MEDICAL ANIMATION TRANSCRIPT: The cardiac cycle is the sequence of contraction and relaxation of the heart chambers during a single heartbeat. The contraction of the heart chambers is called systole. And relaxation of the heart chambers is called diastole. The cycle begins with both the atria and ventricles in diastole. Both atrioventricular valves are open while the pulmonary and aortic semilunar valves are closed. Blood flows into the right atrium through the superior and inferior vena cava. Blood flows from the lungs into the left atrium through the pulmonary veins. Then blood moves from both atria into the ventricles through the open atrial ventricular valves. During atrial systole, the atria contract and force any remaining atrial blood into the ventricles. The ventricles are still in diastole, allowing them to expand and completely fill with blood. During ventricular systole, the ventricles contract, the atrioventricular valves close, preventing backflow or regurgitation of blo...