During which phase of the cardiac cycle does the heart relax to allow filling with blood?

• A. Systole phase
• B. Atrial Systole
• C. Diastole phase
• D. Isovolumetric contraction

Answer: (C)

During diastole, the heart relaxes. After a beat, the ventricles loosen up and their pressure falls, so the atrioventricular valves open and blood flows from the atria into the ventricles, allowing filling. Early in this phase there’s rapid filling due to the pressure gradient, and later the atria may contract (atrial systole) to push in the final amount. The other phases involve contraction or a brief period with no change in volume (isovolumetric contraction) or the atria contracting themselves, not the relaxation that enables filling. So, the relaxation phase responsible for filling is diastole.