During phase 3 of the cardiac cycle, the following events are occurring group of answer choices atria and ventricles are relaxed (diastole); av valves are open while the semilunar valves are closed. atria contract (systole) while the ventricles relax (diastole); av valves are open while the semilunar valves are closed. atria and ventricles are relaxed (diastole); av valves are open while the semilunar valves are closed and blood enters the right atrium through the venae cavae and the left atrium through the pulmonary veins. atria relax (diastole) while the ventricles contract (systole); av valves are closed while the semilunar valves are open. blood enters the right atrium through the venae cavae and the left atrium through the pulmonary veins. quiz