1. Closure of semilunar valves produces the second heart sound, S2.
2. For a moment, however, the semilunar valves remain closed, and the ventricles contract within a closed space.
3. Similarly, semilunar valves open when ventricular pressures are higher than aortic/pulmonary pressures, and close when the reverse is true.
4. When ventricular pressures drop below aortic and pulmonary pressures, the semilunar valves close, marking the end of systole and beginning of diastole.