Anatomy & Physiology I and II

oxygen demands of the heart, even when it is working at maximum capacity. 19.9 Cardiac Cycle Each heartbeat is followed by a temporary resting phase, allowing time for the chambers to relax and prepare for the next heartbeat. The period between the start of one heartbeat and the beginning of the next is a single cardiac cycle . The cardiac cycle, therefore, includes alternating periods of contraction and relaxation. For any single chamber in the heart, the cardiac cycle can be divided into two phases: (1) systole (contraction) and (2) diastole (relaxation). The chamber contracts during 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

• Step 3 Repolarization : As the plateau continues, slow calcium channels begin closing, and slow potassium channels begin opening. As the channels open, potassium ions (K + ) rush out of the cell, and the net result is a period of rapid repolarization that restores the resting potential. The Refractory Period As with skeletal muscle contractions, for some time after an action potential begins, the membrane will not respond normally to a second stimulus. This time is called the refractory period. Initially, in the absolute refractory period, the membrane cannot respond at all because the sodium channels are already open or are closed and inactivated. In a ventricular muscle cell, the absolute refractory period lasts approximately 200 msec, spanning the duration of the plateau and the initial period of rapid repolarization. The absolute refractory period is followed by a shorter (50 msec) relative refractory period. During this period, the voltage-gated sodium channels are closed but can open. The membrane will respond to a stronger-than-normal stimulus by initiating another action potential. In total, an action potential in a ventricular contractile cell lasts 250–300 msec, roughly 30 times as long as a typical action potential in a skeletal muscle fiber. The Energy for Cardiac Contractions When a normal heart is beating, the energy required is obtained by the mitochondrial breakdown of fatty acids (stored as lipid droplets) and glucose (stored as glycogen). These aerobic reactions can occur only when oxygen is readily available. In addition to obtaining oxygen from the coronary circulation, cardiac muscle cells maintain their own sizable reserves of oxygen. In these cells, oxygen molecules are bound to the heme units of myoglobin molecules. Normally, the combination of circulatory supplies plus myoglobin reserves is enough to meet the

Anatomy & Physiology Study Guide

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