during which phase of the cardiac cycle does the first heart sound occur
Cardiac cycle As the ventricular systole begins, pressure within the ventricle rises and the AV valves close producing the 'first heart sound' (S1). The semilunar valves remain closed. The contracting ventricles become closed chambers again and this phase is termed as "isovolumic contraction". As the name implies, there is no change in volume, but intra-ventricular pressure rises. The outflow phase, "ventricular ejection," is when the intra-ventricular pressure has achieved a higher pressure than the blood in the aorta (or the pulmonary trunk), the corresponding semilunar valves open. Ejection phase begins.[2][3]