Anatomy & Physiology I and II

For the AV valves to open and close properly, the chordae tendineae and papillary muscles must exhibit normal function. When the ventricles are at ease, the chordae tendineae loosen, 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 secure. When the semilunar valves shut, the three symmetrical cusps reinforce one another like tripod legs. Sac-like dilations of the bottom 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 wall of the aorta when the valve opens. Serious valve problems can interfere with cardiac function. If valve function deteriorates to the point where the heart cannot maintain sufficient circulatory flow, signs of valvular heart disease (VHD) become noticeable. Congenital malformations may be responsible, but in many cases the condition develops after carditis, an inflammation of the heart, occurs. 19.5 The Blood Supply to the Heart 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. The blood flow to the myocardium may then increase to nine times that of resting levels. The Coronary Arteries The left and right coronary arteries originate at the bottom of the ascending aorta at the aortic sinuses. Blood pressure in this area is the highest in the systemic circuit. Every time the left ventricle contracts, it pushes blood into the aorta. The arrival of more blood at elevated pressures stretches the aorta’s elastic walls. When the left ventricle relaxes, blood no longer travels into the aorta, pressure falls, and the walls of the aorta recoil. This recoil, known as elastic rebound, forces blood both forward into the systemic circuit, and backward, through the aortic sinuses, and into the coronary arteries. The collaboration of elevated blood pressure and elastic rebound assures 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 the right atrium, portions of both ventricles, and portions of the conducting system of the heart, including the sinoatrial (SA) node and the atrioventricular (AV) node. The normal heart rate is established by the cells of these nodes. Inferior to the right atrium, the right coronary artery generally gives rise to one or more marginal arteries, which extend across the surface of the right ventricle. The right coronary artery then continues across the posterior surface of the heart, giving rise to the posterior interventricular artery or posterior descending artery, which runs toward the apex within the

Anatomy & Physiology Study Guide

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