The Heart Cardiac Cycle

The cardiac cycle includes all the events--pressure changes, volume changes and valve action--which occur during one complete period of contraction and relaxation of the heart. Since the complete cycle takes place in a period of one second or less, it is no wonder that early physiologists despaired of ever solving its mysteries. Modern methods of recording rapid changes in volume and pressure have, however, permitted a very exact analysis of events.

A description of the cardiac cycle may begin at any point in the cycle. For convenience, we will start with the phase of diastasis, the period during which the whole heart is completely relaxed. Blood is entering the right auricle from the venae cavae and the left auricle from the pulmonary veins. The auriculo-ventricular (A-V) valves which guard the orifice between the auricle and the ventricle on each side are open and the blood which enters the auricles flows freely through into the relaxed ventricles. The valves leading from the ventricles to the pulmonary artery and the aorta are closed, so that none of the blood entering the ventricles is able to leave. The period of diastasis ends abruptly with the onset of systole (contraction) of the auricles. Filling of the ventricles is already virtually complete when auricular systole occurs, so that it is of minor importance so far as ventricular filling is concerned. Almost immediately after auricular systole is completed, ventricular systole begins. Contraction of the ventricular muscle results in a rapid rise in the pressure of the blood in the ventricle. This very quickly exceeds the auricular pressure (which is always low) and causes a sudden closure of the A-V valves on both sides. The vibrations of these valves as they close set up waves which are transmitted to the surface of the chest, where they may be heard as the first heart sound. As the ventricles continue their contraction the pressure exerted on their contained masses of blood rises steeply, but since all the valves of the heart are closed no blood is pumped out. Since the heart muscle cannot shorten during this period, it is referred to as the isometric phase of systole. As soon as the ventricular pressure rises above the pressure in the pulmonary artery and the aorta, the valves guarding these vessels open and blood is rapidly ejected. During the ejection phase, the ventricular muscle is able to shorten (resulting in a decrease in the size of the ventricular cavities) so that it represents an isotonic contraction.

The termination of ventricular systole marks the onset of ventricular diastole, or relaxation. As the ventricular muscle relaxes, intraventricular pressure falls. When it drops below the pressure in the aorta and the pulmonary artery, the valves guarding their orifices close, giving rise to the second heart sound. The intraventricular pressure continues to fall and eventually drops below intra-auricular pressure, resulting in the opening of the A-V valves. As blood begins to pour from the auricles into the ventricles, the period of diastasis, with which this description of the cardiac cycle began, is reached once more.

A typical cardiac cycle consists, then, of three phases: diastasis (resting period), systole (contraction period) and diastole (relaxation period). Most of the filling of the ventricle occurs during the early part of diastole. This is of importance in the adjustment of heart function in exercise, because the increase in heart rate occurs primarily at the expense of the period of diastasis. When the heart rate becomes excessively high, the filling of the ventricle may be cut short, with a resulting decrease in stroke volume.