Circulatory System

The human circulatory system functions to transport blood and oxygen from the lungs to the various tissues of the body. The heart pumps the blood throughout the body. The lymphatic system is an extension of the human circulatory system that includes cell-mediated and antibody-mediated immune systems. The components of the human circulatory system include the heart, blood, red and white blood cells, platelets, and the lymphatic system.

Functions of the Human Circulatory System

Moving Nutrients

The human circulatory system has the important function of moving nutrients (such as electrolytes and amino acids), gases, hormones, blood cells, and other substances to and from cells.

Removing Debris

The human circulatory system also has an important role in carrying metabolites created from the cellular activity and debris resulting from disease and injury processes and cell death to areas where they can be broken down and eliminated.

Maintaining a Healthy Body

It also functions in the areas of fighting disease, stabilizing body temperature, and controlling the proper pH so that body homeostasis can be maintained.

Circulatory System composed of 

  • Cardiovascular System
  • Lymphatic System

Cardiovascular System

The main components of the human cardiovascular system are the


The human heart is about the size of a clenched fist. It contains four chambers: two atria and two ventricles. Oxygen-poor blood enters the right atrium through a major vein called the vena cava. The blood passes through the tricuspid valve into the right ventricle. Next, the blood is pumped through the pulmonary artery to the lungs for gas exchange. The oxygen-rich blood returns to the left atrium via the pulmonary vein. The oxygen-rich blood flows through the bicuspid (mitral) valve into the left ventricle, from which it is pumped through a major artery, the aorta. Two valves called semilunar valves are found in the pulmonary artery and aorta.

The ventricles contract about 70 times per minute, which represents a person’s pulse rate. Blood pressure, in contrast, is the pressure exerted against the walls of the arteries. Blood pressure is measured by noting the height to which a column of mercury can be pushed by the blood pressing against the arterial walls. Normal blood pressure is a height of 120 millimeters of mercury during heart contraction (systole), and a height of 80 millimeters of mercury during heart relaxation (diastole). Normal blood pressure is usually expressed as “120 over 80.” Coronary arteries supply the heart muscle with blood. The heart is controlled by nerves that originate on the right side in the upper region of the atrium at the sinoatrial node. This node is called the pacemaker. It generates nerve impulses that spread to the atrioventricular node where the impulses are amplified and spread to other regions of the heart by nerves called Purkinje fibers.


Blood is the medium of transport in the body. The fluid portion of the blood, the plasma, is a straw-colored liquid composed primarily of water. All the important nutrients, the hormones, and the clotting proteins as well as the waste products are transported in the plasma. Red blood cells and white blood cells are also suspended in the plasma. Plasma from which the clotting proteins have been removed is serum.

Red Blood Cells

Red blood cells are erythrocytes. These are disk-shaped cells produced in the bone marrow. Red blood cells have no nucleus, and their cytoplasm is filled with hemoglobin. Hemoglobin is a red-pigmented protein that binds loosely to oxygen atoms and carbon dioxide molecules. It is the mechanism of transport of these substances. (Much carbon dioxide is also transported as bicarbonate ions.) Hemoglobin also binds to carbon monoxide. Unfortunately, this binding is irreversible, so it often leads to carbon monoxide poisoning.

A red blood cell circulates for about 120 days and is then destroyed in the spleen, an organ located near the stomach and composed primarily of lymph node tissue. When the red blood cell is destroyed, its iron component is preserved for reuse in the liver. The remainder of the hemoglobin converts to bilirubin. This amber substance is the chief pigment in human bile, which is produced in the liver.

Red blood cells commonly have immune-stimulating polysaccharides called antigens on the surface of their cells. Individuals having the A antigen have blood type A (as well as anti-B antibodies); individuals having the B antigen have blood type B (as well as anti-A antibodies); individuals having the A and B antigens have blood type AB (but no anti-A or anti-B antibodies), and individuals having no antigens have blood type O (as well as anti-A and anti-B antibodies).


Leukocytes are much larger – and present in far smaller quantities – than erythrocytes. They have nuclei and contain no hemoglobin. Several types are present. Leukocytes form part of the immune system and are all involved in destroying harmful microorganisms.


Platelets are small disk-shaped blood fragments produced in the bone marrow. They lack nuclei and are much smaller than erythrocytes. Also known technically as thrombocytes, they serve as the starting material for blood clotting. The platelets adhere to damaged blood vessel walls, and thromboplastin is liberated from the injured tissue. Thromboplastin, in turn, activates other clotting factors in the blood. Along with calcium ions and other factors, thromboplastin converts the blood protein prothrombin into thrombin. Thrombin then catalyzes the conversion of its blood protein fibrinogen into a protein called fibrin, which forms a patchwork mesh at the injury site. As blood cells are trapped in the mesh, a blood clot forms.


This straw-colored fluid constitutes the liquid component of blood and consists of 95% water, as well as dissolved minerals like potassium, calcium, and sodium, vitamins, amino acids, breakdown products like urea, proteins, hormones, and bile salts. Plasma not only makes blood fluid enough to travel to where it is needed but is also the carrier of important nutrients and other substances. Plasma proteins that are known as globulins perform a number of vital functions, especially where immunity against disease is concerned: The immunoglobulins are a group of plasma proteins that serve as antibodies. Other plasma proteins include fibrinogen, which promotes the coagulation of blood, and albumin (egg-white).

Types of blood vessels


Arteries are muscular blood vessels that carry blood away from the heart, oxygenated and deoxygenated blood. The pulmonary arteries will carry deoxygenated blood to the lungs and the systemic arteries will carry oxygenated blood to the rest of the body. Arteries have a thick wall that consists of three layers. The inside layer is called the endothelium, the middle layer is mostly smooth muscle and the outside layer is connective tissue. The artery walls are thick so that when blood enters under pressure the walls can expand.


An arteriole is a small artery that extends and leads to capillaries. Arterioles have thick smooth muscular walls. These smooth muscles are able to contract (causing vessel constriction) and relax (causing vessel dilation). This contracting and relaxing affects blood pressure; the higher number of vessels dilated, the lower the blood pressure will be. Arterioles are just visible to the naked eye.


Capillaries are tiny blood vessels that pass blood from the arteries into the veins. They are very small, the largest being about 10 micrometers in diameter. Their walls are thin which allows materials to pass into the capillaries. Different types of capillaries exist and perform different functions for the body. Primarily, however, the capillaries are able to profuse the tissues of the body with needed oxygen and important nutrients supplied by the blood. Three types of capillaries exist. These are continuous capillaries, fenestrated capillaries, and sinusoidal capillaries. They vary in construction and in the degree to which they will allow things outside the capillaries to get into the capillaries. All capillaries have an endothelial wall with a differing degree of permeability depending upon the type. Continuous capillaries have the thickest endothelial wall. They allow only water, and ions into their pathways. Fenestrated capillaries have “windows” that let larger molecules in and out of the capillaries. Sinusoidal capillaries have the greatest amount of permeability, letting red blood cells and proteins in through the endothelial walls 

Function of Capillaries

While capillaries function in one respect as the “communicators” between arteries and veins, they also are the tiny blood vessels that supply blood to organs. Capillaries supplying blood to an organ, when taken in whole, are called a capillary bed. They are numerous and feed the organ with amino acids, proteins, and most importantly oxygen, without which organ cells could not survive.


Veins systemic system; differing outer bring caused by gravity. The hollow internal cavity in which the blood flows is called the lumen. A muscular layer of purposes,


A venule is a small vein that allows deoxygenated blood to return from the capillary beds to the larger blood veins, except in the pulmonary circuit where the blood is oxygenated. Venules have three layers; they have the same makeup as arteries with less smooth muscle, making them thinner.

What are the three types of blood circulation in your body?

Pulmonary Circulation

Pulmonary Circulation is part of the cardiovascular system which forms a loop of blood vessels that moves oxygen-depleted blood away from the heart to the lungs where carbon dioxide is released. Oxygen is picked up by the red blood cells in the lungs and is then transported from there back to the heart. The pump for the arterial side is the heart and the pump for the venous side is smooth muscle and respiration.

Coronary Circulation

Coronary Circulation is part of the systemic circulatory system and it transports blood to and from the heart.

Systemic Circulation

Systemic Circulation is that portion of the cardiovascular system which carries oxygen-rich blood from the heart to all parts of the body and returns oxygen-depleted blood back to the heart. The primary pump for the arterial side of this system is the heart. The pump for the venous side of this system is smooth muscle, skeletal muscle, and movement and respiration.

What is the lymphatic system what are its functions?

Lymphatic system

The lymphatic system is an extension of the circulatory system consisting of a fluid known as lymph, capillaries called lymphatic vessels, and structures called lymph nodes. Lymph is a watery fluid derived from plasma that has seeped out of the blood system capillaries and mingled with the cells. Rather than returning to the heart through the blood veins, this lymph enters a series of one-way lymphatic vessels that return the fluid to the circulatory system. Along the way, the ducts pass through hundreds of tiny, capsule-like bodies called lymph nodes. Located in the neck, armpits, and groin, the lymph nodes contain cells that filter the lymph and phagocytize foreign particles. 

The spleen is composed primarily of lymph node tissue. Lying close to the stomach, the spleen is also the site where red blood cells are destroyed. The spleen serves as a reserve blood supply for the body. 

The lymph nodes are also the primary sites of the white blood cells called lymphocytes. The body has two kinds of lymphocytes: B-lymphocytes and T-lymphocytes. Both of these cells can be stimulated by microorganisms or other foreign materials called antigens in the blood. Antigens are picked up by phagocytes and lymph and delivered to the lymph nodes. Here, the lymphocytes are stimulated through a process called the immune response. 

Certain antigens, primarily those of fungi and protozoa, stimulate the T-lymphocytes. After stimulation, these lymphocytes leave the lymph nodes, enter the circulation, and proceed to the site where the antigens of microorganisms were detected. The T-lymphocytes interact with the cell of the microorganism to cells and destroy them. This process is called cell-mediated immunity. B-lymphocytes are stimulated primarily by bacteria, viruses, and dissolved materials. On stimulation, the B-lymphocytes revert to large antibody-producing cells called plasma cells. The plasma cells synthesize proteins called antibodies, which are released into the circulation. The antibodies flow to the antigen site and destroy the microorganisms by chemically reacting with them in a highly specific manner. The reaction encourages phagocytosis, neutralizes many microbial toxins, eliminates the ability of microorganisms to move, and causes them to bind together in large masses. This process is called antibody-mediated immunity. After the microorganisms have been removed, the antibodies remain in the bloodstream and provide lifelong protection to the body. Thus, the body becomes immune to specific disease microorganisms.

What are the 6 lymphatic organs?

Primary lymphoid organs

bone marrow

Bone marrow is the spongy tissue inside some of your bones, such as your hip and thigh bones. It contains stem cells. The stem cells can develop into the red blood cells that carry oxygen through your body, the white blood cells that fight infections, and the platelets that help with blood clotting.


The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, thymus cell lymphocytes or T cells mature. T cells are critical to the adaptive immune system, where the body adapts specifically to foreign invaders.

Secondary lymphoid organs

Lymph nodes

Lymph nodes are small glands that filter lymph, the clear fluid that circulates through the lymphatic system. They become swollen in response to infection and tumors. Lymphatic fluid circulates through the lymphatic system, which is made of channels throughout your body that are similar to blood vessels.

The spleen

The spleen is the largest organ in the lymphatic system. It is an important organ for keeping bodily fluids balanced, but it is possible to live without it. The spleen is located under the ribcage and above the stomach in the left upper quadrant of the abdomen. A spleen is soft and generally looks purple.

Peyer’s patches

Peyer’s patches are small masses of lymphatic tissue found throughout the ileum region of the small intestine. Also known as aggregated lymphoid nodules, they form an important part of the immune system by monitoring intestinal bacteria populations and preventing the growth of pathogenic bacteria in the intestines.

Nasal associated lymphoid tissue

Nasal- or nasopharynx-associated lymphoid tissue (NALT) represents an immune system of the nasal mucosa and is a part of mucosa-associated lymphoid tissue (MALT) in mammals. It protects the body from airborne viruses and other infectious agents. In humans, NALT is considered analogous to Waldeyer’s ring.


Adenoids are a patch of tissue that is high up in the throat, just behind the nose. They, along with the tonsils, are part of the lymphatic system. The lymphatic system clears away infection and keeps body fluids in balance. The adenoids and tonsils work by trapping germs coming in through the mouth and nose.


Tonsils are the two lymph nodes located on each side of the back of your throat. They function as a defense mechanism and help prevent your body from getting an infection. When tonsils become infected, the condition is called tonsillitis. Tonsillitis can occur at any age and is a common childhood illness.

Share This