The Circulatory System


The circulatory system consists of blood, a heart, and blood vessels.

Functions of the Circulatory System

The circulatory system functions with other body systems to provide the following:

Transport of materials:

Gasses transported: Oxygen is transported from the lungs to the cells. CO2 (a waste) is transported from the cells to the lungs.

Transport other nutrients to cells - For example, glucose, a simple sugar used to produce ATP, is transported throughout the body by the circulatory system. Immediately after digestion, glucose is transported to the liver. The liver maintains a constant level of glucose in the blood.

Transport other wastes from cells - For example, ammonia is produced as a result of protein digestion. It is transported to the liver where it is converted to less toxic urea. Urea is then transported to the kidneys for excretion in the urine.

Transport hormones - Numerous hormones that help maintain constant internal conditions are transported by the circulatory system.

Contains cells that fight infection

Helps stabilize the pH and ionic concentration of the body fluids.

It helps maintain body temperature by transporting heat. This is particularly important in homeothermic animals such as birds and mammals.

Blood Vessels

heart arteries arterioles capillaries venules veins heart


Arteries carry blood away from heart.

Arteries have a thick, elastic layer to allow stretching and absorb pressure. The wall stretches and recoils in response to pumping, thus peaks in pressure are absorbed.

The arteries maintain pressure in the circulatory system much like a balloon maintains pressure on the air within it. The arteries therefore act as pressure reservoirs by maintaining (storing) pressure.

The elastic layer is surrounded by circular muscle to control the diameter and thus the rate of blood flow. An outer layer of connective tissue provides strength.


Smooth muscle surrounding the arteries and arterioles controls the distribution of blood. For example, blood vessels dilate when O2 levels decrease or wastes accumulate. This allows more blood into an area to bring oxygen and nutrients or remove wastes.


The smallest blood vessels are capillaries. They are typically less than 1 mm long. The diameter is so small that red blood cells travel single file.

The total length of capillaries on one person is over 50,000 miles. This would go around the earth twice.

Not all of the capillary beds are open at one time because all of them would hold 1.4 times the total blood volume of the all the blood in the body. Vasodilation and vasoconstriction refer to the dilation and constriction of blood vessels. The diameter is controlled by neural and endocrine controls. Sphincter muscles control the flow of blood to the capillaries.

The total cross-sectional area of the capillaries is greater than that of the arteries or veins, so the rate of blood flow (velocity) is lowest in the capillaries. Blood pressure is highest in the arteries but is considerably reduced as it flows through the capillaries. It is lowest in the veins.

Interstitial Fluid

The exchange of substances between blood and the body cells occurs in the capillaries. Capillaries are specialized for exchange of substances with the interstitial fluid. No cell in the body is more than 100 micrometers from a capillary. This is the thickness of four sheets of paper.

Interstitial fluid surrounds and bathes the cells. This fluid is continually being replaced by fresh fluid from blood in the circulatory system.

Body cells take up nutrients from the interstitial fluid and empty wastes into the fluid.

By maintaining a constant pH and ionic concentration of the blood, the pH and ionic concentration of the interstitial fluid are also stabilized.

Although fluid leaves and returns to the capillaries, blood cells and large proteins remain in the capillaries.

At the arterial end of capillaries (the left side of the diagram below), blood pressure forces fluid out and into the surrounding tissues. As blood moves through the capillary, the blood pressure decreases so that near the veinule end, less is leaking into the surrounding tissues.

As blood flows through the capillary and fluid moves out, the blood that remains behind becomes more concentrated. The osmotic pressure in the capillary is therefore greater near the veinule end and results in an increase in the amount of fluid moving into the capillary near this end.

The arrows on the diagram above represent the movement of blood into and out of the capillary. Long and thick arrows are used to represent a large amount of fluid movement. The total amount of movement out of the capillary is approximately equal to the amount of movement into the capillary. Notice that more blood tends to leave the capillary near the arteriole end and more tends to enter it near the veinule end.

The lymphatic capillaries collect excess fluid in the tissues.


Capillaries merge to form venules and venules merge into veins.

Venules can constrict due to the contraction of smooth muscle. When they are constricted there is more fluid loss in the capillaries due to increased pressure.


The diameter of veins is greater than that of arteries.

The blood pressure in the veins is low so valves in veins help prevent backflow. 

The contraction of skeletal muscle squeezes the veins and assists with moving blood back to the heart.

The vena cava returns blood to the right atrium of the heart from the body. In the right atrium, the blood pressure is close to 0.

Varicose veins develop when the valves weaken.

Veins act as blood reservoirs because they contain 50% to 60% of the blood volume.

Smooth muscle in the walls of veins can expand or contract to adjust the flow volume returning to the heart and make more blood available when needed.

Portal Veins

Portal veins connect one capillary bed with another.

The hepatic portal vein connects capillary beds in the digestive tract with capillary beds in the liver.

The Heart

Chambers of the Heart

The heart is actually two separate pumps. The left side pumps blood to the body (systemic circulation) and the right side pumps blood to the lungs (pulmonary circulation). Each side has an atrium and a ventricle. See the diagram below

The atria function to receive blood when they are relaxed and to fill the ventricles when they contract.

The ventricles function to pump blood to the body (left ventricle) or to the lungs (right ventricle).


Valves allow blood to flow through in one direction but not the other. They prevent backflow.

Atrioventricular valves (diagram above) are located between the atria and the ventricles. They are held in place by fibers called chordae tendinae. The left atrioventricular valve is often called the bicuspid or mitral valve; the right one is also called the tricuspid valve.

The semilunar valves (diagram above) are between the ventricles and the attached vessels.

The heartbeat sound is produced by the valves closing.

Below: The structure of the mammalian heart is summarized using a model.

heart.jpg (92289 bytes)Click on the images to view enlargements.
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Cardiac Cycle

As the atria relax and fill, the ventricles are also relaxed.

When the atria contract, the pressure forces the atrioventricular valves open and blood in the atria is pumped into the ventricles.

The ventricles then contract, forcing the atrioventricular valves closed.  The pulmonary artery carries blood from the right ventricle to the lungs. The aorta carries blood from the left ventricle to the body.

Electrical Stimulation

The heart does not require outside stimulation.

The sinoatrial (SA) node is a bit of nervous tissue that serves as the cardiac pacemaker. Stimulation from this node causes both of the atria to contract at the same time because the muscle tissue conducts the stimulation rapidly.

The contraction doesn't spread to the ventricles because the atria and ventricles are separated by connective tissue.

As a wave of stimulation (depolarization) spreads across the atria resulting in their contraction, another bit of nervous tissue called the atrioventricular (AV) node also becomes stimulated (depolarized). It conducts the action potential slowly to the ventricles. The slow speed is due to the small diameter of the neurons within the node.

The slow speed of conduction within the AV node ensures that the ventricles contract after the atria contract..

The bundle of His then transmits impulse rapidly from the AV node to the ventricles.

Nervous Control

Details of nervous control of the cardiac cycle are in the chapter on the nervous system.

Coronary Circulation

Coronary arteries supply the heart muscles with blood.

They have a very small diameter and may become blocked, producing a heart attack.

Blood Pressure

The units of measurement are millimeters of mercury (mm Hg). For example, 120 mm Hg/80 mm Hg is considered to be normal blood pressure.

The top number is referred to as the systolic pressure; the bottom number is the diastolic pressure.

Hypertension - High Blood Pressure

High blood pressure is associated with cardiovascular disease.

Normal: Less than 120/80 

Prehypertension: 120-139/80-89 

Stage 1 hypertension: 140-159/90-99 

Stage 2 hypertension: 160 and above/100 and above


Human blood has two parts, liquid (plasma) and cells.


Plasma contains dissolved gasses, nutrients, wastes, salts, and proteins.

Salts and proteins buffer the pH so that it is approximately 7.4 and they maintain osmotic pressure.

Plasma proteins also assist in transporting large organic molecules. For example, lipoproteins carry cholesterol and albumin carries bilirubin (produced from the breakdown of hemoglobin when old blood cells are destroyed).


Red Blood Cells (Erythrocytes)

Red blood cells are biconcave disks filled with hemoglobin.

Red blood cells are continuously produced in the red marrow of the skull, ribs, vertebrae, and ends of the long bones. 

Human red blood cells loose their nucleus as they mature. As a consequence, red blood cells have a life span of approximately 120 days. Phagocytic cells in the liver and spleen remove old cells.

Anemia occurs when there are insufficient numbers of red blood cells or the cells lack sufficient hemoglobin.

White Blood Cells

White blood cells are covered in the chapter on the immune system.

Blood Clotting

Damaged tissue produces spasms of the smooth muscle and these spasms stop the blood flow for a few minutes.

Platelets are fragments of larger cells produced in the bone marrow that assist in forming a clot. They adhere to exposed collagen in damaged blood vessels. This causes some to rupture and release substances that attract more platelets. Platelets and damaged tissue release substances that cause a blood protein called fibrinogen to be converted to fibrin. Fibrin forms a mesh-like structure that traps blood cells and platelets. This results in the formation of a plug that seals the leak.

Details of Blood Clot Formation

When tissue damage occurs, muscles begin to spasm, which temporarily reduces blood flow to the area. Blood flow is also reduced when platelets in the blood adhere to the damaged tissue. Blood clotting is initiated when platelets and damaged tissue secrete prothrombin activator.

The platelets and damaged tissue release a clotting factor called prothrombin activator.

Prothrombin activator and calcium ions catalyze the conversion of prothrombin to thrombin which then catalyzes the conversion of fibrinogen to fibrin threads.

Fibrin threads are sticky and trap more platelets, further sealing the leak.