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General anatomy and physiology 

Understand general anatomy and physiology is the first step of HESI A2 anatomy and physiology study guide  


All living things consist of cells or their basic organizational unit. 

Cells have various functions that they carry out in order to help the growth and survival of an organism. 

Cells take on many different types, but between organisms, they are pretty unique. 

There are some common aspects, however.

For example, no matter what organism a cell comes from, it will all have a membrane that is made up of phospholipids. 

Proteins that help move ions and molecules in and out of the cell do so by making use of transport holes. 

A fluid called cytoplasm (or cytosol) is found in cells.

Organelles are a group of complex molecules found within cells. 

They help cells survive and possess their own membrane.

Compared to the cell membrane, these have a different chemical makeup. 

Larger cells need more organelles to live than smaller cells.

No matter the organism, at a cellular level, all will show structural organization and have a highly organized cellular structure.  

DNA and RNA will be present in all cells and proteins are synthesized through them. 

Cells consist of cytoplasm, nucleic acids, and cell membranes. 

While all the components necessary for life are found in a single-celled organism, cells can become specialized in those that are multi-celled. 

Different functions are taken on by different cells.

When cells are grouped together, it’s in tissues which in turn are grouped together in organs.

Organs grouped together are called systems, while a complete individual is known as an organism. 

Cell structure

Let’s look at the structure of cells and the parts they are made up of.

Ribosomes: The task of synthesizing proteins from amino acids is carried out by ribosomes. They are plentiful throughout cells, in fact, they make up around a quarter of the cell itself. In some cases, ribosomes are embedded in the endoplasmic reticulum while others are mobile.

Golgi complex: This helps with synthesizing proteins and other materials that are moved out of the cell. Consisting of layers of membranes, the Golgi complex is located near the nucleus.

Vacuoles: Taking the form of sacs, vacuoles are used for a range of functions. This includes waste removal, digestion, and storage. In animal cells, vacuoles are small and can be numerous but in plants, there is one large vacuole. 

Vesicle: Within a cell, the vesicle is a small organelle that performs various functions, including moving materials in and around the cell. It has a membrane. 

Cytoskeleton: Comprises microtubules that provide support for the cell and help shape it.

Microtubules: As mentioned above, microtubules form part of the cytoskeleton and are made of protein. They are tasked with providing support for the cell.

Cytosol: Within the cell, cytosol is a liquid material, mostly made up of water. There are some floating molecules contained within it, however. 

Cytoplasm: This pertains to cytosol and substructures and is a general term. These are not found within the nucleus but in the plasma membrane.

Cell membrane: Also called the plasma membrane, this acts as a barrier that brings definition to the cell. Substances outside of the cell are stopped from entering, while the membrane keeps cytoplasm in. What’s allowed to enter and exit the cell is controlled by the membrane. 

Endoplasmic reticulum: There are two types: rough (with ribosomes on the surface) and smooth (without). This is the transport of the system of the cell and that’s carried out through a tubular network. It extends throughout the cytoplasm into the cell membrane and is fused to the nuclear membrane. 

Mitochondrion: In terms of size and quantity, these cell structures vary. In some cells, mitochondrion may number in the thousands, while in others, there may be just one. Mitochondria have numerous tasks to fulfill including generating ATP. They are also involved in both the growth and death of cells. The DNA in the mitochondrion is separate from that found in the nucleus.

Mitochondria functions

Let’s look a little further into what it is that mitochondria do.

They have four main functions:

  • Produce cell energy
  • Cell signaling (or communication through the cell)
  • Cell differentiation (here non-differential cells are turned into ones that have a specific purpose to carry out)
  • Cell cycle and growth regulation (here the cell prepares to reproduce and then carries out that function)

In eukaryotic cells, mitochondria are plentiful.

In a single cell, there may be hundreds or even thousands with their form consisting of both an outer and inner membrane.

One of their main functions, however, is to ensure the cell has energy. 

The matrix is enclosed in the inner membrane. 

Here you will find ribosomes as well as mitochondrial DNA (mtDNA).

Folds or cristae are found between the inner and outer membranes and it’s here that energy is released when chemical reactions occur. 

The folds also control the cell water levels, as well as recycle and then create proteins and fats. 

Within the mitochondria aerobic respiration takes place. 

Nuclear cell parts

Here are the nuclear parts of a cell. 

Nucleus: Contains the chromosomes and is a small structure overall tasked with regulating a cell’s DNA. In a eukaryotic cell, it’s the defining structure and you will find a nucleus in all of these cell types. Genetic traits are passed on between generations by the nucleus and it contains ribosomes, chromatin, nuclear pores, a nucleolus, nucleoplasm, as well as a nuclear envelope.

Chromosomes: These take the form of threadlike rods of DNA that are extremely condensed. DNA is short for deoxyribonucleic acid. A genetic material, DNA will store information.

Chromatin: DNA and protein make up chromatin.

Nucleolus: Consisting of protein, this structure is found in the nucleus. It plays a part in protein synthesis and synthesizes, then stores RNA. The nucleolus is small and round without a membrane. 

Nuclear envelope: The structures of the nucleus are enclosed in this. Inner and outer membranes that form it consist of lipids. 

Nuclear pores: Are part of the material exchange between the cytoplasm and the nucleus.

Nucleoplasm: Similar to cytoplasm, nucleoplasm is the liquid found within the nucleus.

Cell membranes

Consisting of lipids and proteins, the cell membrane, also called the plasma membrane, is thin and semipermeable. 

It allows the cell to communicate with the outside environment and at the same time, provides protection from it. 

It is made up of a double layer called the phospholipid bilayer.

This has an outer layer with hydrophilic ends which face outwards and an inner layer facing inside the cell itself.

Stiffness and flexibility is added by cholesterol in the cell membrane.

Other cells of the organism are recognized through the help of glycolipids while the shape of the cell is helped by the protein found in the membrane. 

Aiding the cell’s communication with the environment outside of it are other special proteins while others aid the cell in transporting molecules across the membrane. 

Differentiation and cell cycle

Cell differentiation 

There are many different types of cells found within the human body. 

Differentiation is the process that helps to decide what cell type each cell will be. 

A group of cells known as zygotes helps control this progress along with genes.

They direct the cells to set themselves apart from others by building protein as well as other pieces.

Cell cycle

The process by which a cell reproduces is known as the cell cycle. 

This entails cell division, cell growth, as well as duplication of genetic material. 

As cells lose their functionality, they are replaced by their complex organisms through the cell cycle process. 

In animals, it’s a 24-hour time frame for this cell cycle. 

Different cells, however, require different time frames, and in humans, skin, for example, is forever reproducing, while other cells don’t divide so frequently. 

Neurons that have reached maturity will no longer grow and don’t divide anymore either. 

It is via mitosis and meiosis that cells reproduce. 

The daughter cell is an exact replica of the parent cell when mitosis is how cells replicate.

If they divide through the process of meiosis, there is a different genetic coding in the daughter cell when compared to the parent cell. 

It is only in specialized reproductive cells called gametes that meiosis occurs. 


We’ve briefly touched on the process of mitosis but let’s look at it in a little more detail.

During mitosis, these are the main events that occur:

  • Interphase: By replicating its cytoplasmic material, as well as its genetic code, a cell readies itself from division. 
  • Prophase: The nuclear membrane begins to break up as the chromatin gets thicker to turn into chromosomes. Spindle fibers start to form as centriole pairs move to opposite sides of the cell. Chromosomes are moved around within the cell via the mitotic spindle, which is formed from cytoskeleton parts. 
  • Metaphase: Chromosome pairs move into alignment along the center of the spindle structure as it moves to the middle of the cell.
  • Anaphase: Sisters, which are pairs of chromosomes, start to pull apart. They may even bend. When separation occurs, they become daughter chromosomes. In the cell membrane, groves will occur. 
  • Telophase: Disintegration of the spindle takes place at which point this is a reformation of the nuclear membrane. This sees the chromosomes transform into chromatin. 
  • Cytokinesis: Here, the cell physically splits, including the cytoplasm, and forms into two cells. 


In terms of phases, meiosis is the same as mitosis although there is one difference and that’s that in the former, the phases happen twice. 

Also, there are different events that occur between meiosis and mitosis. 

During the first phase of meiosis the following events occur:

  • Interphase
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
  • Cytokinesis

When the first phase occurs, there is an exchange of genetic material while chromosomes cross over. 

Also, there is the formation of tetrads of four chromatids, while the nuclear membrane breaks down. 

Separation occurs between homologous pairs of chromatids and they will move to different poles. 

One cell division has taken place at this point and that results in two cells being formed and they in turn go through another cell division. 

This second division consists of the following phases:

  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
  • Cytokinesis.

As a result, meiosis leads to four daughter cells.

Each of these, however, will have different chromosomes. 

Because they contain half the genetic material of their parent cell, these daughter cells are said to be haploid. 

Genetic diversity is encouraged through meiosis. 


As they perform a specific function, a group of cells known collectively as tissues, work together to do so. 

Based on their function, tissues are divided into categories. 

Here are the seven categories that animal tissues are divided into:

  • Epithelial: This is tissue where cells are joined tightly together. A perfect example of epithelial tissue would be the skin.
  • Connective: This tissue type can take on several forms, for example, it can be fatty, loose, or dense. Body parts are protected by this tissue type as well as bound together by it.  Types of connective tissue include tendons, bone tissue, fat, blood, lymph, and ligaments, for example. 
  • Cartilage: Provides body parts with structural support. Fibrous with a base that’s jelly-like.
  • Blood: While it removes waste, blood is a critical tissue as it transports oxygen around the body. Blood also defends against disease and also carries hormones.
  • Bone: Protects softer tissue and organs as well as supports them. This hard tissue produces red blood cells from its marrow.
  • Muscle: While muscle tissue helps the body to move, it also is tasked with providing support. This tissue comes in three different types – skeletal, cardiac, and smooth. 
  • Nervous: Tissue related to nerves is located in various areas around the body including in the nerves themselves, the brain, and the spinal cord. A network is formed from neuron cells and they help control responses based on external and internal environmental changes. 


Performing specific functions, organs work together to do so. 

Organs form multiple systems when grouped together in complex organisms with the heart tasked with moving blood through the body. 

It is from muscle tissue that the heart is mostly made up of, but the blood and membranes do have connective tissue as well, while the heart rate is controlled by nervous tissue. 

The membranes within the heart also include epithelial tissue. 

There are 11 main organ systems:

  • Integumentary 
  • Respiratory
  • Cardiovascular
  • Endocrine
  • Nervous
  • Immune
  • Digestive
  • Excretory
  • Muscular
  • Skeletal
  • Reproductive

Terms of direction

In the medical field, there are several terms of direction that are important.

  • 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 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.

Primary body planes

There are three primary body planes.

Traverse plane: Also known as the horizontal plane. The patient’s body is divided into upper and lower sections (superior and inferior).

Sagittal plane: Divides the body into left and right sections. Can also apply to parts of the body that can be divided in this way. It’s parallel to the midline of the body that the sagittal plane runs.

Coronal plane: Divides the body vertically into front and back sections (anterior and posterior). Can also apply to body parts that can be divided in this way. 

The respiratory system

The respiratory system

Respiratory system structure

It’s into an upper and lower section that the respiratory system can be divided. 

The larynx, pharynx, mouth, nasal cavity, and nose are all part of the upper system. 

The trachea, lungs, and bronchial tree form part of the lower system. 

In addition, respiratory system components can be seen as part of the airways, lungs, and respiratory muscles.

Parts of the airway are the nose, nasal cavity, mouth, larynx, pharynx, bronchi  (as well as the bronchial network), and trachea. 

Cilla within the airways traps debris and microbes which are then pushed back towards the mouth. 

The bronchi and bronchial network are housed in the lungs.

This network extends throughout and ends in air sacs or alveoli. 

These have a one cell thick wall which means that blood capillaries are able to exchange gasses with them. 

In the right lung, there are three lobes while the left lung only has two.

The reason for this is that space is left for the heart. 

Friction between surfaces when breathing occurs is lessened by the pleural membrane which surrounds the lungs. 

The diaphragm – which separates the thoracic and abdominal cavities – and intercostal muscles – located near the ribs – form part of the respiratory muscle network. 

Respiratory system functions

Supplying the body with oxygen is the main function carried out by the respiratory system and coupled to that, it must rid it of carbon dioxide.

These gaseous exchanges take place in the alveoli in the lungs.

These number in the millions and they are surrounded by blood capillaries. 

Another job the respiratory system carries out is to filter air and this is carried out as it passes through the nasal passages where it is warmed as well.

That’s not all the respiratory system does as it is also tasked with speech. 

This occurs as the larynx moves when air passes through the throat before it enters the windpipe.

This ensures the formation of sound due to the vibrating larynx.

The respiratory system also ensures that all foreign particles found in airways and nasal passages are expelled from the body and does this through cough production.

Two other areas where the respiratory system operates are with our smell as well as helping to maintain acid-base homeostasis.

Blood pH during acidosis (low pH) can be increased via hyperventilation while alkalosis (high pH) can be lowered by slow breathing. 

Breathing process

The lungs are expanded and contracted by the diaphragm and intercostal muscles during the breathing process.

The diaphragm will move down as it contracts during inhalation (also called inspiration). 

Because of this, the size of the chest cavity increases and when this happens, the pressure inside it drops. 

Air then rushes into the lungs because the outside air has a greater pressure applied to it than the air inside the lungs. 

The size of the chest cavity decreases when the intercostal muscles and the diaphragm relax. 

The air is then forced out of the lungs and this is known as expiration (or exhalation). 

The medulla oblongata is the portion of our brain that controls breathing and does so by checking on the levels of carbon dioxide in our blood.

If this gets too high, signals are sent for an increase in the rate of breathing. 

Cardiovascular system

Cardiovascular system

As an introduction to the cardiovascular system, let’s first look at what it’s responsible for in the human body. 

Its main role is moving substances to and from the cells.

The following make up this system:

  • Blood: This is a fluid connective tissue comprising water, solutes, and other elements
  • Blood vessels: These are tasked with transporting blood throughout the body. They are tubles of varying sizes and the system itself is closed. 
  • Heart: The heart is critical to the circulatory system as it keeps blood flowing through it. 

The rate of blood moving through the system will slow down as it moves from larger tubules to smaller ones. 

The lymph vascular system is a supplementary one tasked with cleaning proteins and excessive fluids and then bringing them back into the circulatory system.

Arterial and venous systems

Blood vessel walls (but not those found in capillaries) are made up of three layers.

These are:

  • Tunica intima (the innermost layer)
  • Tunica media
  • Tunica adventitia

Now let’s look at the various vessels by means of their structure and function.

Elastic arteries: This includes branches of the Tunica media and the aorta. When compared to all other vessels, they have the most elastin and in the arterial system, they are the largest of all vessels. They recoil under low pressure but when blood is forced out of the heart, they will stretch. 

Muscular arteries: These are arteries that move off of the elastic arteries and Tunica media. They have fewer elastic fibers when compared to elastic arteries but have more smooth muscle cells. Vasoconstriction/vasodilation regulates blood flow.

Arterioles: These lead to capillary beds and are tiny blood vessels where Tunica media is quite thin. It is made up of smooth muscle cells. These vessels deal with vasoconstriction and vasodilation and control blood flowing to the capillaries. 

Venules: These exit the capillary beds and take the form of tiny vessels with very porous and thin walls. They have few elastic fibers and muscle cells. They are tasked with helping to move blood to the large veins.

Veins: These have very thin tunica intima and tunica media and wide lumen. Backflow of blood is prevented by valves. Veins are tasked with ensuring that blood is carried back to the heart.


Blood transports raw materials to the cells and then ensures that waste products are removed. 

It also ensures the internal environment is kept healthy by hosting cells of the immune system and stabilizing internal pH. 

Humans have around five quarts of blood in their bodies which is made up of several components. 

These components include red and white blood cells, platelets, and plasma.

Inside plasma, you will find proteins, amino acids, hormones, glucose, ions, and dissolved gases. 

For blood clotting purposes, platelets play an important role and they are fragments of stem cells. 

The transport of oxygen around the body is carried out by red blood cells.

It’s in the marrow that these are formed and in total, these critical components of blood live for a period of four months. 

The number of red blood cells (which don’t have a nucleus) is kept stable as older ones are constantly replaced. 

Finally, white blood cells remove waste and fight any infections that enter the body. 

White blood cell types include basophils, eosinophils, monocytes, neutrophils, and lymphocytes. 


The heart operates as a muscular pump within the body and is constructed from tissue, specifically cardiac tissue. 

The atrioventricular and sinoatrial nodes send coordinated electrical signals that contract and relax the heart chamber. 

Ventricles are filled by atrial contraction and blood is forced into the arteries leaving the heart via ventricular contraction in a sequence of events termed the cardiac cycle. 

Blood flows in a single direction through the heart via valves and they also prevent the four chambers of the heart from experiencing any potential backwash. 

Here’s the order that deoxygenated blood flows through the heart in:

  • Blood is brought in from the upper body by the superior vena cava while from the lower body, the inferior vena cava handles this task
  • Right atrium
  • Tricuspid valve
  • Right ventricle
  • Pulmonary valve
  • Left and right pulmonary artery
  • Lungs. This is where the gas exchanges take place

Here’s the order that oxygenated blood returns to the body:

  • Left and right pulmonary veins
  • Left atrium
  • Mitral valve
  • Left ventricle
  • Aortic valve
  • Aortic arch
  • Aorta

The septum separates the left and right sides of the heart.

A separate circulation system with its own set of arteries exists in the heart.

Cardiac cycle

The ventricular filling phase is the first phase of the cardiac cycle and here, there is a lower pressure in the ventricle than in the atrium.

This ensures that the atrioventricular valve opens so blood can pass from the atrium into the ventricle. 

At this point, the pressure is higher in the blood vessels leading to the ventricle and because of this, the semilunar valves will close. 

The second phase of the cycle is ventricular contraction.

As the name suggests, at this point, the ventricle will contract and the pressure within it will increase, and as this happens pressure lowers.

This means that the atrial pressure is higher and that closes the atrioventricular valves.

While the semilunar valves close as well initially, they are forced open and ventricular ejection will start when the pressure in the ventricle moves higher than the pressure in those blood vessels (the aorta or pulmonary artery) leaving the heart. 

The cardiac cycle’s third phase is ventricular relaxation. 

When the blood is pushed out of the ventricles, it moves into the blood vessels moving away from the heart and this lowers pressure. 

When this drops lower than that in the blood vessels (the aorta or pulmonary artery), there is the closure of the semilunar valves.

Pressure falls below the pressure of the atrium when the ventricle relaxes.

This causes the atrioventricular valves to open and ventricular filling starts. 

Circulation types

There are three types of circulation within coronary circulation.

These are: 

  • Systemic circulation
  • Pulmonary circulation
  • Coronary circulation

In coronary circulation, we are specifically talking about blood flowing to the heart tissue. 

This occurs when blood moves into the coronary arteries which can be found branching off the aorta.

This blood moves through major arteries and then enters the heart. 

It’s through the cardiac veins that deoxygenated blood moves back into the right atrium at which point it empties into the coronary sinus. 

What about pulmonary circulation?

Well, this deals with blood flowing between the heart and lungs. 

Blood that is deoxygenated flows into the pulmonary arteries from the right ventricle to the lungs.

Blood that is oxygenated flows through the pulmonary veins back to the left atrium. 

Lastly, let’s touch on systemic circulation.

Leaving out pulmonary and coronary circulation, this deals with blood flowing throughout the body. 

To start, the blood will travel out of the aorta when it leaves the left ventricle. 

This branches out into the following: the carotid arteries, subclavian arteries, common iliac arteries, and the renal artery. 

When blood returns to the heart, it is via the jugular veins, subclavian veins, common iliac veins, and renal veins and from here enters into the superior and inferior venae cavae. 

Systemic circulation includes portal circulation.

This handles blood flowing from the digestive system to the heart via the liver. 

It also includes renal circulation which sees blood flowing between the heart and the kidneys. 

Blood pressure

Arterial blood pressure deals with taking oxygen-poor blood into the lungs where it is reoxygenated and then carried out to tissue all around the body. 

The arteries themselves will branch out into arterioles, which are smaller.

Depending on the signals they receive, they will expand and contract. 

Adjustments to blood delivery to certain areas of the body are made in the arterioles and this is based on communication received from systems within the body. 

Capillaries will merge into venules and then into veins which are larger diameter tubules. 

Veins are tasked with carrying blood from various tissues straight back to the heart which is helped by valves found within them. 

Vein walls consist of smooth muscle and are thin.

Additionally, they serve as blood volume reserves. 

Lymphatic system

Returning excess tissue fluid to the bloodstream is the main aim of the lymphatic system.

To do this, it is made up of lymphoid organs and transport vessels. 

Lymph capillaries, lymph vessels, and lymph ducts are all part of the lymph vascular system.

The lymphatic system also carries out the following, over and about returning excess fluid:

  • Returning protein from capillaries
  • Moving fats out of the digestive tract
  • Getting rid of cellular waste as well as debris

The following are lymphoid organs:

  • Lymph nodes
  • Spleen
  • Appendix
  • Adenoids
  • Thymus
  • Tonsils
  • Patches of tissue in the small intestine

Let’s focus a little more on lymph nodes themselves.

They are found in the lymph vessel system, located at certain intervals. 

The nodes themselves are made up of plasma cells and lymphocytes.

Moving onto the spleen, this is charged with stored macrophages and blood cells while hormones are secreted by the thymus which is a large contributor to lymphocyte production. 


We briefly chatted about the spleen early, but let’s look at its functions a little more closely.

The location of the spleen in the abdomen, towards the upper left, below the diaphragm and behind the stomach. 

Weighing about half a pound the spleen is made up of lymphoid tissue and it is around the size of a thick paperback book. 

It is via splenic sinuses, which are modified capillaries, that blood vessels are connected to the spleen.

The spleen is supported by these peritoneal ligaments:

  • The gastrolienal ligament that connects it to the stomach
  • The lienorenal ligament that connects it to the kidneys
  • Phrenicocolic ligament (the middle section thereof) which connects it to the thoracic diaphragm and the left colic flexure

The spleen has two main functions.

First, it helps to fight any infections in the body, and second, it is used to filter the blood of any unwanted materials.

This includes old red blood cells that are no longer effective.

Gastrointestinal system

Gastrointestinal system

The following functions occur in the digestive system:

  • Movement: This not only eliminates waste but also ensures that nutrients are mixed and passed through the system
  • Secretion: It ensures the secretion of hormones, enzymes, and other substances that are needed for digestion. These are secreted into the digestive tract
  • Digestion: Nutrients are chemically broken down into smaller units and enter the internal environment afterwards
  • Absorption: It is through plasma membranes that the nutrients pass where they move into blood or lymph and are then into the body

Mouth and stomach

Our food digestion starts as soon as we begin to chew it in our mouth as it’s here where saliva will mix with nutrients.

This is secreted by the salivary glands.

There are enzymes in saliva which help the process by first ensuring that starch is broken down. 

When the food has been chewed sufficiently, it will be swallowed from which point it moves down to the stomach via the route of the pharynx and esophagus. 

The stomach itself is a muscular sac that’s flexible

There are three functions it will perform:

  • It mixes food as well as stores it
  • Via secretions, it helps to dissolve food
  • It controls the passing of the food into our small intestines

While saliva helps to ensure starch is broken down from the moment we start chewing, for protein, this process only starts when the food enters the stomach. 

Nutrients are made available for absorption when the acid in the stomach begins to break down the food. 

The nutrients end up in the small intestine and it is here that they are absorbed into the body. 


The largest solid organ in the body, as well as the largest gland, is the liver. 

Located on the abdomen’s right side and below the diaphragm, there are four lobes that make up the liver.

These are the left, right, quadrate, and caudate lobes. 

Five ligaments secure it to the abdominal wall and the diaphragm. 

These five ligaments are the falciform, coronary, right triangular, left triangular, and round.

So what does the liver do?

Well, it is tasked with processing blood and does so once nutrients are received from the intestines.

The transfer of nutrients is done through the hepatic portal vein.

The liver functions from oxygen-rich blood which it receives from the hepatic artery. 

The hepatic veins are then used to move the blood out of the liver. 

The functional units within the liver are called lobules.

It is from thin layers of liver cells that they are constructed. 

Using branches of the protein vein and hepatic artery, blood enters these lobules, moving down sinusoids, which are smaller channels within them.

Here are some of the vital functions that the liver performs:

  • It produces bile
  • It produces some blood plasma proteins
  • It produces certain proteins to carry fat, as well as cholesterol
  • It stores glycogen which is excess glucose. When needed this glycogen is converted back to glucose by the liver
  • It regulates amino acids
  • To help the body store iron, it processes hemoglobin
  • It is tasked with converting ammonia to urea (which then leaves the body through urine)
  • Purifies the blood
  • Blood clotting regulation
  • Removes bacteria as well as boosting immune factors to help control infections 


Located at the back of the abdomen behind the stomach, the pancreas, a long organ with a tapered shape, is between six to 10 inches long. 

The right-hand side of the organ is wider and this is called the head with the narrower left-hand side called the tail. 

It’s near the first part of the small intestine, the duodenum, where the head is positioned while the tail ends close to the spleen.

The area between the head and the tail of the pancreas is known as the body. 

In terms of its makeup, the pancreas comprises both endocrine and exocrine tissue.

Digestive enzymes are secreted by the exocrine tissue through a host of ducts.

Together, these ducts run the length of the pancreas to form the main pancreatic duct. 

This is also connected near the duodenum to the common bile duct. 

Hormones such as insulin are secreted into the bloodstream from endocrine tissue. 

The following arteries provide the pancreas with blood: the super mesenteric artery, the gastroduodenal artery, and the splenic artery. 

The role of the pancreas in digestion

By secreting certain enzymes, the pancreas also plays a role in digestion. 

These enzymes go to the small intestine and it deals mostly with proteins and fats as it aids in digestion.

Acini, which are large groups of exocrine cells that produce the precursors to these enzymes (which are known as zymogens).

Through a chemical reaction, they are converted to an active form of the enzyme which are amylase and pancreatic lipase and this happens on entry into the small intestine. 

Sodium bicarbonate is also secreted by the pancreas specifically to help the small intestine by neutralizing any stomach acid that might reach it. 

When food is present, the hormones release help control the exocrine functions of the pancreas.

This is carried out by the stomach as well as the small intestine. 

Any exocrine excretions are carried to the duodenum via the pancreatic duct by flowing into the Wirsing duct (or main pancreatic duct). 

Small and large intestine

The small intestine will absorb most nutrients during the digestive process.

To aid in this process, enzymes are transported from the pancreas, stomach, and liver. 

It is on fats, proteins, carbohydrates, and nucleic acids that these enzymes act.

To break down fats, the liver will release bile, which is very effective in doing so. 

The gall bladder then will store the bile.

Food is already very small molecules when it reaches the small intestines lining. 

Villi cover the small intestine and these small structures are absorptive.

They will help to enlarge the surface area in which the semi-liquid mass of digested food (known as chyme) will interact. 

Microvilli, which are epithelial cells at the villi’s surface help the intestines to absorb nutrients too.

In fact, it is the primary organ in the digestive tract when it comes to absorption. 

Large intestine

The large intestine is also called the colon and it is here that waste material is concentrated, mixed, and stored. The food is moved by peristalsis using smooth muscles which contract and relax to ensure nutrients continue to move too. 

The colon starts on the right side of the abdominal cavity where it is around a meter in length.

Because of this, it moves across to the left side transversely and then drops down where it is connected to the rectum. 

The nervous system will trigger an impulse to have the rectum expel waste as soon as there is distention in the rectal wall caused by the waste material. 

This is achieved when the sphincter muscle at the end of the anus is stimulated which helps the waste matter to be expelled.

The volume of fiber and other undigested material influences the rate at which the colon has waste move through it. 

Nervous system

Nervous system

The nervous system deals with the overall environment of the body and then senses, interprets, and issues commands as a response to various conditions it comes across. 

It’s cells called neurons that make this possible and they are a complex system of communication.

Through a process called action potential, the plasma membrane of neurons has messages sent across it. 

When a neuron is stimulated, these messages occur, but that stimulation will have to be above a necessary threshold. 

It’s from the stimulation point of one neuron to the contact point of another that the stimulation will occur.

This happens in sequence. 

A chemical synapse is then released at the point of contact.

This will either inhibit or stimulate the adjoining cell. 

The framework for our nervous system is a network of nerves made up of neurons that are found throughout the body. 

The Reflex Arc and Somatic Nervous system

Our five senses as well as the voluntary movement of the skeletal muscle system are controlled by the somatic nervous system (SNS). 

All the neurons that are connected to our sense organs are part of this system.

The SNS is helped to operate both senses and skeletal muscle movement by the efferent (motor) and afferent (sensory) nerves.

Signals are brought from the central nervous system to the muscles and sensory organs by different nerves while afferent nerves also carry signals but in the opposite direction. 

Reflex arcs are involuntary movements that the SNS also performs.

The simplest act of the nervous system is a reflex. 

This happens without any conscious thought via the reflex arc when a stimulus is received and is an automatic response. 

The simplest nerve pathway is a reflex arc which we mentioned above. 

This is controlled by the spinal cord and totally bypasses the brain. 

Muscular system

Muscular system

In dealing with the muscular system, we must mention the three types of muscular tissue that you will come across.

They are:

  • Skeletal
  • Cardiac
  • Smooth

The 600 muscles in the human body will come from one of these muscular systems.

All of these muscles will have properties in common which include:

  • Excitability: When stimulated, all muscles have an electric gradient that can reverse
  • Contraction: Muscles can all shorten (or contract)
  • Elongate: Muscles can all relax (or elongate)

Muscular tissue types

Earlier, we mentioned the different types of muscles in the human body, but let’s look into them in greater detail. 

Skeletal muscles: These are voluntary muscles that move parts of the skeleton and operate in pairs. These muscles are constructed of muscle fibers and they are connected to one another in 

parallel bundles. Because of their striped appearance under magnification, these muscles are also known as striated muscles. 

Smooth muscle tissues: Located in the walls of internal organs including the stomach, intestines, and even blood vessels, these are involuntary muscles and nonstriated. When compared to the skeletal muscle fiber, smooth muscle cells are both wider and shorter. 

Cardiac muscles: As their name would suggest, this involuntary striated muscle is only found in the heart.

Of all of these muscles, it’s only the first group that has interaction with the skeleton to make movement in our bodies.

Contraction of skeletal muscles

We’ve mentioned that fibers are what skeletal muscles consist of.

These fibers include myofibril bundles and these are made up of sarcomeres which are repeating contractile units.

In myofibrils, there are two microfilament proteins which are a thin and a thick filament. 

In the thick filament, you will find myosin, which is a protein while the thin filament also has a protein but this is called actin. 

When thick and thin filaments overlap in the muscles, you will see striations or dark bands. 

When thin filament overlaps in the muscles, the bands you will see are lighter in color. 

The sarcomere is shortened when skeletal muscle attraction occurs.

This is a result of the thick filaments sliding over the thicker ones.

Calcium ions are released when the action potential (an electrical signal) is received by a muscle fiber. 

It is to the protein myosin and actin that these calcium ions bind during muscle contraction. 

This in turn will help the myosin heads of the thick filament to bind with the thin filaments actin molecules. 

The energy that’s needed for a contraction is provided by the adenosine triphosphate which is supplied from glucose. 

For the exam, while we won’t go through them here, make sure you do know the major muscles found in the body. 

Reproductive system

Reproductive system

Male reproductive system

This system makes, maintains, and then transfers sperm and semen.

The target of this transfer is the female reproductive system.

It has another purpose, however, and that is male hormone secretion. 

The male reproductive system has three external structures: the penis, scrotum, and testes.

The urethra is found in the penis which when it fills with blood, causes an erection. 

During sexual intercourse, this allows the penis to deposit semen in the female reproductive organs through ejaculation. 

The sack of smooth muscle and skin in which the testes are found is known as the scrotum. 

The idea is that in order to ensure that testes remain in a cooler environment than inside the body, they are kept on the outside in this sack as a way to ensure spermatogenesis. 

The male gonads are the testes and they are tasked with producing testosterone as well as sperm.

The epididymis, vas deferens, ejaculatory ducts, urethra, seminal vesicles, prostate gland, and bulbourethral glands are all part of the internal structure of the male reproductive system.

When sperm matures, it is stored in the epididymis and from there, it will move to the ejaculatory duct, via the vas deferens. 

Alkaline fluids with mucus and protein are also secreted into the ejaculatory duct by the seminal vesicles.

A milky white fluid is added from the prostate gland and this contains enzymes and protein. 

Because the acidity in the urethra can cause harm to sperm, this is neutralized by a fluid secreted into it by the bulbourethral. 

Female reproductive system

The production of an ova (egg cycles or oocytes), transferring that to the fallopian tubes where it is fertilized, taking in the sperm from the male, and then developing an embryo in a protective and nourishing environment is the function of the female reproductive system. 

The labia majora, labia minora, Bartholin glands as well as clitoris make up the external parts of the system. 

The first two are tasked with protecting the vagina while lubricating fluid, necessary during sexual intercourse, is secreted by the Bartholin glands.

The clitoris has many nerve endings and comprises erectile tissue and provides pleasure.

Ovaries, fallopian tubes, uterus, and vagina are all internal parts of the female reproductive system.

Estrogen and progesterone are secreted by the ovaries and they will also produce the ova. 

The mature fertilized egg will be carried in the fallopian tubes towards the uterus.

Once there, it attaches itself to the uterine wall and begins to form the embryo.

The uterus is tasked with providing an environment for the embryo to develop; it also provides nourishment while protecting it. 

The vagina is made from muscle and takes on the form of a tube that receives sperm and semen during intercourse. 

It also acts as a birth canal for the baby. 

Female reproductive cycle

Changes in both the uterine lining as well as the ovaries characterize the female reproductive cycle. 

In the ovarian cycle, there are three stages and these are:

  • The follicular state
  • Ovulation
  • The luteal phase

Let’s look at them in a little more detail.

The follicular phase sees the maturation of the follicle through stimulation by the FSH. 

In turn, it secretes estrogen which will help repair the uterine lining lost due to menstruation. 

During ovulation, a secondary oocyte is released from the ovary. 

This is brought about by an LH surge. 

When the corpus luteum forms from the follicle’s remnants, the luteal phase has started. 

This inhibits both LH and FSH through the production of both progesterone and estrogen, while the thickness of the endometrium is maintained by progesterone as well.

There is a regression in the corpus luteum if a fertilized egg is not implanted while estrogen and progesterone levels will drop. 

The cycle then renews and LH and FSH are no longer inhibited. 

There are three stages in the uterine cycle as well and these are:

  • The proliferative stage
  • The secretory stage
  • The menstrual stage

A renewal of the uterine lining characterizes the proliferative phase.

To get ready for implantation, during the secretory phase, nutrients are secreted and the endometrium gets far more vascular.

The menstrual phase occurs if there is no implantation, with the endometrium being shed. 

Pregnancy, parturition, and lactation

hCG is released when a blastocyst is implanted in the uterine lining. 

The degradation of the corpus luteum is stopped by this particular hormone as the production of progesterone and estrogen continues. 

The maintenance of the uterine lining is necessary and that’s the task of these particular hormones.

In the second trimester, enough estrogen and progesterone is produced by the placenta as a way to support the pregnancy and as hCG levels drop, these will increase as the term goes on.

Parturition: It’s unclear what the exact mechanism that facilitates the start of parturition (birth) is.

Beforehand, however, there is an increase in the levels of fetal glucocorticoids. 

This affects the placenta which, in turn, sees a drop in progesterone but an increase in estrogen.

From the posterior pituitary gland, a release of oxytocin occurs, stimulated by the cervix stretching.

Together with estrogen, oxytocin will then aid in the release of prostaglandins which, in turn with the oxytocin will aid in more uterine contractions occurring. 

Eventually, this will result in birth.

Lactation: As pregnancy progresses, there is an increase in prolactin hormone. 

Estrogen and progesterone, however, will inhibit its effect on the mammary glands.

The levels of these hormones will drop after parturition and then milk production is stimulated by prolactin. 

Milk is ejected due to suckling stimulating the release of oxytocin. 

Integumentary system

Integumentary system

This system comprises the skin but also includes nails, hair, sweat glands, and sebaceous glands.

It is tasked with several functions that are connected to communication, secretion, as well as protection. 

Let’s look at protection, for example, which sees this system help protect from several pathogens.

These include various chemicals, but also, bacteria and viruses. 

When it comes to secretion, our sebaceous glands will aid in waterproofing our skin by secreting sebum.

Our sweat glands help with thermoregulation as well as helping to remove metabolic waste from the body. 

Lastly, in communication, information to the brain regarding temperature, pressure, touch, and pain are sent by sensory receptors found in our skin. 

Vitamin D is also manufactured by the skin and through it, certain chemicals can be absorbed, for example, specific medications. 

Skin layers

From the surface of the skin inwards, the layers of skin are the epidermis and the dermis.

The most superficial layer of skin is the epidermis and it is made up of epithelial cells.

These do not have any blood vessels present. 

The stratum basale, a single layer of skin cells, is the portion of the epidermis found at its deepest point. 

This is undergoing division constantly which sees the production of skin cells. 

Because of this, it’s towards the surface of the skin that older cells will be moved and they are sloughed off when they die.

Also, most of the cells on the epidermal layer are waterproofed through keratin.

Next, look at the layer found under the epidermis and which is known as the dermis and is mostly made up of connective tissue.

In the dermis, you will find the following:

  • Sweat glands
  • Sebaceous glands
  • Hair follicles
  • Sensory receptors
  • Blood vessels

Collagen and elastin fibers also form part of the dermis. 

Connective tissue is what the hypodermis (or subcutaneous layer) consists of and technically, it is not considered to be skin. 

This layer helps to connect the skin to the muscles below it.

This layer also includes fat deposits and their job is to help provide cushioning as well as to provide insulation for the body.

Let’s now look at the various cells found within the dermis and epidermis.

Keratinocytes: Located in the epidermis, this is the most common cell type you will find there. Stem cells in the stratum basale are responsible for their production. Eventually, move to the surface of the skin as they flatten and die. These cells help to make our skin water resistant by producing keratin.

Melanocytes: Located in the epidermis, this provides our skin pigmentation through melanin production and also provides UV radiation protection.

Langerhans cells: Located in the epidermis, these are antigens of the immune system. They are mostly found in the epidermis’s stratum spinosum layer.

Merkel cells: Located in the epidermis, specifically the stratum basale, these are cutaneous receptors and help us with detecting light touch. 

Fibroblasts: Located in the dermis, these cells secrete components of the extracellular matrix, including collagen, elastin, and glycosaminoglycans.

Adipocytes: Located in the dermis, these are fat cells.

Macrophages: Located in the dermis, these help deal with pathogens.

Mast cells: Located in the dermis, these cells release histamine when necessary and play a role in inflammatory response. 

Temperature homeostasis and the involvement of the skin

Via the activation of sweat glands, our skin is involved in temperature homeostasis or thermoregulation, which allows the body to maintain a stable temperature.

A receptor, a control center, and an effector form part of a negative feedback system that helps to control the temperature of the body. 

Located in the dermis of our skin, the receptors are sensory cells, while the hypothalamus, the control center, is found in our brains. 

Sweat glands are part of the effectors, but they also include muscles (which aid with shivering when we are cold) and blood vessels. 

The body can be cooled via sweat evaporation.

Another way that body temperature is lowered is when vasodilation of the blood vessels occurs near the skin’s surface. 

Endocrine system

Endocrine system

Both long and short term regulation of the body is carried out by the endocrine system which uses the secretion of hormones to do so.

The nervous system and endocrine system work together in close harmony, for example, the pituitary gland and the hypothalamus operate as a neuroendocrine control center.

Many different signals will trigger hormone secretion and these can include environmental cues, chemical reactions, as well as hormone signs. 

Hormonal influence, however, only benefits those cells that have certain receptors in what is called the “key in the lock” model.

In some target cells, gene activation as well as protein synthesis is triggered by steroid hormones.

The activity of existing enzymes in cells can be changed by protein hormones.

In some cases, like with the hormone insulin, action is quick when the body receptor activates that the hormone is needed.

Longer, gradual, and in some cases, permanent body changes are down to slower acting hormones. 

The overall well-being of the body is linked to eight major endocrine glands which we will look at a little more closely:

  • Adrenal cortex: This aids in protein and lipid metabolism and monitors blood sugar levels
  • Adrenal medulla: Controls the size of blood vessels, increases blood sugar, and deals with cardiac function
  • Thyroid gland: Functions in development and growth and regulates metabolism
  • Parathyroid: Control blood calcium levels
  • Pancreas islets: Deals with the metabolism of carbohydrates and also controls levels of blood sugar
  • Thymus gland: Deals with immune response
  • Pineal gland: Influences sexual activity and daily biorhythms
  • Pituitary gland: Heavily involved in overall growth and development

Pituitary and hypothalamus hormones

The link between the endocrine system and the nervous system is governed by the hypothalamus.

Found in the brain, its position is inferior to the thalamus and superior to the pituitary. 

Releasing hormones (RH) and inhibiting hormones (IH) are secreted by the hypothalamus as a way of communicating with the pituitary gland. 

Hypothalamus hormones include:

  • Gonadotropin RH – GnRH: This stimulates the release of FSH and LH from the pituitary gland
  • Growth hormone RH – GHRH: This stimulates the release of GH from the pituitary gland
  • Growth hormone IH (somatostatin) – GHRH: This stops the pituitary gland from releasing GH
  • Thyrotropin RH – TRH: The gets the pituitary gland to release thyrotropin 
  • Prolactin RH – PRH: This stimulates the release of prolactin from the pituitary gland.
  • Corticotropin – RH: This stimulates the release of ACTH from the pituitary gland
  • Oxytocin: Stimulates contractions in the uterus. Also helps with milk secretion by targeting the mammary glands
  • Antidiuretic hormone – ADH: Increases water retention by targeting kidneys and blood vessels

“Master gland” is another name given to the pituitary gland because of the role it plays in hormone secretion which then has an effect on other glands. 

In terms of location, it is found in the sella turcica of the sphenoid bone.

This is below the hypothalamus.

Hanging from a stalk known as the infundibulum, the pituitary gland is around the same size as a pea. 

It is divided into the anterior lobe and posterior lobe. 

The anterior of the pituitary gland produces the following hormones:

  • Thyroid stimulating hormone – TSH: This affects the thyroid, getting it to secrete its hormones
  • Adrenocorticotropic hormone – ACTH: This affects the adrenal cortex, getting it to release both mineralocorticoids as well as glucocorticoids
  • Growth hormone – GH: This stimulates growth by targeting muscle and bone
  • Follicle stimulating hormone – FSH: This affects the gonads so that there is maturation in sperm cells/ovarian follicles
  • Luteinizing hormone: This affects the gonads so as to stimulate sex hormone production and ovulation in females
  • Prolactin – PRL: This stimulates milk production by targeting the mammary glands

The posterior of the pituitary gland produces the following hormones:

  • Oxytocin: This is released by the posterior pituitary gland but is produced by the hypothalamus. This stimulates contractions within the uterus as well as milk secretion in the mammary glands
  • Antidiuretic hormone – ADH: This increases water retention by targeting the blood vessels and kidneys

Head and neck hormone sources

Let’s look at the hormone sources of the head and neck areas.

  • Pineal gland: This is located in the brain where the two halves of the thalamus join. It produces melatonin and helps to regulate our daily rhythms
  • Thyroid gland: This is attached at the isthmus, which are two lobes located on the trachea (in the anterior thereof). This produces T3 (triiodothyronine), T4 (thyroxine), and calcitonin. This will work on most cells by promoting cellular metabolism. 
  • Parathyroid gland: Found in the posterior aspect of the thyroid, this produces PTH (parathyroid hormone). This helps to raise blood calcium by working on bones and kidneys. 

Abdomen hormone sources

Let’s move on to abdomen hormone sources.

  • Thymus gland: Found between the heart and the sternum. It produces the hormone thymosin which will help activate the production of T-cells by targeting lymphatic tissues.
  • Pancreas: This is located posterior to the stomach mostly. It produces insulin, glucagon, and GHIH (growth hormone IH). Insulin decreases blood glucose by targeting adipose tissue, the liver, and muscles. Glucagon helps the liver by increasing blood glucose while GHIH helps to impede insulin and glucagon secretion. 
  • Adrenal medulla: Found on the top of the kidneys, this produces epinephrine and norepinephrine. This increases heart rate, as well as blood sugar (in a response of fight or flight) by targeting the lungs, liver, blood vessels, and heart. 
  • Adrenal cortex: This term is given to the outer area of the adrenal gland. It produces glucocorticoids and androgens. The former will target most bodily tissues which helps to develop secondary sex characteristics. The latter also targets tissue, and when long-term stressors occur, for example, blood glucose going up, it will be released.
  • GI tract: This produces gastrin (stimulates the release of HCI in the stomach), secretin (stimulates the liver and pancreas to release bile and digestive enzymes), and CCK or cholecystokinin which will lower the level of bile and digestive enzymes produced by the liver and pancreas when necessary. 
  • Kidneys: They help produce erythropoietin (helps bone marrow produce red blood cells) and calcitriol (helps the intestines reabsorb CA2+)
  • Heart: This produces ANP (atrial natriuretic peptide) which works on the adrenal cortex, as well as the kidneys to lower blood pressure as well as reducing NA+ reabsorption.
  • Adipose tissue: This produces leptin which works on the brain to lower appetite. 

Reproductive system hormone sources

Last in this section, we look at the hormone sources of the reproductive system.

  • Ovaries: Found on each side of the uterus in the pelvic cavity, the ovaries produce estrogen, progesterone, and inhibin. Estrogen works with the brain, mammary glands, and ovaries in many ways, including contributing to secondary sex characteristics, managing the menstrual cycle, and helping with the growth of the uterine lining.
  • Placenta: Like the ovaries, this also produces estrogen, progesterone, and inhibin. It also produces human chorionic gonadotropin (hCG).  This helps the ovaries produce progesterone and estrogen.
  • Testes: These produce inhibin (which we’ve covered above) and testosterone which promotes secondary sex characteristics and spermatogenesis by targeting the testicles.

Urinary system

Urinary system

Excess substances not needed by the body are eliminated via the urinary system. 

The kidneys, bladder, and urinary ducts all form part of this system.


Our particular focus for the HESI A2 exam is on the kidneys. 

You will find these just under the diaphragm in the back of the abdominal cavity. 

They consist of three areas:

  • The outer layer which is known as the renal cortex
  • The inner layer which is known as the renal medulla
  • The renal pelvis, which funnels the waste products collected from the nephrons to the ureter

Over one million nephrons make up the renal cortex and these nephrons are very tiny filters.

In each nephron, you will find a glomerulus which is a group of capillaries.

The glomerulus is encircled by the Bowman’s capsule, which is cup-shaped, and in turn, this leads to a tubule. 

Renal arteries, which come off from the aorta, provide the kidneys with blood which flows into the arterioles and then the glomerulus and it is here that it is filtered, one of the main functions of the kidneys. 

The kidneys will then go through the following process:

  • Filtering the blood
  • Materials that are needed are reabsorbed
  • Waste products are excreted along with excess water through urination

We touched on the filtering process above, but reabsorption is also a key function. 

Here, the renal vein will reabsorb glucose, ions, water, and other organic materials which are then returned to the bloodstream.

This takes place between the peritubular capillaries and the tubules.

The distal convoluted tubule will remove any other substances – drugs for example – from the blood while through the secretion of hydrogen ions, the blood’s pH level can be adjusted.

Any materials that are not absorbed will move as urine from the renal medulla’s collecting tubules to the renal pelvis. 

There it is drained to the urinary bladder and expelled. 

Immune system

Immune system

Invading pathogens, such as bacteria, fungi, protists, and viruses will invade the human body.

Protection from these pathogens comes from our immune system. 

A critical part of this system is the lymphatic system which comprises:

  • Lymph
  • Lymph capillaries
  • Lymph vessels
  • Lymph nodes

There is also red bone marrow as well as white blood cells (leukocytes). 

So how do these systems work together?

Well, to start, the lymph capillaries are filled with tissue fluid and the lymph vessels are formed when these combine. 

Through contractions within skeletal muscles, the lymph is moved in one direction right through the system where it ends up in the lymph ducts. 

These return to the venous blood supply as well as along the lymph vessels, specifically into the lymph nodes. 

The nodes themselves will filter various matter, including pathogens.

Lymph nodes are found in the following areas:

  • Groin
  • Head
  • Armpits

Lymphatic tissue, like our tonsils, adenoids, spleen, thymus, and Peyer’s patches are found outside of the lymphatic vessel system where they reside in lymphatic tissue.

Tonsils, for example, help to stop various pathogens from getting into our bodies and the spleen helps remove pathogens, along with dead cells. 

The immune system includes the following:

  • Skin: A barrier against bacteria is formed by an intact epidermis and dermis
  • Ciliated Mucous Membranes: Pathogens are moved out of the respiratory tract by cilia
  • Glandular secretions: Bacteria are destroyed by secretions from the exocrine glands
  • Gastric secretions: Pathogens are destroyed by gastritis acid
  • Normal bacterial populations: Compete with gut and vagina pathogens

Also, infections are stopped through chemical reactions and the mobilization of white blood cells, while other pathogens and bacteria can also be repelled by plasma proteins.

White blood cell types include:

  • Macrophages
  • T lymphocytes
  • B Lymphocytes

Leukocyte types

White blood cells are also called leukocytes.

Their production is carried out within red bone marrow. 

Leukocytes are further categorized as:

  • Granulocytes
  • Monocytes
  • T lymphocytes
  • B lymphocytes
  • Natural killer cells

Found fixed in lymphatic tissue or traveling in the lymph, the largest, long-living phagocytes that encompass and then destroy pathogens are macrophages. 

Antigens are presented to T-cells by dendritic cells, while any invasion of bacteria or pathogens will see a quick response from neutrophils.

When an invasion does occur, basophils will alert the body. 

If a multicellular invasion occurs, large phagocytes called eosinophils will help deal with it. 

T lymphocytes, more commonly called T-cells are split into several groups:

  • Helper T-cells: Infections are fought within the body when these cells produce various chemicals, including antibodies, to do so
  • Killer T-cells: Tumor cells or those infected with a pathogen or virus are destroyed by these cells
  • Suppressor T-cells: When the battle is over, these cells will suppress other T-cells
  • Memory T-cells: Should the invading pathogen attack again, these cells remain in the bloodstream

Lastly, antibodies are produced by B lymphocytes. 

Antigen and immune responses

When the immune system is stimulated, it happens via antigens. 

Usually, these antigens are found on the surfaces of fungi, viruses, and bacteria where they take the form of proteins.

While that’s where antigens are usually found, they can also be toxins, foreign particles, or even drugs. 

If the antigens are unfamiliar, the human body will attack them, but should they recognize those produced by their own cells, this is not the case. 

Whenever a pathogen enters our body, it responds by producing an antigen. 

Let’s look at how this works with a typical immune response.

To start, a macrophage will engulf the foreign object or pathogen. 

After that, the macrophage is joined by a helper T-cell and this, in turn, will activate the killer T-cells as well as B-cells. 

While the B-cells will move on to become memory and plasma cells, the killer T-cells actively will look for cells having the same antigens and then destroy them. 

The plasma cells that generate from B-cells are important because depending on what the pathogen is, it will produce the necessary antibodies that are specific to it.

These antibodies then cling to the pathogen and this is an indication to other phagocytes that they will need to be destroyed. 

Passive and active immunity

An individual is protected from pathogens by an innate immune system at birth. 

They develop adaptive immunity that reacts to pathogens when they encounter infection or have immunization. 

While adaptive immunity is acquired, passive and active immunity can result naturally, as well as artificially. 

An acquired active immunity that is natural occurs without immunization and happens because a person builds immunity after exposure.

An acquired active immunity that is artificial sees a person build immunity to a pathogen after a vaccine is administered to them. 

It is during pregnancy that naturally acquired passive immunity occurs. 

Here the fetus will receive antigens from its mother via the bloodstream usually.

Antigens, however, can also be transferred when the baby is born and drinking breast milk.

An excellent example of artificially acquired passive immunity would be vaccinations taken during the COVID-19 pandemic. 

Skeletal system

Appendicular and Axial skeleton

Our skeletons are divided into two sections:

  • The appendicular skeleton 
  • The axial skeleton

Appendicular skeleton

This is made up of 126 bones which include appendages, pelvic girdle, and pectoral girdle.

  • Pectoral girdle: This is made up of the scapulae or shoulders and the clavicle or collarbones.
  • The pelvic girdle: This is made up of our pelvic or hip bones
  • The appendages: Include the humerus, radius, ulna, carpals, metacarpals, and phalanges of the upper appendages as well as the femur, patella, fibular, tibia, tarsals, metatarsals, and phalanges of the lower appendages. 

Axial skeleton

This is made up of 80 bones which include the vertebral column, skull, rib cage, sternum, and hyoid bone. 

  • The vertebral column: This is made up of 33 vertebrae including the sacral, lumbar, thoracic, and cervical vertebrae. 
  • The rib cage: This has two pairs of floating ribs and 10 pairs of ribs making 12 paired ribs in total. The sternum is made up of the corpus sterni, manubrium, and xiphoid process.
  • The skull: This is made up of the cranium and facial bones.

Structure of joints

Where two or more elements of the skeleton connect, you will find joints. 

When it comes to classifying these joints, it’s either by the material that they are held together with or their range of motion. 

Let’s start with functional classification.

  • Synarthrosis: These joints can be cartilaginous or fibrous and have zero range of motion (immovable). Examples include teeth/mandibles and skull sutures.
  • Amphiarthrosis: These joints can be cartilaginous or fibrous and have a slight range of motion. Examples include the distal tibiofibular joint and intervertebral discs.
  • Diarthrosis: These joints are always synovial and have free movement when it comes to their range of motion. Examples include shoulders, knees, and wrists. 

Next, we move on to structural classification. 

  • Fibrous: Fibrous connective tissue holds these joints together. There are various types, each with a different range of motion. They include suture (immovable, e.g. the skull), gomphosis (immovable, e.g. teeth/mandible), and syndesmosis (move slightly, e.g. distal tibiofibular joint). 
  • Cartilaginous: Cartilage holds these joints together. They include synchondrosis (hyaline cartilage which is nearly immovable, e.g. first rib/sternum) and symphysis (fibrocartilage which is slightly movable, e.g. pubic symphysis, intervertebral disc)
  • Synovial: These joints are most common. Here, the cavity of the joint is filled with synovial fluid. They include pivot (can rotate, e.g. the atlantoaxial joint), hinge (movement in one plane, e.g. knee), condyloid (no axial rotation, but can pivot in two planes, e.g. radiocarpal joint), and ball and socket (the highest range of motion in a joint, e.g. the hip).

Skeletal system functions

Other than providing structural support, there are several other functions that the skeletal system is responsible for. 

These include protection, movement, blood cell production, and storage (of substances such as minerals and fat). 

Let’s look at those a little more closely.

Muscles and organs need to be supported, and the skeletal system does this by providing structure. 

The weight of the upper body is transferred to the lower appendages by the axial skeleton while the skeletal system in general uses muscular systems as well as joints to provide movement. 

Attachment points for muscles – of which there are two points for each – are provided by bones. 

These attachment points are known as the origin and the insertion.

The bone that moves during contraction and relaxation of the muscle is known as the insertion, while the origin is immobile. 

In terms of protection, specific parts of the skeletal system protect specific parts of the body.

For example, the brain is protected by the cranium while the vertebrae ensure the spinal cord is protected, while the organs such as the heart and lungs are protected by the rib cage. 

Both red and white blood cells are manufactured by red marrow, while fat is stored by yellow marrow.

Lastly, minerals such as calcium and phosphorus are stored in the skeletal system. 

Bones, as we have seen, play a critical part in the overall skeletal system and they are classified as irregular, flat, long, or short. 

They are connective tissue with a pulp base.

In this base, both living cells and collagen are found. 

As the mineral composition thereof changes, bones will continually regenerate themselves.

While this ensures that the correct calcium levels are maintained within the body, it also ensures that during growth periods, compensation is made. 

As we get older, bone regeneration can decrease significantly, especially in females and this can lead to osteoporosis. 

Adjacently areas connected to skeletal bones are the joints found within a body which we have already covered extensively. 

Spongy and compact bone

Cortical (compact) bone is extremely strong.

Thanks to the fact that it comprises tightly packed cells, it is also very rigid and dense. 

In this type of bone, you will find Haversian canals that run vertically through it.

Concentric circles of bone tissue surround them and these are known as lamellae. 

Lacunae is the term given to the spaces found between lamellae. 

The Haversian system is the collective name given to these lamellae along with their associated nerve endings, lymph vessels, veins, and arteries. 

This system is important as it provides the blood with a reservoir for both phosphorus as well as calcium. 

The white, smooth appearance of bones are as a result of the thin outside layer of compact bone. 

Cancellous (spongy) bone is made up of a network of girders. 

These are known as trabeculae and the spaces between them are filled with red bone marrow. 

Spongy bone is incredibly lightweight when compared to compact bone. 

One of the main reasons for this is the fact that it is porous. 

Long bones have their diaphysis made up of compact bone which is found all around the marrow cavity.

The epiphyses contains spongy bone in which red marrow is found. 

Depending on their specific classification within the skeletal system, bones will have differing amounts of spongy and compact bone.

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