<P> As the ventricular systole begins , pressure within the ventricle rises and the AV valves close producing the ' first heart sound ' ( S ) . 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 . </P>
<P> As the ventricular systole begins , pressure within the ventricle rises and the AV valves close producing the ' first heart sound ' ( S ) . 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 . </P>
during which phase of the cardiac cycle does the first heart sound occur
during which phase of the cardiac cycle does the first heart sound occur
isovolumic contraction
isovolumic contraction