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 blood into the atria, the pulmonary semilunar valve opens, and the right ventricle expels blood into the pulmonary arteries to the lungs. Likewise, the aortic semilunar valve opens and the left ventricle expels blood into the aorta and out to the rest of the body. After ventricular systole, the cardiac cycle begins again as both the atria and ventricles enter diastole to allow the heart to fill with blood. Normally, this cycle repeats 60 to 100 times a minute. The right-side of the heart produces pulmonary circulation. This is the movement of deoxygenated blood from the body through the right atrium and ventricle, out through the pulmonary artery and to the lungs. Blood oxygenated in the lungs' alveoli returns to the heart through the pulmonary veins. The left side of the heart produces systemic circulation. This is the movement of oxygenated blood returning from the lungs to the left atrium and ventricle and out through the aorta to be distributed to the rest of the body.