<Tr> <Td_colspan="2"> Animalia with the exception of Porifera, Cnidaria, Ctenophora, Platyhelminthes, Bryozoan, Amphioxus . </Td> </Tr> <P> The cardiac cycle is the performance of the human heart from the beginning of one heartbeat to the beginning of the next . It consists of two periods: one during which the heart muscle relaxes and refills with blood, called diastole (die - ASS - toe - lee), followed by a period of robust contraction and pumping of blood, dubbed systole (SIS - toe - lee). After emptying, the heart immediately relaxes and expands to receive another influx of blood returning from the lungs and other systems of the body, before again contracting to pump blood to the lungs and those systems . A normally performing heart must be fully expanded before it can efficiently pump again . Assuming a healthy heart and a typical rate of 70 to 75 beats per minute, each cardiac cycle, or heartbeat, takes about 0.8 seconds to complete the cycle . </P> <P> There are two atrial and two ventricle chambers of the heart; they are paired as the left heart and the right heart--that is, the left atrium with the left ventricle, the right atrium with the right ventricle--and they work in concert to repeat the cardiac cycle continuously, (see cycle diagram at right margin). At the "Start" of the cycle, during ventricular diastole--early, the heart relaxes and expands while receiving blood into both ventricles through both atria; then, near the end of ventricular diastole--late, the two atria begin to contract (atrial systole), and each atrium pumps blood into the ventricle' below' it . During ventricular systole the ventricles are contracting and vigorously pulsing (or ejecting) two separated blood supplies from the heart--one to the lungs and one to all other body organs and systems--while the two atria are relaxed (atrial diastole). This precise coordination ensures that blood is efficiently collected and circulated throughout the body . </P> <P> The mitral and tricuspid valves, also known as the atrioventricular, or AV valves, open during ventricular diastole to permit filling . Late in the filling period the atria begin to contract (atrial systole) forcing a final crop of blood into the ventricles under pressure--see cycle diagram . Then, prompted by electrical signals from the sinoatrial node, the ventricles start contracting (ventricular systole), and as back - pressure against them increases the AV valves are forced to close, which stops the blood volumes in the ventricles from flowing in or out; this is known as the isovolumic contraction stage . </P>

What is the relaxation phase of the cardiac cycle called