Biology Spl

The cardiac cycle refers to a complete heartbeat from its generation to the beginning of the next beat, and so includes the diastole, the systole, and the intervening pause. The frequency of the cardiac cycle is described by the heart rate, which is typically expressed as beats per minute.At a normal heart rate of 75 beats per minute, one cardiac cycle lasts 0.8 seconds. Under resting conditions, systole occupies 1/3 and diastole 2/3 of the cardiac cycle duration.During systole, atrioventricular valves are closed and semilunar valves are open. In diastole the opposite is true. Atrioventricular valves are open to allow ventricular filling and semilunar valves are closed to prevent backflow of blood to the heart from either systemic circulation or pulmonary circulation.