Final answer:
Isovolumetric contraction is an initial phase of ventricular systole during which the ventricles contract with no change in blood volume, as the pressure inside them rises but is not high enough to open the semilunar valves, leading to the closure of atrioventricular valves.
Step-by-step explanation:
Isovolumetric Contraction Phase
During the isovolumetric contraction phase of ventricular systole, the ventricles of the heart begin to contract, leading to an increase in pressure within the ventricles. This phase occurs after the atria have finished contracting (atrial systole) and are now in a state of relaxation known as atrial diastole. The pressure within the ventricles rises but is not yet sufficient to open the semilunar valves (pulmonary and aortic valves), which leads to the ejection of blood out of the heart. The rising ventricular pressure also exceeds that of the relaxed atria, causing the atrioventricular valves (tricuspid and mitral valves) to close. As a result, no blood can enter or leave the ventricles, maintaining the volume of blood within them constant, hence the term 'isovolumetric' or 'isovolumic'. This stage is a critical part of the cardiac cycle and sets the stage for the subsequent ventricular ejection phase, where blood is eventually pumped out of the heart.