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Cardiac cycle. Image 1 of 5. Computer artwork of a healthy heart during atrial systole of the cardiac cycle (heartbeat). Blood enters the atria (upper chambers, centre left and right) from the vena cava (top and bottom left) and pulmonary veins (centre right). During atrial systole both atria contract, pushing blood into the ventricles (lower chambers, bottom centre and right). To reach the ventricles the blood passes through the open atrioventricular valves (cream). The valve (T-shaped) between the right ventricle and the pulmonary artery (beneath aortic arch, top centre) is closed at this stage. For a sequence showing the cardiac cycle see P216/544 to P216/548. (KEYSTONE/SCIENCE PHOTO LIBRARY/MEDI-MATION)