<Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (August 2012) (Learn how and when to remove this template message) </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 second . </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 traverse the cardiac cycle continuously, (see circular 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 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 circular 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>

The sequence of contraction of the heart chambers is
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