What causes the AV valves to close?

The closure of the atrioventricular (AV) valves is primarily caused by the contraction of the ventricles, which leads to a rise in ventricular pressure. When the ventricles contract during systole, the blood is forcefully pumped out into the aorta and pulmonary artery. The increase in pressure within the ventricles exceeds the pressure in the atria, causing the AV valves (the mitral and tricuspid valves) to close. This mechanism is crucial because it prevents the backflow of blood into the atria during ventricular contraction, ensuring that the blood flows efficiently out of the heart and into circulation.

In contrast, the other options highlight processes that do not directly trigger the closure of the AV valves. For example, while the contraction of the atria does play a role in the overall cardiac cycle, it primarily facilitates the filling of the ventricles and does not contribute to the closure of the AV valves. Decreased ventricular volume would typically occur after the ventricles have expelled their blood, which is after the valves are already closed. Moreover, electrical impulses from the brain are responsible for signaling heart activity but do not directly cause the AV valves to close. Thus, the rise in ventricular pressure due to ventricular contraction is the definitive factor in