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Cardiac cycle. Image 4 of 5. Computer artwork of a healthy heart during early ventricular diastole of the cardiac cycle (heartbeat). During this stage both atria (upper chambers, centre left and right) and ventricles (lower chambers, bottom left and right) are relaxed and the atria passively fill with blood. The atrioventricular valves (cream, centre) between the ventricles and the atria are closed, preventing flow between the two. The valve (T-shaped) between the right ventricle and the pulmonary artery (beneath aortic arch, top centre) and between the the left ventricle and aorta (upper centre) are also closed. For a sequence showing the cardiac cycle see P216/544 to P216/548. (KEYSTONE/SCIENCE PHOTO LIBRARY/MEDI-MATION)