To understand isovolumic diastole, we must first understand isovolumic systole, one of the phases of blood flow and circulation within the heart. During isovolumic systole, the intraventricular pressure is high, the volume is relatively small, and the aortic and atrioventricular valves are closed, so the amount of blood in the ventricle is basically unchanged, or isovolumic. The isovolumic diastole is the end of the isovolumic systole, when the aortic and atrioventricular valves originally closed open and blood flows from the ventricles into the arteries, while the blood in the arteries is pushed by the pressure causing the arterial valves to close, which serves to prevent blood reflux. This opening period is brief, when the intraventricular pressure becomes low and the volume becomes large. When this opening period is over, the systolic period is over again, so that the systolic and diastolic periods form a good balance of blood flow between the body and pulmonary circulations, and the heartbeat can be felt with the hand when the diastolic period is equal to the capacity.