The series of events that occur within the heart during each heartbeat constitutes the cardiac cycle. Let's list, in order, the changes that occur. AV valves start to bulge back into the atria. Papillary muscles prevent the valves from bulging too far. This forces semilunar valves open.
Blood leaves ventricles.
As ventricular pressure falls, blood starts to flow back toward ventricles, but is stopped by the semilunar valve cusps.