Diastole is the phase of the cardiac cycle characterized by which event?

• A. Ventricular relaxation
• B. Ventricular contraction
• C. Atrial contraction
• D. Valve opening

Answer: (A)

Diastole is the phase when the heart muscle relaxes, especially the ventricles, allowing them to fill with blood from the atria. That relaxation and subsequent filling define this phase, so ventricular relaxation is the best way to describe what diastole is. During this time the atrioventricular valves are open to let blood into the ventricles, while the semilunar valves stay closed to prevent backflow from the arteries. Ventricular contraction happens in the opposite phase, called systole, and atrial contraction occurs later in diastole as a final push to finish filling. Valve opening by itself isn’t the defining aspect; the key idea is the ventricles relaxing and filling.