The lungs are the primary organs of the human respiratory system. From the lungs, oxygenated blood is transported by the pulmonary veins towards the left side of the heart. The heart then pumps the oxygenated blood through the aorta, which transports it to the blood vessels and capillaries throughout the body. Capillaries are the small blood vessels where oxygen and carbon dioxide are exchanged in the tissues. Blood becomes deoxygenated as a result of this exchange, and is thus transported back to the heart via the vena cava superior and inferior. The heart then pumps the blood through the pulmonary arteries towards the lungs once again, where it can release the carbon dioxide it has taken up and exchange it for oxygen.