The human heart consists of four chambers: The left side and the right side each have one atrium and one ventricle. Each of the upper chambers, the right atrium (plural = atria) and the left atrium, acts as a receiving chamber and contracts to push blood into the lower chambers, the right ventricle and the left ventricle. The ventricles serve as the primary pumping chambers of the heart propelling blood to the lungs or to the rest of the body.
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There are two distinct but linked circuits in the human circulation called the pulmonary and systemic circuits. The pulmonary circuit transports blood to and from the lungs, where it picks up oxygen and delivers carbon dioxide for exhalation. The systemic circuit transports oxygenated blood to virtually all of the tissues of the body and returns to the heart.
The right ventricle pumps deoxygenated blood into the pulmonary trunk which leads toward the lungs and splits into the left and right pulmonary arteries. These vessels in turn branch many times before reaching the pulmonary capillaries where gas exchange occurs: Carbon dioxide exits the blood and oxygen enters. The pulmonary trunk arteries and their branches are the only arteries in the body that carry relatively deoxygenated blood (blue blood). Highly oxygenated blood returning from the pulmonary capillaries in the lungs passes through a series of vessels that join together to form the pulmonary veins—the only veins (red) in the body that carry highly oxygenated blood. The pulmonary veins (red) bring blood into the left atrium which delivers the blood into the left ventricle which in turn pumps oxygenated blood into the aorta and on to the many branches of the systemic circuit. Eventually, these vessels will lead to the systemic capillaries where exchange with the tissue fluid and cells of the body occurs. In this case, oxygen and nutrients exit the systemic capillaries to be used by the cells in their metabolic processes, and carbon dioxide and waste products will enter the blood.
The blood that has traveled throughout the body is lower in oxygen concentration than when it entered. The superior vena cava and the inferior vena cava return blood to the right atrium. The blood in the superior and inferior venae cavae flows into the right atrium, which delivers blood into the right ventricle. This process of blood circulation continues as long as the individual remains alive. (Figure).
Blood flows from the right atrium to the right ventricle where it is pumped into the pulmonary circuit. The blood in the pulmonary artery branches is low in oxygen but relatively high in carbon dioxide. Gas exchange occurs in the pulmonary capillaries (oxygen into the blood, carbon dioxide out), and blood high in oxygen and low in carbon dioxide is returned to the left atrium. From here, blood enters the left ventricle which pumps it into the systemic circuit. Following exchange in the systemic capillaries (oxygen and nutrients out of the capillaries and carbon dioxide and wastes in), blood returns to the right atrium and the cycle is repeated.
Right AtriumThe right atrium serves as the receiving chamber for blood returning to the heart from the systemic circulation. The two major systemic veins are the superior and inferior venae cavae. The superior vena cava drains blood from regions superior to the diaphragm: the head, neck, upper limbs, and the thoracic region. It empties into the superior and posterior portions of the right atrium. The inferior vena cava drains blood from areas inferior to the diaphragm: the lower limbs and abdominopelvic region of the body. It, too, empties into the posterior portion of the atria but inferior to the opening of the superior vena cava.
After exchange of gases in the pulmonary capillaries, blood returns to the left atrium high in oxygen via one of the four pulmonary veins. Blood flows nearly continuously from the pulmonary veins back into the atrium which acts as the receiving chamber and from here through an opening into the left ventricle. The opening between the left atrium and ventricle is guarded by the the Left AV or bicuspid valve. The myocardium of the thick walled left ventricle pumps strongly creating enough pressure to force blood flow through the entire body. As the blood leaves the left ventricle, it passes through the aortic valve into the aorta. Although both sides of the heart will pump the same amount of blood, the muscular layer is much thicker in the left ventricle compared to the right.
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The Left Ventricle
Recall that, although both sides of the heart will pump the same amount of blood, the muscular layer is much thicker in the left ventricle compared to the right. Like the right ventricle, the left also has trabeculae carneae, but there is no moderator band. The bicuspid valve is connected to papillary muscles via chordae tendineae. There are two papillary muscles. The left ventricle is the major pumping chamber for the systemic circuit; it ejects blood into the aorta through the aortic semilunar valve.