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Cardiac cycle. Image 2 of 5. Computer artwork of a healthy heart during early ventricular systole of the cardiac cycle (heartbeat), when the ventricles (lower chambers, bottom centre and right) contract. During this stage the atrioventricular valves (cream, centre) between the ventricles and the atria (upper chambers, centre left and right) are shut, preventing the backflow of blood into the atria. The valve (T-shaped) between the right ventricle and the pulmonary artery (beneath aortic arch, top centre) and the valve between the left ventricle and aorta (upper centre) are also closed at this early stage. For a sequence showing the cardiac cycle see P216/544 to P216/548. (KEYSTONE/SCIENCE PHOTO LIBRARY/MEDI-MATION)