What happens during atrial systole?

During atrial systole, the atria contract, which leads to the filling of the ventricles with blood. This phase occurs just before the ventricles contract and is crucial for ensuring that the maximum amount of blood is delivered to the ventricles.

As the atria contract, they push blood through the open atrioventricular (AV) valves (the mitral and tricuspid valves) into the ventricles. This process enhances ventricular filling and contributes to efficient heart function.

In the context of other options, the AV valves remain open during atrial systole, and it is during ventricular systole — not atrial systole — that the ventricles contract and the semilunar valves (aortic and pulmonary valves) open to allow blood to exit the heart. Thus, the filling of the ventricles with blood during atrial systole is an essential step in the cardiac cycle, setting the stage for effective circulation.