Final answer:
When the pressure in the pulmonary trunk and aorta is greater than in the ventricles, the semilunar valves close to prevent backflow of blood.
Step-by-step explanation:
When the pressure in the pulmonary trunk and aorta exceeds the pressure in the right and left ventricles, respectively, the semilunar valves close. These valves are essential in preventing the backflow of blood into the ventricles after the ventricles contract and begin to relax. The semilunar valves include the pulmonary semilunar valve, between the right ventricle and pulmonary trunk, and the aortic semilunar valve, between the left ventricle and aorta. These valves operate passively, closing when the pressure in the arteries surpasses that in the ventricles, ensuring unidirectional blood flow within the circulatory system.