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

The correct answer is diastole, which is the phase of the cardiac cycle when the heart fills with blood. During diastole, the heart muscles relax after contraction, allowing the chambers of the heart—specifically the atria and ventricles—to expand and fill with blood.

In this phase, the atrioventricular (AV) valves are open, and blood flows from the atria into the ventricles. This is crucial for ensuring that there is enough blood in the ventricles before the next contraction during systole, which is when the heart pumps blood out to the body and lungs.

Understanding this phase is fundamental in cardiology, as it plays a significant role in the effectiveness of cardiac function and the overall circulation of blood throughout the body. Proper filling of the heart chambers during diastole is essential for maintaining adequate stroke volume and cardiac output during subsequent contractions.