What event is triggered by rising ventricular pressure?

Rising ventricular pressure is a critical event in the cardiac cycle, particularly during the phase of ventricular contraction known as systole. As the ventricles contract, the pressure generated inside them increases significantly. This increase in pressure serves to ensure that the blood is ejected efficiently from the heart.

When the pressure in the ventricles exceeds that in the atria, it causes the atrioventricular (AV) valves to close. This closure is a protective mechanism that prevents the backflow of blood from the ventricles into the atria during ventricular contraction. The closing of the AV valves is also responsible for the characteristic "lub" sound of the heartbeat.

In contrast, the opening of the semilunar valves, which occurs shortly after the AV valves close, allows blood to be ejected from the ventricles into the aorta and pulmonary artery when the ventricular pressure exceeds the pressure in these arteries. Therefore, it is the rising ventricular pressure that primarily triggers the closing of the AV valves, marking a decisive transition in the cardiac cycle.