Nursing Entrance Exam

Achieve of 175 The Heart Valves The heart has two pairs of one-way valves that prevent the backflow of blood as the chambers contract. In order for the AV valves to open and close properly, the chordae tendineae and papillary muscles function normally and properly. When the ventricles relax, the chordae tendineae become loose, and the AV valves offer no resistance to the flow of blood from the atria into the ventricles. When the ventricles contract, blood moving back toward the atria swings the cusps together, closing the valves. At the same time, the chordae tendineae are tensed by the contraction of the papillary muscles to prevent the cusps from swinging into the atria. If the chordae tendineae are cut or the papillary muscles are damaged, backflow (regurgitation) of blood into the atria occurs each time the ventricles contract. Unlike the ventricles, the arterial walls do not contract and, therefore, the semilunar valves do not require muscular structure and the relative positions of the cusps are stable. When the semilunar valves close, the three symmetrical cusps support one another like the legs of a tripod. Sac-like dilations of the base of the ascending aorta are adjacent to each cusp of the aortic valve. These sacs, called aortic sinuses, prevent the individual cusps from sticking to the aortic wall when the valve opens. Serious valve problems can disrupt cardiac function. If valve function deteriorates to the point at which the heart cannot maintain adequate circulatory flow, symptoms of valvular heart disease (VHD) appear. Congenital malformations may be responsible, but in many cases, the condition develops after carditis (inflammation of the heart) occurs. The heart beats continuously; thus, cardiac muscle cells need a continuous supply of oxygen and nutrients. A large volume of blood flows through the chambers of the heart; yet the myocardium needs its own, separate blood supply. The heart muscle tissue is nourished by the coronary circulation. The cardiac circulation includes an extensive network of coronary blood vessels. During maximum exertion, the oxygen demand increases significantly; in fact, blood flow to the myocardium may increase to nine times that of resting levels. The right and left coronary arteries begin at the base of the ascending aorta at the aortic sinuses. Here, blood pressure is the highest in the systemic circuit. Each time the left ventricle contracts, it forces blood into the aorta. The arrival of additional blood at elevated pressures stretches the elastic walls of the aorta. When the left ventricle relaxes, blood no longer flows into the aorta, pressure declines, and the walls of the aorta recoil. This recoil, called elastic rebound, pushes blood both forward, into the systemic circuit, and backward, through the aortic sinuses then into the coronary arteries. Thus, the combination of elevated blood pressure and elastic rebound ensures a continuous flow of blood to meet the demands of active cardiac muscle tissue. However, myocardial blood flow is not steady – it peaks while the heart muscle is relaxed, and almost ceases while it contracts. Following the path of the coronary sulcus around the heart, the right coronary artery supplies blood to (1) the right atrium, (2) portions of both ventricles, and (3) portions of the conducting Page 129

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