In the circulatory system, veins are blood vessels that carry blood toward the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood. They differ from arteries in structure and function; for example, arteries are more muscular than veins and they carry blood away from the heart.
Veins serve to return blood from organs to the heart. In systemic circulation oxygenated blood is pumped by the left ventricle through the arteries to the muscles and organs of the body, where its nutrients and gases are exchanged at capillaries, entering the veins filled with cellular waste and carbon dioxide. The de-oxygenated blood is taken by veins to the right atrium of the heart, which transfers the blood to the right ventricle, where it is then pumped through the pulmonary arteries to the lungs. In pulmonary circulation the pulmonary veins return oxygenated blood from the lungs to the left atrium, which empties into the left ventricle, completing the cycle of blood circulation.
Veins are classified in a number of ways, including superficial vs. deep, pulmonary vs. systemic, and large vs. small.
Superficial veins are those whose course is close to the surface of the body, and have no corresponding arteries. Deep veins are deeper in the body and have corresponding arteries. The pulmonary veins are a set of veins that deliver oxygenated blood from the lungs to the heart. Systemic veins drain the tissues of the body and deliver deoxygenated blood to the heart.