Anatomy & Physiology I and II

same time. Ventricular systole lasts 270 msec. During this period, blood is pushed through the systemic and pulmonary circuits and toward the atria. The heart then 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 are relaxed. The next cardiac cycle begins with atrial systole and the completion of ventricular filling. As the heart rate increases, all the phases of the cardiac cycle are shortened. The greatest reduction occurs in the length of time spent in diastole. When the heart rate climbs from 75 bpm to 200 bpm, the time spent in systole drops by less than 40 percent, but the duration of diastole is reduced by almost 75 percent. 19.10 Heart Sounds Listening to the heart, a technique called auscultation , is a simple and effective method of cardiac assessment. Clinicians use an instrument called a stethoscope to listen for normal and abnormal heart sounds. Where the stethoscope is placed depends on which valve is under examination. Valve sounds must pass through the pericardium, surrounding tissues, and the chest wall, and some tissues muffle sounds more than others. As a result, the placement of the stethoscope differs somewhat from the position of the valve under review. There are four heart sounds, designated as S1 through S4. If you listen to your own heart with a stethoscope, you will hear the first and second heart sounds. These sounds accompany the closing of your heart valves. The first heart sound, known as “lubb” (S1), lasts a little longer than the second, called “dubb” (S2). S1, which marks the start of ventricular contraction, is produced as the AV valves close; S2 occurs at the beginning of ventricular filling when the semilunar valves close. Third and fourth heart sounds are usually

very faint and seldom are audible in healthy adults. These sounds are associated with blood flowing into the ventricles (S3) and atrial contraction (S4), rather than with valve action. The surges, swirls, and eddies that accompany regurgitation create a rushing, gurgling sound known as a heart murmur . Minor heart murmurs are common and inconsequential. When considering cardiac function over time, physicians are most interested in the cardiac output (CO) , the amount of blood pumped by the left ventricle in one minute. In essence, cardiac output is an indication of the blood flow through peripheral tissues—without adequate blood flow, homeostasis cannot be maintained. The cardiac output provides a useful indication of ventricular efficiency over time. The body precisely adjusts cardiac output such that peripheral tissues receive an adequate circulatory supply under a variety of conditions. When necessary, the heart rate can increase by 250 percent, and stroke volume in a normal heart can almost double. The Atrial Reflex The atrial reflex, or Bainbridge reflex, involves adjustments in heart rate in response to an increase in the venous return. When the walls of the right atrium are stretched, the stimulation of stretch receptors in the atrial walls triggers a reflexive increase in heart rate caused by increased sympathetic activity. Thus, when the rate of venous return to the heart increases, the heart rate, and hence the cardiac output, rises as well. Venous Return In addition to its indirect effect on heart rate via the atrial reflex, venous return also has direct effects on nodal cells. When venous return increases, the atria receive more blood, and the walls are stretched. The stretching of the cells of

Anatomy & Physiology Study Guide

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