During which phase do the ventricles relax and fill with blood?

The correct choice is diastole, which is the phase of the cardiac cycle characterized by the relaxation of the heart muscle, particularly the ventricles. During diastole, the ventricles open their valves, allowing blood to flow in from the atria. This passive filling occurs as the pressure in the ventricles drops below that in the atria, promoting blood flow.

Diastole is essential for ensuring that the ventricles have enough blood volume to pump effectively during the subsequent contraction phase, known as systole. The timing of diastole is crucial for maintaining efficient circulation, as it allows for the essential filling of the ventricles in preparation for the next heartbeat. Understanding this phase is important for grasping how the heart functions effectively within the cardiac cycle.