Describe the cardiac cycle, referring to the valves of the heart.

Deoxygenated blood enters the right atrium of the heart via the superior & inferior vena cavae. The atria contract and the right atrioventricular valve opens, allowing the blood to enter the right ventricle. Once the blood is in the right ventricle, the valve snaps shut in order to prevent back-flow of blood. The ventricles of the heart then contract and the right semi-lunar valve opens, allowing blood to leave the right ventricle and exit the heart via the pulmonary artery. The right semilunar valve then snaps shut to prevent the back-flow of blood. The blood is then oxygenated at the lungs and once this process of oxygenation is complete, the blood returns to the heart via the pulmonary veins to enter the left atria. The left atrium contracts and the left atrioventricular valve opens, allowing blood to enter the left ventricle. The valve then snaps shut to prevent back-flow. Finally, the left ventricle contracts in order to pump the oxygenated blood around the body. It passes through the left semilunar valve and exits via the aorta. The left semilunar valve then shuts. The cycle then starts again