In which phase of the heartbeat does the heart primarily fill with blood?

In which phase of the heartbeat does the heart primarily fill with blood?

• A. Systole
• B. Diastole
• C. Atrial systole
• D. Isovolumetric relaxation

Answer: (B)

During diastole, the heart relaxes and the ventricles fill. The atrioventricular valves are open, allowing blood to flow from the atria into the ventricles. Most ventricular filling occurs passively during early diastole as pressures balance. Later in diastole, the atria contract to push in any remaining blood, but this atrial kick is a smaller contribution compared with the initial filling that happens as the ventricles relax. The other phases involve contraction or no filling: systole is when the heart pumps blood out, and isovolumetric relaxation occurs with all valves closed, so no blood enters the ventricles.

During diastole, the heart relaxes and the ventricles fill. The atrioventricular valves are open, allowing blood to flow from the atria into the ventricles. Most ventricular filling occurs passively during early diastole as pressures balance. Later in diastole, the atria contract to push in any remaining blood, but this atrial kick is a smaller contribution compared with the initial filling that happens as the ventricles relax. The other phases involve contraction or no filling: systole is when the heart pumps blood out, and isovolumetric relaxation occurs with all valves closed, so no blood enters the ventricles.