues and organs.  

Neurulation is the process by which an ectodermal neural plate breaches the mesoderm and gives rise to a neural tube. 

Around the third week, a primitive heart begins to form and begin to beat, continuing the process of organogenesis. 

Internal organs, including the digestive system, the placenta, and the umbilical cord, develop at this time. 

At the end of the first trimester, the embryo has developed into a fetus with fully functional organ systems. 

The reproductive system matures, the limbs learn to work together, the bones solidify, and subcutaneous fat expands during the fetal period. 

The average gestational period is 40 weeks after fertilization.

As it moves from the protective confines of the uterus to the wider world outside, the fetus must quickly adjust to its new surroundings. 

Hormones, especially cortisol and catecholamines, play a crucial role in this change. 

The newborn’s lungs are collapsed and filled with fluid before birth, so they must rely on oxygen from the mother’s blood. 

Fetal lung fluid secretion decreases and reabsorption increases as labor nears. 

At birth, air enters the lungs, and any excess fluid drains out. 

This initial breath sets off a chain reaction in the circulatory system. 

The shunts that allow blood to bypass the lungs and liver close or constrict, resulting in a decrease in pulmonary resistance and an increase in pulmonary blood flow. 

The pulmonary bypass opening known as the foramen ovale closes with the first inhale. 

Both the ductus venosus (which bypasses the liver) and the ductus arteriosus (which bypasses the lungs) narrow and close shortly after birth. 

After the placenta stops delivering nutrients, the newborn will subsist on breast milk and liver glycogen reserves.  

The newborn will raise its metabolic rate, using energy by moving its muscles and metabolizing its brown fat, in order to stay warm.

Biology | Development

Eukaryotic cells: Tissue formation  

We start this section by looking at epithelial cells.

All epithelial cells share the property of being avascular, despite their wide range of morphology and function. 

They get their sustenance from the basement membrane, a layer of connective tissue that benefits from the diffusion of oxygen and nutrients from nearby capillaries. 

The body’s lining and covering tissues are called epithelial tissues.

Their location determines the role they play, which may be protection, filtration, absorption, or secretion.    

These tissues are classified according to the shape and arrangement of their cells, and you can read up on this in your coursework.

Different kinds of epithelial tissues can be found throughout the body. 

Frictional environments like the mouth, esophagus, and skin’s surface support the presence of stratified squamous epithelial tissues. 

The digestive tract is lined with simple columnar epithelia that contain goblet cells responsible for mucus production. 

Membranes involved in filtration or diffusion, like the alveoli of the lungs, are formed by simple squamous epithelia.  

What about connective tissue cells?

Throughout the body, you will find connective tissues, with most being highly vascular. 

Those that are include tendons, cartilage, and ligaments.

In general, they feature a nonliving matrix that serves to support and protect the body. 

Connective tissue cells secrete a matrix that includes ground substances (water, proteins, and carbohydrates) and protein fibers like collagen, elastin, or reticular fibers. 

Note, however, that there is a wide range of consistency among these connective tissues. 

Blood, which consists of red blood cells and plasma, is a connective tissue that carries oxygen, carbon dioxide, nutrients, and wastes around the body. 

Cushioning and insulating the body are functions of the fat cells that make up adipose tissue. 

Osseous tissue, also known as bone, is made up of osteocytes embedded in a dense matrix of calcium salts and collagen. 

While both cartilage and bone serve as supportive connective tissues, cartilage is more pliable because it is composed of specialized cells called chondrocytes. 

Collagen fibers make up the bulk of the dense fibrous connective tissue that makes up ligaments and tendons.

Development mechanism

Cell-cell communication in development is what we need to look at first in this section.

For an embryo to develop normally, cell-cell communication is essential. 

Competent cells can respond to cues from neighboring cells to differentiate into a specific type. 

Cells in development may also release their own inducing factors.

Differentiation occurs between two types of cells: inducers, which secrete signal molecules, and responders, which differentiate in response to those molecules. 

The vast majority of these cues come from growth factors that only influence cells in certain tissues. 

The action of autocrine signals is limited to the cell that secretes them, while cells in close proximity will react to paracrine signals. 

However, in order to reach distant tissues, endocrine signals must first enter the bloodstream, while direct contact between cells is necessary for juxtacrine signals. 

When two cells come into contact, they exchange signals that bind to each other’s receptors. 

There are cases in which signals from two distinct tissues can mutually promote differentiation, and this is known as reciprocal induction.  

Let’s move onto cell migration.

Migration of cells, which begins during gastrulation and continues our whole lives, is necessary for normal embryonic development. 

Malformations, diseases, or even the death of the embryo can result from a deviation in the migration pathway. 

Signaling molecules initiate migration by causing cells to detach from their substrate. 

When the cell polarizes, a leading edge is defined.

This causes the cytoskeleton’s actin filaments to polymerize, propelling the cell forward.

Lamellipodia are flat, sheet-like projections formed by a rearrangement of the cytoskeleton at the leading edge. 

Filopodia are protrusions beyond the lamellipodia that usually point in the movement’s direction. 

Actin-myosin interactions lead to cellular contraction. 

With the help of chemical messengers, cells are guided to their proper destination at just the right time. 

Some cells (like epithelial and mesenchymal cells) migrate singly, while others (like fibroblasts) migrate in groups.

Our next subject is stem cell pluripotency.  

The capacity to differentiate is what is meant by potency in cells. 

Starting with totipotent cells, which have the most potential for differentiation, the spectrum continues down through pluripotent, multipotent, oligopotent, and finally unipotent cells.

Pluripotent stem cells lack the complete potency of totipotent cells (the zygote and cells arising after the first few divisions), but they still have extensive differentiation potential. 

Except for placental cells, they can differentiate into any other type of cell. 

Cells that were once totipotent undergo cleavage in the zygote and subsequent blastomeres, resulting in the development of two distinct lineages.

The ectoderm, mesoderm, and endoderm are all derived from embryonic stem cells, which are found in the trophoblast. 

Although these germ layers give rise to all of the hundreds of different types of human cells, pluripotent cells cannot form an entire organism.

That’s a result of their inability to produce placental tissues.

Let’s move onto gene regulation in development.

The differentiation of cells and the subsequent growth of an organism are both driven by differential gene expression.

The expression patterns of genes are controlled by a number of interrelated factors. 

Cells that are meant to receive signals (such as growth factors) released by other cells do so through cell-cell communication. 

Membrane receptors undergo a conformational change when they are bound by signaling molecules. 

Phosphorylation of cytoplasmic proteins continues the signal transduction pathway and ultimately activates transcription factors. 

DNA-binding transcription factors can regulate gene expression in two ways, either through promotion or suppression. 

The expression of genes can also be controlled by other means, however.

DNA methylation and histone modification are two mechanisms for epigenetic regulation, and the structure of the chromosome can be altered due to these heritable modifications.

The process of regulation does not stop at transcription, however. 

For instance, a single transcript can be used to generate multiple proteins by splicing together its coding regions (exons) in a variety of ways. 

It’s possible that the proteins synthesized during translation need to be activated at a later point in their lifespan. 

Programed cell death is something we need to look into as well. 

This process, called apoptosis, plays a crucial role in the development of the embryo. 

Signals that activate proteases known as caspases trigger this, and as a result, a series of events are set in motion when specific cytoplasmic proteins are cleaved.

This causes the cell to detach from its surrounding cells as well as shrink. 

The cell membrane swells and protrudes, forming blebs, which cause the chromatin to condense. 

Blebs separate from the cell, taking some of the cytoplasm with them, and the DNA and organelles fragment.

Phagocytic cells consume and digest the blebs, which are now known as apoptotic bodies. 

In contrast to necrosis, in which an injured cell releases its contents into the surrounding environment, during this process, no intracellular components will leak.

Abnormal, misplaced, or displaced cells can be eliminated through this controlled procedure, while certain structures are sculpted through it as well.

For instance, to simplify the transmission of electrical impulses, many neural cell progenitors are removed. 

Forming of the fingers and toes is also aided by apoptosis. 

Apoptosis can be triggered by teratogenic agents, which can then cause birth defects or even fetal death.

Lastly, in this section, let’s look at aging and senescence.

Biological aging causes a gradual loss of function, and this is known as senescence. 

Both whole organisms and cells that have stopped dividing but are still alive can be described by this term. 

To lessen the likelihood of cancer, oncogene activation or tumor suppressor gene inactivation can trigger senescence. 

In addition to telomere shortening, DNA damage, and oxidative stress can all induce this quiescent state.  

Telomeres are repetitive, non-coding DNA sequences that are located at the ends of chromosomes and serve to preserve the integrity of the coding.

Due to DNA polymerase’s inability to replicate the chromosome ends, they get shorter with each cell division. 

When telomeres get too short, the cell enters senescence to protect its DNA from being damaged.

Telomerase is an enzyme that can add nucleotides to these endangered end portions. 

Embryonic stem cells, germ cells, malignant cells, and even adult stem cells (albeit at low levels) are the only cell types in which it is detected, however. 

Although senescent cells become more common as we age, there is growing evidence that they play a role in embryonic development.

Here, they stop the growth of specific tissues and also help in the shaping of the embryo.

Biology | Development

Genetic material

When we talk about genetic material, DNA is probably the first thing that pops into your mind.

Genes, the building blocks of DNA, and what they are composed of are contained within our chromosomes.

DNA is a type of nucleic acid found in cellular nuclei, and that’s true of every cell in our body.

But that’s not the only place you will find it, as it resides in the mitochondria too. 

DNA replicates so that genetic information can be passed on, and almost all cells will have the same DNA. 

It has a secondary function as well, and that’s protein biosynthesis.

There’s a very specific model that you would have seen for DNA, where its structure takes on the form of a double helix.  

When two congruent curves are joined by horizontal members, they are said to form a double helix, which looks much like a spiral staircase where the railing is on the right.  

This double-helix model was formulated by James Watson and Francis Crick, and they were able to do so using x-ray diffraction provided by British scientist Rosaline Elsie Franklin. 

Let’s look at DNA structure a little closer. 

DNA has a compact double helix shape of a twisted ladder and is made up of nucleotides. 

These, in turn, are made up of:

• A five-carbon sugar (pentose), 
• A phosphate group 
• A nitrogenous base. 

The rungs of the ladder are formed by the pairing of the two bases. 

The side rails are also referred to as the backbone.

Sugar and phosphate are covalently bonded in their structure. 

Hydrogen bonds hold the bases together and can be broken easily, allowing replication to take place without too much trouble. 

A phosphate and a sugar are joined to each base, too.

• In total, nitrogenous bases come in four different kinds.

These are:

• Adenine (A) – pairs with thymine (T)
• Guanine (G) – paris with Cytosine (C)
• Cytosine (C)  – paris with guanine (G)
• Thymine (T) – paris with Adenine (A)

In total, within human DNA, you will find around 3 billion bases, which, while generally the same in everyone, have a differentiating order, and that’s what diversifies us.

Next, let’s look at purines and pyrimidines.

According to their chemical makeup, the five bases found in DNA and RNA are classified as either pyrimidine or purine.  

Cytidine, thymine, and uracil are all examples of pyrimidine bases, with all having a single ring shape and six sides. 

Adenine and guanine are examples of purine bases; each has two interlocking rings, one having six sides and the other five.  

Any of the five bases can be converted into a nucleoside by adding a sugar. 

Purine nucleosides end in “osine,” while pyrimidine nucleosides end in “idine” and examples of these nucleosides are thymidine and adenosine.  

The building blocks of DNA and RNA are the bases, then the nucleosides, and finally the nucleotides.

Then there are codons.

Groups of three nucleotides (called codons) on messenger RNA can be thought of as a ladder.

The code for a single amino acid is contained in a codon, of which there are 64, while there are only 20 amino acids.  

This means that to synthesize a necessary amino acid, more than one combination is possible.  

Codons for lysine include sequences like AAA (adenine-adenine-adenine) and AAG (adenine-adenine-guanine) as examples. 

These sets of three occur frequently in strings and can be interpreted as frames. 

Codons for lysine (AAA), serine (UCU), and threonine (UCG) are represented by the triplets at the beginning of the sequence AAAUCUUCGU. 

Reading the same sequence in threes from the second position yields AAU (asparagine), CUU (proline), etc., and as a result, there would be very different amino acids. 

That’s why we have start and stop codons.

Their task is to mark the start and end of a frame.

The start codon is AUG, which encodes methionine. 

There are three types of stop codons: UAA (also known as ocher), UGA (also known as opal), and UAG (also known as amber).

Next up is DNA replication.

DNA is packaged in chromosomes in tandem in order to maximize space, but it begins to unwind when replication starts. 

Enzymes regulate each stage of the DNA replication process. 

By disrupting hydrogen bonds between bases, helicases facilitate the separation of the two strands of DNA, and this starts the process. 

Since there are only two hydrogen bonds found here, the break occurs at the A-T bases (adenine and thymine). 

There are three chemical bonds in the cytosine-guanine base pair, however. 

The phrase “origin of replication” describes the first site of DNA division. 

The replication fork is the section of DNA that is uncoiled during the replication process, and this is what will be replicated.  

There is mRNA that transcribes each strand of DNA, and this will copy the DNA onto itself in a complementary manner.

This occurs base by base, although there is an exception when thymine is replaced by uracil. 

Let’s talk a little more about RNA.

In addition to assisting DNA, RNA serves a variety of other purposes. 

Ribosomal RNA, transfer RNA, and messenger RNA are all examples of different types of RNA. 

RNA can be used by viruses to transport their genetic information to DNA. 

It is believed, however, that over time, ribosomal RNA has not changed that much, and because of this, the relationship between organisms is studied through this particular type of RNA.

A strand of DNA is duplicated and transported from the nucleus to the cytoplasm by messenger RNA. 

The process by which DNA is copied into RNA by RNA polymerization is called transcription. 

When RNA is being assembled, DNA unwinds to act as a template, and it’s to the RNA that the molecules of DNA are copied. 

Using RNA that has been transcribed, ribosomes then construct the protein. 

The cytoplasm contains transfer RNA, a molecule essential to translation.

So what are the differences between RNA and DNA?

The structure and function of RNA and DNA are not the same, as a different sugar is present in RNA (ribose sugar compared to deoxyribose sugar). 

Adenine (A), Guanine (G), Cytosine (C), and Uridine (U) are the nitrogenous bases found in RNA. 

DNA contains thymine, but RNA only contains uracil, another major difference. 

They differ in the number of strands they have too, with a single strand found in RNA and a double strand in DNA, which also has two side rails when straightened out. 

The sugar and phosphate groups that make up RNA’s backbone are singular in nature.  

The sugar used by RNA, pentose, is fully hydroxylated, whereas the sugar used by DNA, deoxyribose, lacks an oxygen. 

The work done by DNA is backed up by RNA, and this includes helping with transportation, replication, and gene expression. 

While we won’t cover it here, while working through this DNA section, make sure you cover genetic inheritance as well.

Biology | Macromolecules

Macromolecules

These are substantial in size and complexity, and they perform crucial roles in cellular structure and function. 

Anabolic reactions result in the formation of carbohydrates (polysaccharides), nucleic acids, proteins, and lipids, the four fundamental organic macromolecules. 

Catabolic reactions produce glucose, amino acids, fatty acids (glycerol), and nucleotides, the four fundamental components of all living organisms. 

When simple molecules are combined to form larger and more complex ones, this process is called an anabolic reaction. 

The opposite of anabolic reactions is called catabolic, and here it is into simpler, smaller molecules that larger ones are broken down. 

Also note that while anabolic reactions will require energy, catabolic ones will release it. 

The chemical reactions that absorb heat are called endothermic reactions, while the ones that give off heat are called exothermic reactions.

Let’s look at various macromolecules, starting with carbohydrates.

Carbohydrates, which are quickly converted to glucose, are the body’s primary source of energy. 

The majority of a cell’s energy comes from the oxidation of carbohydrates. 

Fermentation by glycolysis, or respiration, can help to break down glucose further.  

Photosynthesis and respiration, two metabolic energy cycles, both rely on carbohydrates.

Carbohydrates are composed of carbon, hydrogen, and oxygen, so their molecular structure is typically represented by a variation of CH2O. 

The polysaccharides found in carbohydrates are metabolized into glucose and other simple sugars.

Glucose, fructose, and galactose are examples of monosaccharides, while disaccharides are the other type of simple sugar, but both are types of carbohydrates.  

One sugar monomer (a small molecule) is found in disaccharides, while two are found in monosaccharides. 

In the case of monosaccharides (CH2O), there is one carbon for every two H2O molecules.

Monomers are the building blocks of polymers, and they exist as a single compound that joins with others via chemical bonds. 

The monomers in a polymer are repeated, creating larger and larger molecules. 

Macromolecular polymers include carbohydrate, protein, and nucleic acid molecules. 

Next, we can look at lipids.

Despite being soluble in nonpolar solvents, lipid molecules are hydrophobic and therefore do not form strong bonds with water or dissolve evenly in aqueous solutions. 

Numerous C-H bonds are present in lipids; thus, they are comparable to hydrocarbons.  

Lipids serve primarily as energy stores and structural components. 

Lipids can be any kind of fat, phospholipid, steroid, or wax. 

Long strands of fatty acids (three fatty acids bound to a glycerol) are what make up fats, also known as triglycerides. 

Chains of reduced carbon and a carboxylic acid group make up fatty acids. 

Soap is a good representation because it is composed of free fatty acids and sodium salts. 

Lipids that lack a fatty acid but instead contain a phosphate group are called phospholipids, while another kind of lipid is glycerides (such as oil). 

Fatty acids and glycerol (an alcohol) combine to form glycerides.

Then there are proteins.

Amino acids combine to form proteins.

They are polypeptides, which are chains of multiple peptides, anything from 10 to 100. 

Condensation reactions (those where water is lost when two molecules combine) are responsible for the peptide bonds. 

The opposite of this type of reaction sees water added, and this is known as a hydrolysis reaction. 

Two or more amino acids can combine to form a peptide. 

When protein is partially hydrolyzed, an amide bond is created, and amino acids are formed. 

An amine and a carboxylic acid are responsible for this partial hydrolysis. 

Amino acids have two types of functional groups on their carbon chains: carboxylic acid (-COOH) and amine (- NH2). 

Between them is a hydrogen-bonded carbon atom; each amino acid has its own unique “R” group (side chain). 

The properties of the proteins are determined by the “R” group. 

What about enzymes, another form of macromolecule?

Proteins with high catalytic power are called enzymes. 

They significantly speed up the time it takes for certain reactions to reach equilibrium. 

Enzymes don’t kick off chemical reactions that wouldn’t happen anyway; they just make them happen more quickly and reliably. 

Sometimes this acceleration is so great that reactions occur a million times faster than normal. 

The substrates of each enzyme’s specific reaction are different. 

All enzymes are extremely picky about the substrates they interact with, as they only accept molecules that fit into their active sites. 

Enzymes can only catalyze reactions when they reshape themselves to fit their substrates, and this can happen even when the substrates are a good match. 

Enzymes are unique in that they are not completely used up in the reactions they accelerate. 

They can be recycled indefinitely, so cells always have access to a ready supply.

Because of this, the variety and velocity of cellular reactions can be greatly expanded.

Lastly, let’s look at nucleic acids.

Nucleic acids are large molecules made up of subunits called nucleotides. 

Through the process of hydrolysis, water is split into hydrogen cations (H+) and hydroxide anions (OH-). 

To produce RNA and DNA oligonucleotides, this is a step in the breakdown of nucleic acids by enzymes. 

Nucleosides are smaller sugar-nitrogenous units that are extracted from oligonucleotides. 

Since the sugar has been separated from the nitrogenous base, cells are able to digest them.  

This then results in the creation of the five different nitrogenous bases, sugars, and other precursors needed for RNA and DNA synthesis.  

Nucleotides are monomeric units that are joined together via phosphodiester bonds to form larger nucleic acid polymers. 

Protein synthesis from amino acids and DNA replication both require the use of cellular energy, which is provided by ATP. 

Protein and nucleotide synthesis both require nitrogen fixation. 

In the process of nitrogen fixation, the enzyme nitrogenase converts dinitrogen gas (N2) into ammonia (NH3).

Nucleic acids are crucial catalysts in addition to their storage and informational roles. 

RNA is responsible for facilitating the translation of genetic instructions from DNA into the amino acid sequences that make up proteins. 

Adenosine triphosphate (ATP) is a nucleotide found in RNA. 

Nucleic acids are also constructed from nucleotides. 

Nucleotides consist of a nitrogenous base, one or more phosphates, and a five-carbon sugar like ribose or deoxyribose. 

Multiple phosphate nucleotides can also store energy in their bonds.

Biology | Microorganisms and disease

Infection cycle

Pathogens (disease-causing microorganisms) require a favorable environment in which to establish and spread infection.

When they find one, the infection cycle begins.  

The ability to spread from one host to another is just as important as the ability to grow and reproduce in this cycle. 

There are two modes of transmission in this cycle: direct and indirect. 

When an infected host transfers the disease directly to a susceptible host, this is called direct transmission. 

Indirect transmission can occur in a number of ways. 

The contamination of an object (a fomite) can spread the infection to anyone who comes into contact with it.

In order to infect and spread to another host, pathogens can use vectors to infect and colonize intermediate hosts. 

It’s possible that the pathogen could even become airborne and then infect a new host. 

In order for there to be transmission, the pathogen needs access to the new host and the new host needs to be susceptible to infection.

Let’s talk about the reservoir and the role it plays in infections.

In order to effectively combat disease transmission, medical professionals need knowledge of all five stages of the infection cycle. 

In order for an infection to take place, there must be a reservoir host, an exit portal, a means of transmission, an entry route, and a susceptible host. 

When a microbe (pathogen) invades a living organism, this is the first stage. 

A reservoir host can be any living thing, including a human, an insect, or an animal. 

The reservoir host provides the pathogen with the nutrients it needs to survive and potentially multiply within the host body. 

Humans are particularly effective reservoir hosts because infected humans often do not realize they are infected and can spread the disease unknowingly to others. 

Evidence of disease in a reservoir host may increase people’s vigilance about the importance of hand washing and other disease-prevention measures.

Let’s move onto the portal of exit.

The second necessary condition for infection is the availability of an exit portal, which is provided by the reservoir host.  

This describes how the microbe escapes from the reservoir host and then goes on to infect the susceptible host. 

The mouth, nose, blood, urine, vaginal or seminal fluid, feces, and even the eyes are portals the pathogen can use to exit the body. 

The fourth stage of an infection’s life cycle is leaving through a portal that is the same as the portal through which the pathogen entered. 

Transmission modes

Our first mode of transmission are droplets.

If the reservoir host sneezes or coughs, the mucus droplets released could spread the pathogen to other people. 

It’s common knowledge that droplet particles can travel several feet in the air, so the reservoir host doesn’t have to be right next to the susceptible host for transmission to occur. 

Inhaling droplets from an infected person can spread respiratory diseases like influenza and tuberculosis.

It’s crucial to use the right procedures to stop the spread of these diseases through the air, as they can quickly spread.

It is common practice to have patients with respiratory infections wear masks to contain the spread of the virus.

The second transportation mode is direct contact.

When the skin or mucous membranes of a susceptible host are broken or damaged, blood from an infected reservoir host can enter the body and cause an infection. 

In order to avoid getting blood on mucous membranes or into a cut in the skin, healthcare workers should always take universal precautions and wear the necessary PPE.

In a hospital, hepatitis B virus (HBV), hepatitis C virus (HCV), and the human immunodeficiency virus (HIV) are the most common bloodborne pathogens that can be transmitted.

All bodily fluids should be assumed to be contaminated at all times, and PPE should be properly managed and disposed of.

Sexually transmitted infections (STIs) like gonorrhea, herpes, and syphilis can also be passed directly from mother to child via the placenta or during a vaginal delivery.

Our third transmission mode is via the airborne route. 

Airborne transmission is the process by which microorganisms are spread from one host to another through the inhalation of tiny particles expelled from the respiratory system of a reservoir host.  

Inhaling droplets from an infected person’s cough or sneeze is an example of this.

It’s common knowledge that cough and sneeze droplets can travel several feet.

This is a typical mode of transmission for many infectious diseases, including the flu, TB, and even chickenpox. 

Aspiration of bacteria or fungi from contaminated water is another potential route of infection. 

This is not transmitted through casual contact, but rather through inhalation of contaminated water droplets. 

This is a common occurrence in places where the water supply has been tainted, like hotels, resorts, and apartment complexes.

The tainted water can even be spread through air conditioning units. 

Our next transmission mode is vehicle-borne fomite.

Any nonliving thing that can transfer disease from one human to another is called a fomite. 

Door handles, water fountains, glasses, pens, toys, books, and shopping carts are all examples of fomites that can spread disease. 

These illustrations highlight the ease with which infectious diseases can spread in institutions like schools and daycares. 

Indirect transmission occurs because the membranes of the bodies involved do not have to come into direct contact. 

Instruments used in clinical care settings, such as those used in surgery or patient care, are potential examples of vehicle-borne fomites.

Blood, biopsy samples, and organs and tissues used for transplants or grafting material also fall under this category.

Our last transmission mode is vector-borne mechanical, or biological.

When pathogens are transferred from one living thing to another, this is known as a vector-borne method of transmission. 

The insects (vectors) are responsible for spreading bacteria and other common pathogens from one person to another. 

The mosquito, the fly, the tick, and the flea are all examples of vectors. 

Since mosquitoes, ticks, and fleas all get sick by sucking blood from their hosts, we classify them as biological vectors.

Portal of entry

The fourth necessary condition for infection is the presence of an entry point. 

To infect a new host after leaving its reservoir host, a microbe needs a way in. 

Any mucous membrane, including the nose, mouth, rectum, or vagina, can serve as an exit or an entry point. 

When the integumentary system is compromised, these pathogens can also enter the body. 

The eyes are a major entry point for many infectious diseases, such as conjunctivitis.

Another common type of infection, especially for women, is an infection of the urinary tract. 

This happens because the urethra is so close to the rectum that bacteria from the rectum can easily travel to it. 

Wiping down the toilet or washing one’s hands afterward are just two examples of how to maintain good hygiene and stop the spread of disease-causing microbes.

Susceptible host

A host that can take the infection is the cycle’s fifth and final stage. 

When an infected person transfers the pathogen to another person, the cycle continues because the second host is susceptible and not able to fight it effectively.  

The susceptibility of a host organism to infection depends on a number of different factors.  

Examples of such factors are how well you’re eating and how healthy you are in general. 

Infants and the elderly are especially vulnerable to certain diseases, so age is also an important consideration. 

Infection risk factors also include, but are not limited to, poor hygiene and unsanitary living conditions. 

The host may, for instance, take great care when washing their hands, but they may be using tainted water because of the presence of pests in their home. 

In some cases, even a healthy host can fall prey to an infection from a microorganism that is too powerful for its immune system to handle.

For further information regarding microorganisms and disease, make sure you scan through your coursework with specific reference to viruses, fungi, and parasites, as well as the sections on medical and surgical asepsis. 

Chemistry

Chemistry | Chemical bonds and atomic structure

Atomic structure: The basics

Let’s start by talking about the pieces an atom is made up of. 

The smallest unit of matter is the atom, and it is made up of the nucleus and electrons. 

Protons and neutrons are the components of the nucleus. 

Both an electrical charge and mass are the measurable parts of these pieces of an atom.

Since protons are positively charged, the nucleus is as well.

Electrons circle the nucleus, but they are negatively charged.

When compared to the electrons around it, the nucleus has far more mass.

Molecules are the result of atoms bonding together.

Atoms with an identical amount of protons and electrons do not carry an electrical charge.

Ions are atoms with an unequal amount of protons and electrons.

Depending on whether there are more protons over electrons or vice versa, an ion will have either a positive or negative charge.

We move onto the model of atoms. 

Atoms are minuscule in size. 

For example, the diameter of a hydrogen atom is approximately 5 x 108 mm. 

You could fit as many as five trillion hydrogen atoms on the tip of a needle, according to some calculations. 

The phrase atomic radius is used to describe the gap found between an atom’s nucleus and its furthest electron. 

The electrons in atomic models, which also feature a proton and nucleus, are generally depicted as being very near to the nucleus and circling around it like the Earth orbits the sun.

The phrase electron cloud refers to an alternative theory in which the Earth serves as the nucleus and its atmosphere as the electrons. 

Electrons can also be thought of as swarming about the nucleus.

These models of the atom are not to scale, so keep that in mind. 

A nucleus of 2 cm in diameter placed in a stadium would be a more accurate scale, with all the electrons then found in the stands.

According to the periodic table, the radius of an atom grows with the addition of energy levels but shrinks with the addition of protons.

This is a result of protons attracting electrons toward the nucleus. 

Francium (Fr) has the largest atomic radius, and Helium (He) has the smallest.

From the left and toward the bottom of the table, you should note that the atomic radius increases. 

What about the atomic number?

The number of protons in the nucleus of an atom represents what is meant by its atomic number and each number is unique to the element it represents.

To represent the atomic number of an element, we use the letter Z.

When an atom’s charge is neutral, the number of its electrons is its atomic number.

Let’s move on to atomic mass. 

The mass number is another term used for atomic mass, and to determine this, we take the nucleus of the atom and determine the number of protons and neutrons found in it. 

When referring to the atomic mass of an element, we usually just represent this with the letter A.  

The sum of the number of protons (Z) and neutrons (N) is equal to the atomic mass (A) and is simply expressed by the equation: A=Z+N.

Due to their negligible contribution to the atomic mass, electrons’ mass can be ignored. 

Though it is sometimes used interchangeably with the phrase relative atomic mass, atomic weight and atomic mass are two entirely different concepts, so don’t get confused between the two. 

The atomic weight of a sample (which may contain many isotopes of the same element) is defined as the ratio of that sample’s average atomic mass to one-twelfth the mass of a carbon-12 atom.

Next up are isotopes. 

Atoms of the same element can have different isotopes or variations in the number of neutrons. 

All of the isotopes of a given element share the same atomic number because they all contain an identical amount of protons. 

The element symbol is used to represent them, with the mass number and atomic number appended in superscript and subscript, respectively. 

Stable, or non-radioactive, isotopes are those that have not been reported to decay. 

Some stable isotopes may have extremely lengthy half-lives, making decay observations impossible, but this is difficult to establish. 

Eighty elements have at least one stable isotope.

For now, there are 256 stable isotopes that have been discovered.

Let’s look at a quick example of isotopes with carbon in mind.

There are three distinct isotopes of carbon, two stable and one unstable. 

The two stable isotopes are carbon-12 and carbon-13 and one unstable one is carbon-14. 

Because of their nuclei’s instability, radioactive isotopes are capable of releasing particles or radiation on their own. 

A single nucleus’ decay time cannot be predicted, but the decay rates of a huge number of nuclei with the same composition can.

The age of objects that contain radioactive isotopes can be estimated with the help of decay rate data.

Then there are electrons. 

These subatomic particles can be thought of as layers, shells, or clouds that orbit the nucleus. 

Only a small percentage of an atom’s total mass can be attributed to the electrons in orbit around it. 

They’re considerably tinier than the nucleus itself, have wave-like characteristics as well as a negative charge.

Electrons belong to the elementary particle family.

The electrons in atoms can settle into a variety of orbits around the nucleus, each one looking for the lowest energy level that they can occupy. 

A stable electron configuration will see an atom with all electrons taking up the lowest available positions that they can.  

In its uncombined form, an atom’s outermost electron shell is known as its valence shell. 

Bonding behavior is established by the total number of valence electrons present. 

Between an electron or electrons and the nucleus (of one atom) or nuclei (of more than one atom) there is a negative-positive attraction that takes place in a chemical bond.

The attraction not only helps keep the atom together but also makes it possible for atoms and molecules to bond with one another. 

There is a maximum number of electrons that can occupy each of the atom’s four energy levels/shells.

Before adding electrons to the valence level, all other levels must be full. 

An electron’s energy increases as it moves away from the nucleus. 

Each successive shell has an increasing capacity for electrons, with the K-shell holding a maximum of 2, the L-shell holding 8, the M-shell holding 18, and the N-shell holding 32. 

Subshells may be present within the shells as well.

When one or more electrons in an atom’s outer valence shell are gained, lost, or shared, chemical bonds are formed and broken. 

A polar bond is one that has an obvious separation of charge, and this is a covalent bond where one pole is positive and the other negative.

An example of this is the bond found in water between hydrogen and oxygen.

While in this section, let’s talk about ions as well. 

The nucleus’s protons are positively charged, whereas the orbiting electrons are negatively charged, which makes most atoms neutral. 

When atoms collide, they exchange electrons with one another. 

This results in a positively or negatively charged molecule or atom because the number of electrons is not equal to the number of protons. 

Gaining one electron causes the formation of a negative ion, while losing one results in the formation of a positive ion. 

Ions with opposite charges attract one another, creating an ionic connection, and the compound that results will then be neutral. 

The transformation of neutral particles into charged particles is known as ionization. 

Ionization can produce either partial or complete ionization of gasses and plasmas.

Let’s move on to the chemical bonds between atoms. 

It is possible for atoms of the same element to form molecules or crystalline solids by bonding together.

A compound is the result of chemical bonding between two or more different atom types. 

The chemical and physical properties of compounds are indications of the molecular interactions that have taken place. 

The atomic makeup of molecules, as well as the distances and angles between their constituent atoms, determine these interactions.

Chemical bonding is the joining of atomic electron structures. 

When atoms form bonds, each partner may gain, lose, or share electrons. 

Here are three examples of chemical bonds.

• Ionic bonding: Ionization occurs when an atom acquires or loses an electron, creating a negatively or positively charged ion. Two ions with opposite charges form a strong link called an ionic bond. 

• Covalent bonds: These are formed between atoms when their electrons are shared. A non-polar bond occurs when electrons are shared equally, while a polar bond occurs when electrons are shared unequally. 

• Hydrogen bonding: When an atom in a molecule connects with a hydrogen atom in the same region, this is known as hydrogen bonding. The structure of DNA and other big molecules relies on hydrogen bonding between various regions of the same molecule.

When an atom loses an electron, it transforms into a cation, which is also known as a positive ion. 

When an atom gets an electron, the result is a negative ion known as an anion.

In this section, we also need to cover ionic bonding. 

Ionic bonding refers to the process by which electrons are transferred between atoms. 

Ions are atoms that have gained or lost an electron.

An ion’s charge is determined by whether or not it has gained or lost electrons. 

Let’s look at a sodium (Na) and chlorine (Cl) atom. 

There are 11 electrons in total found in sodium, with just one occupying the outermost shell. 

There are 17 electrons in chlorine, with 7 located in their outermost shell. 

Since there are the same number of protons and electrons in an atom, we can deduce that sodium has an atomic number of 11. 

When NaCl is formed, an electron is transferred to chlorine by sodium.

Because ions carry electric charges, they will either have a + or – sign at the start of their name.

When ions in a compound have opposite charges, they are drawn to each other.

Let’s also discuss covalent bonds a little more. 

When one or more pairs of electrons are shared between an atom and another covalent bond, or between two atoms, a covalent bond is in place.  

The resulting attraction-repulsion balance keeps the molecules in place.

In order to ensure that the outer electron shells of all atoms are filled, atoms will often share electrons with one another. 

While similar in strength to ionic bonds, the resulting covalent bonds are stronger than intermolecular hydrogen bonds.  

Most often, atoms with similar electronegativity form covalent bonds with one another. 

Since it is easier for metals to release an electron, nonmetals are more likely to form ionic bonds than covalent ones.

When two entities with identical electronegativity come into contact, electrons are exchanged. 

Both process chemistry and industrial catalysis rely heavily on metals’ ability to form covalent bonds.

Let’s also look at electronegativity and what that entails. 

The ability of an atom to attract a pair of bonding electrons is quantified by its electronegativity. 

A dipole results when one atom in a bond produces a slightly greater force than the other.

A polar covalent bond is formed between atoms with a low electronegative difference. 

When the gap is large, ionic bonds form between the atoms. 

Electronegativity-free covalent bonds are completely nonpolar.

Make sure you read through your coursework with regards to chemistry, organic compounds, and the periodic table. 

Chemistry | Matter: States and property

Everything with mass and volume is considered to be matter. 

According to the standard definition, there are three distinct phases that matter can exist in: solid, liquid, and gas. 

The energy that holds molecules or atoms together can take on a variety of forms, each of which is the result of slight variations in distance and orientation, and that’s how these states arise.

Rigid or nearly rigid structures with strong connections are considered solid. 

In liquids, the molecules or atoms float more freely, and while their bonds are weak, they are easily broken. 

Gas molecules and atoms normally move in their own directions, and do not bond as they keep their distance from one another.

According to the standard definition, there are actually four different types of matter, with plasma as this fourth, which is not often mentioned.

This is an ionized gas in which certain electrons are not attached to any atoms or molecules and are so called free. 

Make sure you look through your coursework to totally understand the three states of matter and their various characteristic properties.

Heat can be added or removed to move between the three states of matter. 

When a solid is heated past its melting point, for instance, it begins to transform into a liquid. 

However, at the melting point, extra heat is required to overcome the latent heat of fusion and make the transition from solid to liquid. 

The liquid can start to form a gas with further heating to its boiling point, but more heat must be applied at the boiling point to compensate for the latent heat of vaporization.

When frozen, water has a lower density than when it is liquid. 

This is easily shown by observing the behavior of an ice cube in a glass of water, where it will float near the surface. 

If this weren’t the case, then lakes and rivers everywhere wouldn’t freeze over.

Many aquatic species would perish if lakes and rivers froze from the bottom up.

Ice’s low density results from a number of factors, including the shape of water molecules and the strength of their hydrogen bonds. 

In ice, two covalent bonds and two hydrogen bonds connect each oxygen atom to a total of four hydrogen atoms. 

This stops molecules from getting close to each other because it creates a loosely organized tetrahedral structure.

Therefore, the low density of ice can be attributed to the presence of empty spaces within the structure.

Changes in matter states

Melting refers to the process by which a solid transforms into a liquid. 

Reversing this process from liquid to solid is referred to as freezing. 

Vaporization is the process through which a liquid changes into a gas. 

Condensation describes the reversal of this process. 

Direct transitions from gas to solid and from solid to gas don’t happen that often, but they are possible under the right circumstances. 

Sublimation is the process by which a solid is transformed into a gas, and deposition is the process in reverse.

When a liquid is heated above its boiling point, all of its molecules vaporize, and the liquid becomes a gas. 

This process is known as evaporation.

Despite the cohesive forces exerted by their neighbors, some molecules at the surface of a liquid retain enough thermal energy to escape. 

When heated, the substance’s molecules rearrange themselves at such a rapid rate that they generate enough kinetic energy to change phase from liquid to gas form. 

When a considerable amount of a liquid’s surface area is exposed (as in an ocean), the rate of evaporation increases. 

Some of the molecules that evaporate from a liquid will condense back if there is already a lot of water vapor in the air around the liquid. 

Increased air pressure also slows the rate of evaporation.

The process of a gas changing into a liquid is called condensation, and it is the polar opposite of evaporation.

The molecules in a gas, like water vapor, will drop when the temperature drops.

Hydrogen bonds are formed in water when the molecules are drawn closer together by intermolecular cohesive forces as a result of the reduced mobility of the molecules. 

Increasing the pressure applied to a gas can also cause it to lose volume (the amount of space between its particles decreases), and therefore, it undergoes condensation. 

This kicks off the hydrologic cycle when rising, warm air with water vapor in it cools. 

This causes atmospheric convection, weather fronts, and the uplift of air over high ground.

Substances and their characteristic properties

Let’s start by looking at intensive and extensive properties.

There are two types of physical properties: intensive and extensive. 

There is no correlation between the size of a sample or the amount of matter and its intensive properties. 

So if the sample size is increased or decreased, intensive properties do not change.  

Color, hardness, melting point, boiling point, density, ductility, malleability, specific heat, temperature, concentration, and magnetism all fall under the category of this property type.

The sample size and amount of matter, however, will have an effect on the extensive properties. 

As a result, if the sample size is increased or decreased, there will be a change in these properties. 

So the property will increase when the sample size increases and vice versa.

Volume, mass, weight, energy, entropy, number of moles, and electrical charge are all examples of extensive characteristics.

Then we have the physical properties of matter.

Any attribute of matter that can be directly observed or quantified is considered a physical property. 

Color, flexibility, mass, volume, and temperature are all examples of these property types.

An object’s mass indicates how much substance it has.

The force of Earth’s gravity is quantified by an object’s weight. 

A substance’s volume indicates how much room it takes up. 

If you measure the amount of water an irregular form displaces, you can calculate its volume. 

The amount of mass per unit volume is the measurement of an object’s density.

Density (D) is calculated by dividing mass (m) by volume (V) or D=m/V.

Grams per cubic centimeter (g/cm3) is one common unit of measurement for density.

A substance’s specific gravity can be calculated by dividing its density by the density of water.

Specific gravity, density in volume, mass, and weight

Let’s look at density a little more closely. 

There is a distinction between the density of an object and the density of a material. 

Steel is around eight times denser than water, which itself has a density of one gram per cubic centimeter. 

A steel object may float even if its density is substantially higher than water. 

The air inside a hollow steel sphere contributes to the object’s overall density, making flotation effortless.

What about specific heat capacity?

Heat capacity per unit mass is known as the specific heat capacity. 

In terms of temperature, every substance is unique. 

To raise the temperature of the same mass of magnesium and lead by one degree Celsius, for instance, requires different amounts of heat energy. 

Q = mcAT is the relationship between thermal energy and mass, where m is the mass of the item and c is its specific heat capacity.

We must touch on conduction too. 

It is a universal law that heat moves from hotter to colder areas. 

When two areas have the same temperature, there is no net heat transfer since they are in thermal equilibrium. 

Conduction is a contact-based heat transmission mechanism. 

Heat can be transported from one place or object to another through touch because it is a measure of kinetic energy, most typically vibration, at the atomic level.

Lastly, let’s look quickly at matter’s chemical properties.

A property is considered a chemical one if it requires a chemical change to be observed and measured. 

For instance, water is produced when hydrogen gas is burned with oxygen. 

No amount of heating or cooling the water will allow the hydrogen to be recovered.

Chemistry | Chemical reactions

Chemical Reactions: An overview

Human time scales can be used to accurately gauge how long it takes for chemical reactions to occur.

This can be anything from a split second to millions of years in some cases.   

How often the atoms and molecules involved in a reaction collide is what determines how quickly the reaction proceeds. 

Temperature and the shape and characteristics of the reacting materials, as well as other factors, play a role in reaction rates. 

Chemical reactions can be sped up with the help of catalysts, whereas they can be slowed down using inhibitors, while heat and light are two forms of energy that can be released as a result of a reaction.

The sharing of electrons or hydrogen ions between ions, molecules, or atoms can result from some chemical reactions. 

In others, heat or light is used to break chemical bonds, releasing reactive radicals that can easily form new bonds.

Radical reactions regulate processes like the production of greenhouse gasses and ozone in the environment and the utilization of fossil fuels.

Understanding chemical equations and balancing them

Chemical reactions can be described using chemical equations. 

The arrow points to the products, which are located to the right of the reactants, which are found on the left side of the arrow, with its direction showing the change or reaction itself. 

If you look at the number before the element, you’ll see the stoichiometric coefficient, which expresses the mole ratio of reactants to products. 

To give just one example, the chemical formula for the creation of water from hydrogen and oxygen is 2H2 (g) + O2 (g) 2H2O (1). 

There are 2 moles of hydrogen and 2 moles of water, with the 2 serving as the coefficient. 

The mole of oxygen is 1, however this does not need to be written out. 

For clarity, the letters within parenthesis denote different states of matter: g for gas, I for liquid, s for solid, and aq for an aqueous solution. 

Only individual ions, not ionic compounds, have their charges indicated by a superscript. 

To avoid confusion between the ion and its count, we put polyatomic ions in parentheses.

Since matter cannot be created or destroyed but only altered, an unbalanced equation violates the principle of conservation of mass. 

The stoichiometric coefficients on either side of the arrow won’t point to the same amount of atoms if the equation is unbalanced.

To solve an equation, start by looking at each species in the reaction and writing formulas for it. 

Check if there are an equal number of atoms on both sides by counting them. 

Whole numbers are required for coefficients, while it’s not possible for half molecules or other fractional amounts.

Half a molecule is not feasible, nor are other fractional amounts. 

When balancing an equation, it is best to use the smallest whole integer coefficient available for each term. 

An unbalanced equation example is H2+O2H2O. 

Two moles of hydrogen and one mole of oxygen combine to make two moles of water, as shown by the balancing equation 2H2 + O2 → 2H2O.

Let’s talk a little about the law of conversation of mass.

Common expressions of the Law of Conservation of Mass in chemical reactions go as follows: There is no creation or destruction of matter during a chemical process.

This means that the overall mass of matter remains unchanged after any given reaction. 

Since the number of each type of atom is the same on both sides of a balanced equation, we can use this to predict how molecules will combine. 

For instance, water is formed when two hydrogen molecules and one oxygen molecule join. 

Since there are exactly the same number of each kind of atom on both sides of the arrow, we can safely call this a balanced chemical equation. 

Since the reaction follows the Law of Conservation of Mass, it must be in equilibrium.

Reaction Mechanisms: The basics

Exchange of electrons between atoms or molecules is a common mechanism for chemical reactions. 

The tendency of atoms to gain or lose electrons until the outer energy levels contain eight is known as the octet rule.

This rule determines the nature of reactions and reactivity. 

One or more new products may be created as a result of a reaction, which can see a change in the content or configuration of a component or substance. 

For example, one set of substances (CH1+O) was changed into a new set of substances (CO2 + H2O) in the reaction between oxygen and methane (CH).

For a reaction to take place, there must be a substance that will change (a reactant), a partner in the reaction (a reagent) as well as a catalyst.

This leads to a product, the result of the reaction.

Environmental factors, or reaction circumstances, are also crucial elements. 

Temperature, pressure, concentration, whether or not the reaction takes place in solution, the nature of the solution, and the presence or absence of catalysts are all examples of such conditions. 

The standard format for a chemical reaction is as follows:

Reactants → Products

While we won’t cover them here, make sure you consult your coursework regarding the five basic types of chemical reactions.

They are:

• Combination reactions
• Decomposition reactions
• Single replacement reactions
• Double replacement reactions
• Combustion reactions

Reaction rates

Rate of reaction

The first thing to influence this is activation energy.

In order to react, this is the minimum amount of energy that reactant atoms or molecules must have.

That’s because to either dissolve or create bonds between atoms, there is an energy cost. 

Without the proper activation energy, reactants, even though they collide repeatedly, will never be able to break or create bonds. 

Catalysts speed up chemical reactions by decreasing activation energies.

Next, let’s look at the reaction mechanism. 

It is common practice to provide solely the net reactions, when looking into certain reactions.

The reaction mechanism depicts the realistic sequence of primary processes that make up a reaction. 

The stages involved in a chemical reaction are laid out in detail in reaction mechanisms. 

The pace varies from sluggish to very fast, but there is an individual reaction mechanism for each stage. 

The highest activation energy is found in the reaction mechanism’s slowest step, also called the rate-determining step. 

Catalysts can play a major role as well. 

It boosts the rate of a chemical reaction without itself getting depleted in the process, and it is normally added to the reaction to do so. 

Catalysts cannot force a reaction that wouldn’t normally happen to occur, however.

They simply speed things up by lowering the activation energy.  

More of the reactant molecules are able to undergo the reaction when activation energy (the minimum energy required for molecules to react) is lowered.

What factors can affect the rate of a reaction?

Concentration, surface area, and temperature are all variables that can.

The reaction rate is proportional to the product of the reactant concentration and the number of collisions between the reactants. 

The reaction rate increases when surface areas are increased, as this means more collisions between the reactants. 

Last but not least, raising the reaction temperature boosts the collision rate.

This also means the kinetic energy of the reactants increases, which, in turn, raises the percentage of molecules that can exceed the activation energy threshold.

There will be more molecules able to complete the reaction if there are more of them at the activation energy level.

Next, we look at chemical equilibrium.

When the rate of the forward reaction is equal to the rate of the reverse reaction, we have reached chemical equilibrium, and that means that a reaction is reversible. 

While the relative concentrations of the reactants and products always remain constant, there is a continuous occurrence of forward and reverse reactions.

Both the reactants and the products may not have the same concentration levels, however.

This can only happen if the system in which the reaction is taking place is completely sealed, with no way for any heat to enter or leave the system.

Lastly, let’s discuss Le Châtelier’s Principle.

This states that every stressor to a system in equilibrium will elicit a response from the system that, at least in part, tries to balance it.

Temperature changes, changes in the concentration of a reactant or a product, and pressure changes are all examples of stressors. 

When one is applied, it is common to speak of a system shifting left or shifting right.

For instance, a system will shift to the right to use up heat when the forward reaction in an endothermic equilibrium is increased.

Only when the number of moles of gasses in the reactants is different from the number of moles of gasses in the products will the equilibrium shift in response to a change in pressure. 

A change in pressure will have no effect on equilibrium if there are no gasses present or if the number of moles of gasses in the reactants and the products is equal.

While we won’t cover it here, make sure you cover the chemistry section on solutions, acids, and bases.

Scientific Reasoning

The last section to be covered in this module is scientific reasoning. 

Metric system

The metric system is a system of measurement used around the globe.

With a common reference point, people located anywhere can simply and accurately understand metric measurements.

The meter is the basic unit of length, the liter is the basic unit of volume, and the gram is the basic unit of mass in the metric system.

The increments in the metric system are multiples of 10, beginning with the base unit. 

To signify a numerical value, add the prefix to the base unit.

For example, 10 times the base unit is a deka.

Dekameters are 10 meters in length, dekaliters are 10 liters in volume, and dekagrams are 10 grams in weight. 

Hecto- denotes a multiple of 100; kilo denotes a multiple of 1,000 of the base unit. 

A deci is a prefix that shows a fraction of a base unit, and here centi is 1/100 and milli is 1/1000 of the base unit.

SI measurement units

The second (s) is the unit of time.

Metric prefixes like “milli” (1/1,000 of a second) and “nano” (1/1,000,000,000 of a second) are commonly used to express fractions of seconds. 

Minutes and hours, both multiples of 60 and 24, are used to measure durations longer than a second. 

For example, in the 800-meter freestyle, a time of 7 minutes, 32 seconds, and 67 one-hundredths of a second would be written as 7:32.67.

The ampere (A), kelvin (K), candela (cd), and mole (mol) are other base units and are used to measure electrical current, thermodynamic temperature, luminous intensity, and molecular mass, respectively. 

Length is measured in meters, and weight is measured in kilograms.

The use of metric prefixes for multiples and subdivisions is likely to be tested, so be careful to learn and memorize them.

Also, browse through the various apparatus that’s used in a lab.

Review: Scientific explanation using logic and evidence

Collecting data

Every legitimate experiment has measurable outcomes. 

Forming data tables and collecting thorough, specific information for each test is essential. 

The researcher must first identify the specific data needs and the rationale behind those needs. 

In advance, it’s necessary to know not only why the data is needed but also how it will be collected. 

Other than accuracy, which is a must, data must be easily reproduced as well as easily repeatable. 

The researcher must guarantee a consistent and reliable data collection process. 

In other words, it’s essential to run mock experiments, check that all instruments are properly calibrated, and repeat these steps at regular intervals during data collection.

Scientific process skills

Observation is a crucial ability when working in a scientific field.

The ability to collect unbiased data from controlled lab settings or the natural world is essential for scientists. 

The ability to form plausible hypotheses is also crucial. 

In order to predict the outcomes of their own experiments, scientists need to synthesize the information they have gleaned from theory and previous experiments.

Ordering and categorizing are crucial in data analysis too.

The information collected needs to be presented in a way that’s easy to understand and highlights the most important findings. 

Comparing is a skill that can be combined with the two above. 

It is important for scientists to be able to compare their work to that of others. 

Drawing logical conclusions from their findings is critical too.

They should be able to use their theoretical and empirical understanding to plan well-reasoned experiments and spot outliers in the data.

Finally, scientists need to be good at explaining their findings to others. 

When researchers are able to critique and improve upon one another’s findings, science advances rapidly.

Scientific statements

When a reasonable guess is made about what is likely to occur in an experiment, that is called a hypothesis.

They give researchers a place to begin when planning an experiment. 

They may be deduced from theoretical knowledge or from the findings of previous experiments.

For the sake of an experiment, assumptions are statements that are accepted without evidence. 

They could be completely accurate, or they could be accurate only for the specific experimental conditions.

In fact, many experiments can’t even be done without making some assumptions first, and they can help simplify the experimental process.

Mathematical statements used to chronicle certain types of behavior are known as scientific models. 

Our understanding of a system determines how accurate a model can be. 

When fresh evidence disproves an existing model, it is often abandoned for a new version.

They are still helpful for simplifying complex systems down to their essential components, even though they can never accurately represent reality.

Natural behavior statements that have been tested repeatedly and proved to yield consistent, reliable results are considered scientific laws. 

When all available evidence is consolidated, then that statement is known as a theory.  

Theories, like rules, are used to explain natural world behavior and, much like laws, are open to being disproved because they are less well-established.

If theories can be verified via experimentation, they just might become law.

Objects and events

Let’s look at events first. 

The term effect refers to the result of a certain action or occurrence, while cause refers to the action or event itself.

Some English words, such as since, because, and due to, imply a cause even when one is not stated explicitly. 

Effect-signaling words include then, so, and because of this.

Multiple outcomes can result from a single cause (e.g., Single cause: Your dog ate your homework because you left it on the table). 

There are a number of knock-on implications, from failing a class to being barred from seeing friends or going to the movies to missing out on a potential romantic partner, and these are multiple effects. 

Multiple factors may contribute to a single outcome (Single outcome: Alan has a fever). 

Multiple factors contributed to Alan falling ill, however. 

For example, Alan failed to take his multivitamin before an unexpected cold front arrived, and he caught a cold because of it.

In what is known as a chain of causes and effects, one effect might be the catalyst for yet another. 

Scale is critical too. 

Objects can range in size and shape, from the brightest stars in the galaxy to the smallest cells in the human body. 

There are numerous ways that these things can be measured. 

To accurately record dimensions and masses, one must be familiar with the appropriate units of measurement.

Taking a patient’s measurements is one instance where this skill is critical.

The standard unit of measurement for determining the height of a patient or the length of an extremity is the meter. 

However, the standard unit of measurement for vein diameter is millimeters. 

In the same way that a patient’s weight would be reported in kilograms, the weight of a human heart would be reported in grams. 

Scale also applies to lengths of time. 

The standard units of measurement for a patient’s expected lifespan are months and years. 

However, the standard unit of measurement for the number of breaths a patient takes is minutes (e.g., breaths per minute).

Scientific inquiry

Let’s talk about the methodology used in science.

Here, research that helps to evaluate hypotheses in the laboratory to either prove or disprove them based on the results is carried out.

A problem that will be checked in detail is the first stage of the scientific method.

In order to conduct a more precise analysis, it is crucial to establish precise boundaries for what is to be examined.

A hypothesis must be developed after the issue is specified.

The most accurate guess ought to represent a potential answer to the issue defined in the initial phase.

The next move is to conduct experiments to see if the idea holds up, and usually a full experiment is needed to do this.

Observation is the key to getting the most out of an experiment. 

Quantitative observations involve the use of numbers to quantify a phenomenon, while qualitative assessments rely on the observer’s subjective interpretation of the data. 

Discernible patterns or trends are then determined from the data that’s collected.

In order to predict further results, scientists will draw inferences from the collected data.

The experiment is considered complete if these results back up the initial hypothesis.

The next process is peer review, when others can repeat the experiment to see if they draw the same conclusions. 

A revised theory can be drawn if other findings don’t back up the hypothesis.

This will include new experiments, however.

Designing experiments

Not everyone has the skills necessary to design tests that yield useful information. 

Taking the necessary data in a safe and reliable manner requires meticulous planning at every stage of the experiment.

In a perfect experiment, every variable would be controlled except the one that needs to be manipulated.

This helps eliminate any potential bias introduced by unforeseen changes in the environment. 

The placebo group in a clinical trial is an excellent example of this.

The other groups are given the drug as usual, but this one is excluded.

The experiment should be planned with data gathering in mind, in addition to good control. 

For example, a thermometer or other temperature measuring instrument if temperature is one of the measurements.

It is important to double-check the data for obvious mistakes at regular intervals as it is being collected. 

There may be experimental mistakes in data collection or condition control if there are data points that are several orders of magnitude out of the intended value.

It is time to examine the results after all data collection is carried out.

The nature of the data and the trends that have been noticed determine how this is carried out. 

Curve fitting can help you figure out if the trends share a common mathematical shape. 

Statistical analysis of the data may also be required to establish the significance of the observed effects. 

Finally, the data must be presented clearly.

Controls

Careful regulation is a must for a reliable experiment. 

Except for the one being tested, all other conditions must be maintained in the correct manner.

That’s why it’s crucial to control for every factor when it comes to other variables (but not the independent variable).

Also, a control group will need a set of data of their own.  

The control group is not subject to the experimental manipulation but instead, reflects the normal condition of the variable. 

Negative or positive controls can be used, depending on the experiment type.

When conducting an experiment, it is common practice to include positive controls, or factors that the researcher anticipates will affect results. 

Experiments can be checked for accuracy by comparing results with a positive control group. 

Placebos are usually negative control groups that don’t actually do anything.

If a variable is not affecting the results of the experiment, a negative control group will indicate this.

Results will be more reliable the more tightly an experiment can be kept under control.

If the researcher controls for all factors save the one under study, then they have a better chance of reaching a reliable conclusion.

Variables

There are many potential factors at play in every given experiment, but only one should be targeted for manipulation and analysis, and this is referred to as the independent or manipulated variable. 

There is another variable, however, the dependent variable, and this is observed and recorded.  

Constants are all other factors that can’t be allowed to change during the experiment.

The solubility of a solute, for instance, would be the dependent variable, and the temperature at which the solubility was tested would be the independent variable. 

Everything else about the experiment remains the same, including the pressure, the amount of stirring, the solvent, the solute, and the solute’s particle size.