Cardiac Cycle

The cardiac cycle is a term referring to all or any of the events related to the flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next. The frequency of the cardiac cycle is described by the heart rate. Each beat of the heart involves five major stages. The first two stages, often considered together as the "ventricular filling" stage, involve the movement of blood from atria into ventricles. The next three stages involve the movement of blood from the ventricles to the pulmonary artery (in the case of the right ventricle) and the aorta (in the case of the left ventricle).

The first, "early diastole," is when the semilunar valves close, the atrioventricular (AV) valves are open, and the whole heart is relaxed. The second, "atrial systole," is when the atrium contracts, 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.

Throughout the cardiac cycle, blood pressure increases and decreases. The cardiac cycle is coordinated by a series of electrical impulses that are produced by specialized heart cells found within the sinoatrial node and the atrioventricular node. The cardiac muscle is composed of myocytes which initiate their own contraction without help of external nerves (with the exception of modifying the heart rate due to metabolic demand). Under normal circumstances, each cycle takes approximately one second.