What occurs when the semilunar valves close?

When the semilunar valves close, it leads to the prevention of backflow of blood from the aorta and pulmonary trunk back into the ventricles. These valves close at the end of ventricular systole, which is when the ventricles contract and pump blood into the aorta and pulmonary artery. When the contraction ends and the pressure in the ventricles falls below the pressure in the aorta and pulmonary trunk, the semilunar valves shut tight.

The closure of these valves creates a brief moment where the blood that has just been ejected is prevented from returning to the heart chambers. This closure ensures unidirectional flow of blood, contributing to maintaining pressure in the systemic and pulmonary circulation. Importantly, it also signifies the transition from systole (ventricular contraction) to diastole (ventricular relaxation), thus playing a critical role in the cardiac cycle.

In essence, this action supports the proper flow dynamics necessary for efficient heart function, emphasizing the importance of valve function in cardiac physiology.