The Circulatory System
Lesson 8, Page 2 of 12

The heart's pace maker is the Sinoatrial (SA) Node.

The Heart Sounds

The closure of the heart valves and the contraction of the heart muscle produce sounds that can be heard through the thoracic wall by the unaided ear, although they can be heard better when amplified by a stethoscope. The sounds of the heart may be represented as lubb-dubb-pause-lubb-dubb-pause. The lubb sound indicates the closing of the valves between the atria and ventricles and the contracting ventricles; the dubb sound indicates the closing of the semilunar valves. In addition, there may also be cardiac murmurs, especially when the valves are abnormal. Some heart murmurs, however, may also occur in healthy persons, mainly during rapid or pronounced cardiac action. The study of heart sounds and murmurs furnishes valuable information to physicians regarding the condition of the heart muscle and valves.

Coronary Circulation

The coronary arteries supply blood to the heart muscle. These vessels originate from the aorta immediately after the aortic valve and branch out through the heart muscle. The coronary veins transport the deoxygenated blood from the heart muscle to the right atrium. The heart's energy supply is almost completely dependent on these coronary vessels. When the coronary vessels become blocked, as in arteriosclerosis or hardening of the arteries, blood flow to the cardiac muscle is compromised. This is when the common "bypass surgery" is performed where the coronary arteries are "bypassed" by replacing them with, for example, a vein from the leg. A "double bypass" is when two coronary arteries are bypassed. A "triple bypass" is when three are bypassed, etc.

The Heartbeat

The heart muscle pumps the blood through the body by means of rhythmical contractions (systole) and relaxations or dilations (diastole). The heart's left and right halves work almost synchronously. When the ventricles contract (systole), the valves between the atria and the ventricles close as the result of increasing pressure, and the valves to the pulmonary artery and the aorta open. When the ventricles become flaccid during diastole, and the pressure decreases, the reverse process takes place.

The Pulmonary Circulation

From the right atrium the blood passes to the right ventricle through the tricuspid valve, which consists of three flaps (or cusps) of tissue. The tricuspid valve remains open during diastole, or ventricular filling. When the ventricle contracts, the valve closes, sealing the opening and preventing backflow into the right atrium. Five cords attached to small muscles, called papillary muscles, on the ventricles' inner surface prevent the valves' flaps from being forced backward.

From the right ventricle blood is pumped through the pulmonary or semilunar valve, which has three half-moon-shaped flaps, into the pulmonary artery. This valve prevents backflow from the artery into the right ventricle. From the pulmonary artery blood is pumped to the lungs where it releases carbon dioxide and picks up oxygen.

Blood Flow

The Systemic Circulation

From the lungs, the blood is returned to the heart through pulmonary veins, two from each lung. From the pulmonary veins the blood enters the left atrium and then passes through the mitral valve to the left ventricle. As the ventricles contract, the mitral valve prevents backflow of blood into the left atrium, and blood is driven through the aortic valve into the aorta, the major artery that supplies blood to the entire body. The aortic valve, like the pulmonary valve, has a semilunar shape.

The aorta has many branches, which carry the blood to various parts of the body. Each of these branches in turn has branches, and these branches divide, and so on until there are literally millions of small blood vessels. The smallest of these on the arterial side of the circulation are called arterioles. They contain a great deal of smooth muscle, and because of their ability to constrict or dilate, they play a major role in regulating blood flow through the tissues.

The major artery that supplies blood to the body is the ________.

superior vena cava
inferior vena cava

(Select the best answer and click on the "Continue" button.)