During which phase of the cardiac cycle does the heart fill with blood?

The heart fills with blood during diastole, which is the phase of the cardiac cycle where the heart muscle relaxes after contraction. During this relaxation period, the chambers of the heart expand, allowing blood to flow into the atria from the veins, and then into the ventricles. This filling phase is crucial for ensuring adequate blood volume is available for the next contraction, which is critical for maintaining proper circulation and delivering oxygen to the body.

In contrast, during systole, the heart is actively contracting to pump blood out of the ventricles into the arteries, which is not a filling phase. The terms "resting phase" and "contraction phase" are less precise in describing specific stages of the cardiac cycle and do not accurately represent the phases related to blood filling. This distinction emphasizes the importance of the diastolic phase in the overall function of the heart.