In which phase of the cardiac cycle do ventricles become filled with blood?

During diastole, the ventricles of the heart experience a phase where they become filled with blood. This phase follows systole, during which the ventricles contract to pump blood out of the heart. In diastole, the heart muscle relaxes, allowing the chambers of the heart, including the ventricles, to expand and fill with blood. Blood flows from the atria into the ventricles due to the pressure gradient created by the filling of the atria and the relaxation of the ventricular myocardium.

Atrial contraction typically occurs at the end of diastole, further aiding in the filling of the ventricles, but the main filling occurs during the entire diastolic phase. Isovolumetric contraction occurs during systole and involves the ventricles contracting with no change in volume, as all heart valves are closed. Thus, the unique characteristic of diastole as the period in which the ventricles fill with blood distinguishes it from the other phases or actions within the cardiac cycle.