<P> Throughout the cardiac cycle, blood pressure increases and decreases . The movements of cardiac muscle are coordinated by a series of electrical impulses produced by specialised pacemaker cells found within the sinoatrial node and the atrioventricular node . Cardiac muscle is composed of myocytes which initiate their internal contractions without applying to external nerves--with the exception of changes in the heart rate due to metabolic demand . </P> <P> The cardiac cycle involves four major stages of activity: 1) "Isovolumic relaxation", 2) Inflow, 3) "Isovolumic contraction", 4) "Ejection". (See Wiggers diagram, which, label-wise, presents the stages as 3, 4, 1, 2, left - to - right .) </P> <P> Stages 1 (Isovolumic relaxation) and 2 (Inflow: "Rapid inflow", "Diastasis", and "Atrial systole") together complete the ventricular "Diastole" period, during which blood returning to the heart flows through the atria into the ventricles . Near the end of the "Diastole", both atria pump blood into the ventricles during atrial systole, aka atrial kick . Stages 3 and 4 are the ventricular "Systole" period, which is the simultaneous pumping of separate blood supplies from the two ventricles--one to the pulmonary artery and one to the aorta . </P> <P> The time-wise increases / decreases of the heart's blood volume (see Wiggers diagram), are instructive to follow . The red - line tracing of "Ventricular volume" provides an excellent track of the two periods and four stages of one cardiac cycle . Start with the Systole (see the vertical bar marked "A - V valve closes") at the short - time plateau of Isovolumic contraction stage, then to the rapid loss of blood volume (or the vertical drop of the red - line tracing) during the Ejection stage . Now the Diastole period: the low plateau of Isovolumic relaxation stage, followed by the rapid rise, then two slower rises, of the Inflow stage, returning to the plateau of Isovolumic contraction stage . </P>

The cardiac event that completes ventricular filling is referred to as