Final answer:
The semilunar valves close during the early phase of ventricular diastole when the pressure inside the ventricles falls below the pressure in the aorta and pulmonary trunk, preventing backflow of blood into the ventricles.
Step-by-step explanation:
The semi-lunar valves close during the early phase of ventricular diastole when the ventricles relax and the pressure inside them drops below the pressure in the aorta and pulmonary trunk. As blood begins to backflow towards the heart, the increased pressure fills the cusps of the semilunar valves, causing them to close. This action prevents blood from re-entering the ventricles, ensuring a one-way blood flow through the heart and contributing to the efficiency of the cardiovascular system.
Both the aortic valve and the pulmonary valve exhibit this closing mechanism, which can also be detected as an audible sound. The closure of these valves also marks the onset of the isovolumic ventricular relaxation phase, where there is no change in the volume of blood in the ventricles since the atrioventricular valves also remain closed during this time.