In which phase does isovolumetric contraction occur?

Isovolumetric contraction occurs during ventricular systole, which is the phase of the cardiac cycle when the ventricles contract. During this stage, the pressure within the ventricles rises significantly as the cardiac muscle fibers contract, but the volume of blood in the ventricles remains constant because both the atrioventricular (AV) valves and the semilunar valves are closed. This ensures that no blood enters or leaves the ventricles during this phase, allowing the pressure to build up sufficiently to eventually open the semilunar valves and eject blood into the aorta and pulmonary artery.

In this context, the other phases listed do not involve the isovolumetric contraction. During diastole, the heart is relaxed, and the chambers are filling with blood. Atrial systole refers to the contraction of the atria, which occurs after isovolumetric contraction and aids in filling the ventricles. Ventricular filling is the phase when blood enters the ventricles, also not involving a change in volume while the valves are closed. Thus, the unique characteristics of the isovolumetric contraction phase—where the volume of blood in the ventricles does not change despite significant pressure increase—make it a distinct part of ventricular systole.