The circulatory system of a pig is much like that of a human, with a four-chambered heart and a system of arteries and veins that carry blood. All vertebrates use a similar system to transport and distribute oxygen molecules to the cells of the body.
All living cells need oxygen. Single-celled animals and small multicellular organisms can draw oxygen directly into their cells from the surrounding environment, but if the body's tissues are more than a few cells thick, the interior cells die without a supply of oxygen. Arteries carry oxygenated blood from the lungs to the tissues of the body. Veins carry deoxygenated blood back to the heart and the lungs where it becomes oxygenated again.