THE CARDIAC CYCLE
Cardiac cycle is the term referring to all or any of the events related to the flow or pressure of blood that occurs from the beginning of one heartbeat to the beginning of the next. The frequency of the cardiac cycle is the heart rate. Every single 'beat' of the heart involves five major stages: First, "Late diastole" which is when the semi lunar valves close, the Av valves open and the whole heart is relaxed. Second, "Atrial systole" when atria is contracting, AV valves open and blood flows from atrium to the ventricle. Third, "Isovolumic ventricular contraction" it is when the ventricles begin to contract, AV valves close, as well as the semi lunar valves and there is no change in volume. Fourth, "ventricular ejection", Ventricles are empty, they are still contracting and the semi lunar valves are open. The fifth stage is: "Isovolumic ventricular relaxation", Pressure decreases, no blood is entering the ventricles, ventricles stop contracting and begin to relax, semilunars are shut because blood in the aorta is pushing them shut. Throughout the cardiac cycle, the 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 sino-atrial 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.