Anatomy & Physiology

Anatomy & Physiology Study Guide systole and pushes blood into an adjacent chamber or an arterial trunk. Systole is followed by diastole , or relaxation. During diastole, the chamber is filled with blood and arranges for the next cardiac cycle. 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. There is very little backflow into the veins, even though the connections with the venous system lack valves because blood 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 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.

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