Why do the atrioventricular (AV) valves close at the beginning of systole?

The atrioventricular (AV) valves close at the beginning of systole primarily to prevent regurgitation of blood into the atria while the ventricles are contracting. During systole, which is the phase of the cardiac cycle where the heart muscle contracts, the ventricles generate pressure as they contract to pump blood out of the heart—in the case of the left ventricle, into the aorta, and in the case of the right ventricle, into the pulmonary artery.

When the ventricles contract, the pressure within them exceeds the pressure in the atria. The closure of the AV valves (the mitral and tricuspid valves) is essential at this point to create a one-way flow of blood. This closure ensures that the blood being pushed out of the ventricles does not flow back into the atria, which would hinder effective circulation and reduce the heart's efficiency.

Other options, while related to the heart's function, either miss the direct reason for the valve closure or pertain to different aspects of cardiac physiology. For instance, while preventing backflow is mentioned in other options, the specific term "regurgitation" encompasses the physiological process more accurately as it describes the failure to prevent blood from flowing