Nursing Entrance Exam

system of the heart, including the atrioventricular (AV) node and the sinoatrial (SA) node. The cells in these nodes establish the normal heart rate for an individual. The right coronary artery, located inferiorly to the right atrium, generally gives rise to one or more marginal arteries, which extend across the right ventricle’s surface. The right coronary artery then continues across the posterior surface of the heart; it gives rise to the posterior interventricular artery, or posterior descending artery, which runs toward the apex within the posterior interventricular sulcus. The posterior interventricular artery supplies blood to the interventricular septum and to the adjacent portions of the ventricles. The left coronary artery supplies blood to the interventricular septum, left atrium, and left ventricle. As it reaches the anterior surface of the heart, it gives rise to a circumflex branch and an anterior interventricular branch. The circumflex artery curves to the left around the coronary sulcus, eventually meeting and fusing with the small branches of the right coronary artery. The much larger anterior interventricular artery, or left anterior descending artery (LAD), swings around the pulmonary trunk and runs along the surface within the anterior interventricular sulcus. The anterior interventricular artery supplies small tributaries continuous with those of the posterior interventricular artery. Called arterial anastomoses, these interconnections between arteries are common and allow for continuous, adequate blood flow to the cardiac muscle even with regular changes in pressure of the right and left coronary arteries as the heart beats. 7.4.4. Phases of the Cardiac Cycle There are four phases of the cardiac cycle: atrial systole, atrial diastole, ventricular systole, and ventricular diastole. When the cardiac cycle begins, all four chambers are relaxed and the ventricles are partially filled with blood. During atrial systole, the atria contract, filling the ventricles completely with blood. Atrial systole lasts 100 msec. Over this period, blood cannot flow into the atria because atrial pressure exceeds venous pressure. Yet, there is very little backflow into the veins, even though the connections with the venous system lack valves, as blood tends to takes the path of least resistance. Resistance to blood flow through the broad AV connections and into the ventricles is less than that through the smaller, angled openings of the large veins. The atria next enter atrial diastole, which continues until the start of the next cardiac cycle. Atrial diastole and ventricular systole begin at the same time. Ventricular systole lasts 270 msec. During this period, blood is pushed toward the atria through the pulmonary and systemic circuits. Next, the heart enters ventricular diastole, which lasts 530 msec (the 430 msec remaining in this cardiac cycle, plus the first 100 msec of the next). For the rest of this cycle, filling occurs passively, and both the atria and the ventricles relax. The next cardiac cycle begins with atrial systole and the completion of ventricular filling. All phases of the cardiac cycle are shortened when heart rate increases.

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