Final answer:
After ventricular systole, the semilunar valves close during ventricular diastole, making the 'dub' sound known as S2, which prevents backflow of blood into the heart.
Step-by-step explanation:
Following ventricular systole, the semilunar valves (aortic and pulmonary valves) close once the ventricles begin to relax during ventricular diastole. This closing of the semilunar valves produces the second heart sound, S2 or "dub." This sound occurs because as the pressure in the ventricles drops below that in the aorta and pulmonary trunk, blood begins to flow back toward the ventricles, filling the cusps of the semilunar valves and causing them to snap shut in order to prevent backflow into the heart. This action is part of the cardiac cycle that ensures one-way flow of blood and maintains efficient circulation.