Oxygen-depleted blood filled with carbon dioxide exits the organs and ends up in the superior or inferior vena cava. This blood is transported to the right atrium of the heart, pushed to the right ventricle, and ends up in the lungs via the pulmonary artery. Once it’s in the lungs, the carbon dioxide is removed from the blood and eventually exhaled out of the body. During the gas exchange process, the lungs add oxygen to the blood once again. The blood is transported from the lungs to the left atrium of the heart via the pulmonary vein, and pushed to the left ventricle where the oxygen-rich blood is pumped to the organs via the aorta. The cardiac cycle can be divided into two parts: systole and diastole. During systole, the left and right ventricles are filled with blood and contract to push blood to the lungs and the rest of the body. During diastole, the heart relaxes and refills with blood, additionally making sure that the heart gets enough oxygen through the coronary arteries on its surface.