The primary function of the heart and blood vessels is to transport oxygen, nutrients, and byproducts of metabolism. Oxygenated and nutrient rich blood is distributed to tissues via the arterial system, which branches into smaller and smaller blood vessels from arteries to arterioles to capillaries (where most exchange occurs). Deoxygenated blood and metabolic byproducts are returned from capillaries via venules and then vein. The heart functions as a pump to maintain circulation. The heart is a discrete organ, which in humans has four distinct chambers. Conceptually, there is the right side of the heart (right atrium and right ventricle) which receive blood returning from the periphery and send it to the lungs (via the pulmonary artery) for re-oxygenation. Once blood is re-oxygenated in the lungs it is returned to the left side of the heart via the pulmonary veins. After entering the left atrium, blood enters the left ventricle and is pumped into the aortic arch for distribution to the entire body. The illustration below provides a schematic representation.