Which phase completes ventricular filling aided by atrial contraction?

• A. Isovolumetric relaxation
• B. Ventricular ejection
• C. Atrial systole
• D. Isovolumetric contraction

Answer: (C)

Understanding how ventricular filling is completed hinges on the atrial contraction that follows passive filling. As the ventricles relax and fill, blood moves from the atria into the ventricles through open AV valves. Right at the end of this filling, the atria contract (atrial systole), pushing the final amount of blood into the ventricles—the atrial kick. This completes ventricular filling and increases the end-diastolic volume, setting the preload for the next heartbeat.

The other phases don’t complete filling: isovolumetric relaxation happens after the ventricles finish ejecting blood, with all valves closed so the volume doesn’t change; ventricular ejection is when blood is pumped out of the ventricles; isovolumetric contraction is the start of systole when the ventricles contract but all valves are closed, so again no change in volume.