Circulation Through the Heart

Blood enters the right atrium through two large veins. A vein is a blood vessel that carries blood to the heart. The superior vena cava brings blood from the upper regions of the body; the inferior vena cava brings blood from the lower body. The blood entering the heart through these veins is dark red because it is deoxygenated—that is, without oxygen.

About 70 percent of the blood in the right atrium flows directly into the right ventricle. The remaining blood is forced into the ventricle by a mild contraction of the atrium. When the right ventricle contracts, the tricuspid valve closes, and blood is forced into the pulmonary artery. An artery is a blood vessel that carries blood away from the heart. The semilunar valve closes. The blood travels from the pulmonary artery into its two branches, one to each lung. In the lungs, die exchange of carbon dioxide from the deoxygenated blood and oxygen from freshly inhaled air takes place. The blood, now bright red and saturated with oxygen, enters die left atrium via the pulmonary veins. The path of blood from heart to lungs and back is called the pulmonary circulation.

The path of blood through the left side of the heart is similar to that through the right side. The contraction of the left ventri­cle is very powerful because it must force blood to the farthest regions of the body. Blood rushes from the left ventricle into the aorta, the largest artery. From the aorta, blood flows to all parts of the body through a system of increasingly smaller arteries.

The Heartbeat

The heart is really two separate pumps that operate simultaneously at about 70 contractions — heartbeats — per minute. Blood flows into both atria at the same time, and the atria contract together. Similarly, the ventricles contract to­gether. A ventricular contraction is called systol. Relaxation is called diastole.

The activity within the heart causes the heartbeat, a sound usually described as "lubb dup." The "lubb" sound is related to the closing of the tricuspid and mitral valves. The shorter and higher pitched "dup" comes very shortly thereafter and is re­lated to the closing of the semilunar valves. Certain types of heart disorders can be detected through irregularity in one or both sounds.

What causes the heart to beat regularly without any con­scious control? The heart has its own automatic pacemaker. It is a small region of muscle called the sinoatrial, or SA, node in the back wall of the right atrium. The SA node triggers each heartbeat with an impulse that causes the atria to contract. Within a tenth of a second, the impulse reaches the atrioventricular, or AV, node at the base of the right atrium. Within milliseconds, the AV node triggers an impulse that causes the ventricles to con­tract. In a disorder called fibrillation, contractions become ir­regular and rapid. These uncoordinated contractions affect the ventricles, and therefore the pumping of blood to the body.

Blood Vessels

Blood is carried to all parts of the body through 112,000 km (70,000 mi.) of blood vessels. Different types of vessels vary in size and structure.

Arteries have especially elastic, muscular walls. These walls consist of three layers of tissue. Ar­teries branch into smaller and smaller arteries until they become tiny vessels called arterioles. Arterioles continue to decrease in diameter until they branch into capillaries — vessels so narrow that red blood cells must pass through them in single file.

Capillaries are the smallest and most numerous blood ves­sels in the body. Every body cell is within 0.13 mm (0.005 in.) of one or more capillaries. Although other blood vessels trans­port nutrients and waste products, the actual exchange of these products between blood cells and body cells takes place through capillary walls. Capillary walls are only one cell thick. As a result, diffusion of nutrient molecules, waste products, and gases can take place quickly. Capillary walls also allow plasma to filter out of the blood to become tissue fluid.

Deoxygenated blood travels from capillaries into small veins called venules. Veins increase in size as they approach the superior vena cava and inferior vena cava. Like artery walls, vein walls consist of three layers of tissue. However, the middle layer is less muscular than that of arteries.

Blood in veins generally must flow against the force of gravity — for example, from the feet to the heart. Two features prevent blood from flowing backward, away from the heart. The first is location. Many veins run through skeletal muscles.

As the muscles contract, the veins are squeezed and blood is pushed along. The second is a series of valves that keeps the blood from moving backward.

Circulatory system

Within the circulatory system are several subsystems. The path­way of blood from the heart to the lungs and back to the heart is called the pulmonary circulation. The pathway of blood from the heart to other parts of the body and back to the heart is called the systemic circulation.

Systemic circulation also has subsystems. Coronary circu­lation, for example, supplies the heart itself with blood. The left and right coronary arteries branch off the aorta and provide the heart continuously with oxygen and nutrients. The blood returns to the right atrium by way of a large vein called the coronary sinus.

Heart tissue must be nourished continuously. When some­thing prevents blood from reaching the cardiac muscle, the lack of oxygen causes the muscle cells to die. This condition, known as a heart attack, is one of the leading causes of death in the United States. A heart attack may result from a blood clot that blocks a blood vessel or from a gradual buildup of cholesterol, fibrin, and other cellular material inside the blood vessels. This buildup, called atherosclerosis, narrows the openings inside blood vessels.

Another part of systemic circulation is renal circulation, which carries blood to and from the kidneys. The left and right renal arteries branch from the aorta and enter the kidneys. Ni­trogenous waste products filter out of the bloodstream into renal capillaries. The blood then travels through renal veins to the inferior vena cava.

Hepatic portal circulation, a third part of systemic circula­tion, involves the digestive tract and liver. Mesenteric arteries carry blood from the aorta to the intestines, where water and molecules from digested food enter the capillaries. The blood, which is now enriched with nutrients, travels via the hepatic portal vein to the liver, where some nutrients are stored as glycogen. The hepatic artery supplies the liver with oxygenated blood. Blood leaves the liver and reaches the inferior vena cava through hepatic veins.