Plate from 18th century encyclopedia showing an illustration of the human circulatory system. In the circulatory system, veins (from the Latin vena) 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 to the heart. Veins differ from arteries in structure and function; for example, arteries are more muscular than veins, veins are often closer to the skin and contain valves to help keep blood flowing toward the heart, while arteries carry blood away from the heart.