w The Cardiac Cycle Piyp。 Ventricular contraction causes the atrioventricular (AV)valves to close,which signals the beginning of ventricular systole.The semilunar valves were closed during the previous diastole and remain closed during this period. Copyright OThe MeGraw Hill Companies,Inc 心脏机械活动由一连串基本相同的心动周期组成心脏机械活动由一连串基本相同的心动周期组成