Cardiac cycle is the all events related to the flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next.Each beat of the heart involves five major stages: The first, "late diastole", is when the semilunar valves close, the atrioventricular (AV) valves open, and the whole heart is relaxed. The second, "atrial systole", is when the atrium contracts, the AV valves open, and blood flows from atrium to the ventricle.The third, "isovolumic ventricular contraction", is when the ventricles begin to contract, the AV and semilunar valves close, and there is no change in volume. The fourth, "ventricular ejection", is when the ventricles are empty and contracting, and the semilunar valves are open. During the fifth stage, "Isovolumic ventricular relaxation", pressure decreases, no blood enters the ventricles, the ventricles stop contracting and begin to relax, and the semilunar valves close due to the pressure of blood in the aorta.
The cardiac cycle is coordinated by a series of electrical impulses that are produced by specialized heart cells found within the sino-atrial node and the atrioventricular node.
Atrial systole is the contraction of the myocardia of the left and right atria.Normally, both atria contract at the same time.70% of the blood flows passively down to the ventricles, so the atria do not have to contract a great amount.Electrical systole of the atria begins with the onset of the P wave on the ECG.Ventricular systole is the contraction of the myocardia of the left and right ventricles.Cardiac Diastole is the period of time when the heart relaxes after contraction in preparation for refilling with circulating blood. Ventricular diastole is when the ventricles are relaxing, while atrial diastole is when the atria are relaxing.Together they are known as complete cardiac diastole.