The Heart and the Circulation

Figure 7. Representation of the circulation.

The blood flow around our body is called our circulation. The heart connects the two major portions of the circulation's continuous circuit, the systemic circulation and the pulmonary circulation. The blood vessels in the pulmonary circulation carry the blood through the lungs to pick up oxygen and get rid of carbon dioxide, while the blood vessels in the systemic circulation carry the blood throughout the rest of our body (Figure 7).

Figure 8a. The cardiovascular system.

The heart actually has two separate sides, one designed to pump deoxygenated blood into the pulmonary circulation where the blood becomes oxygenated, and one designed to pump the oxygenated blood into the systemic circulation where the blood flows throughout the body (Figure 8a). Each side of the heart has two chambers or compartments. The top chamber on each side is called the atrium. The right atrium receives incoming deoxygenated blood from the body and the left atrium receives incoming oxygenated blood from the lungs. The thin-walled atrium on each side bulges as it fills with blood, and as the lower heart muscle relaxes, the atrium contracts and squeezes the blood into a second chamber, the thick muscular ventricle. The ventricle is the pumping chamber that, with each muscular contraction, pushes the blood forcefully out and into the lungs (right ventricle) and the rest of the body (left ventricle).

The atrium and ventricle on each side of the heart are separated by tissue flaps called valves. The structure of these valves prevents blood from flowing backward into the atrium as the ventricle squeezes blood out. The valve on the right side, between the atrium and the ventricle, is called the tricuspid valve. The valve on the left side, between the atrium and the ventricle, is called the bicuspid or mitral valve. There are two other important valves that help to keep the blood Rowing in the proper direction. These two valves are located at the two points where blood exits the heart. The pulmonary valve is located between the right ventricle and the pulmonary artery that carries the deoxygenated blood from the heart to the lungs, and the aortic valve is located between the left ventricle and the aorta, the major artery that carries the oxygenated blood from the heart to the rest of the body.

The arteries are the blood vessels that transport blood out of the heart under high pressure to the tissues. The arterioles are the last small branch of the arterial system through which blood is released into the capillaries. The capillaries are very small, thin-walled blood vessels where the exchange of gases, nutrients, and waste takes place between the cells and the blood. Blood flows with almost no resistance in the larger blood vessels, but in the arterioles and capillaries, considerable resistance to flow does occur because these vessels are so small in diameter that the blood must squeeze all its contents through them. The venules collect blood from the capillaries and gradually feed into progressively larger veins. The veins transport the blood from the tissues back to the heart. The walls of the veins are thin and very elastic and can fold or expand to act as a reservoir for extra blood, if required by the needs of the body.

Figure 8b. The path of a typical RBC through the heart.

Let us follow a single red blood cell (RBC) through one full cycle along the circulatory pathway (Figure 8b). Remember that RBCs carry oxygen throughout the body. Since the blood travels endlessly, an arbitrary choice must be made of a starting point to describe the RBC's route. We will begin at the point where the RBC has delivered its oxygen to a cell in need and is on its return back to the heart.

1. Once the deoxygenated red blood cell (RBC) returns to the heart, it enters either through the superior vana cava or the inferior vena cava. The superior vena cava returns deoxygenated blood from the upper part of the body to the heart. The inferior vena cava returns deoxygenated blood from the lower part of the body to the heart. These large veins lead into the right atrium.

2. The RBC passes through the tricuspid valve into the right ventricle.

3. The RBC is then pumped through the pulmonary valve into the pulmonary artery and on to the lungs. There the RBC gives off carbon dioxide and picks up oxygen.

4. The RBC returns to the heart through a pulmonary vein, enters the left atrium, passes through the mitral valve, and flows into the left ventricle.

5. The left ventricle pumps the fully oxygenated RBC through the aortic valve, into the aorta, the body's main artery, and out to the body.

6. From the aorta, the RBC flows into one of the many arteries of the body, through the arterioles, and then to the capillaries, where the RBC will deliver oxygen and nutrients to the cells and remove wastes and carbon dioxide. Next it moves through the venules, veins, and on to the vena cava in a deoxygenated state, and returns to the heart, only to begin its repetitive journey once again. This whole process has taken approximately 20 seconds!

That single RBC will travel about 950 miles (more than 1500 kilometers) in its brief 4-month lifetime!