Cariac cycle is the term referring to all or any of the events related to the flow of blood that occur 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 three major stages: atrial systole, ventricular systole and complete cardiac diastole. The term diastole is synonymous with relaxation of a muscle. Throughout the cardiac cycle, the blood pressure increases and decreases. The cardiac cycle is co-ordinated by a series of electrical impulses that are produced by specialised heart cells founds within the sino-atrial node and the atrioventricular node. The cardiac muscle is composed of myocytes which initiate their own contraction without assistance of external nerves (with the exception of modifying the heart rate due to metabolic demand).