What happens during atrial systole?

• A. Atrial contraction pushes blood into relaxed ventricles.
• B. Ventricles fill during atrial systole.
• C. AV valves close.
• D. Semilunar valves open.

Answer: (A)

Atrial systole is the phase when the atria actively contract to top off the ventricles with blood. Because the ventricles are still relaxed and their pressure is low, the atrioventricular (AV) valves are open, allowing blood to flow from the atria into the ventricles. This adds the final portion of ventricular filling, often called the atrial kick, increasing the end-diastolic volume. The semilunar valves stay closed during this time because ventricular pressure has not yet risen above arterial pressure to push blood into the aorta or pulmonary artery.