<Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> 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 to the heart . In contrast to veins, arteries carry blood away from the heart . </P> <P> Veins are less muscular than arteries and are often closer to the skin . There are valves in most veins to prevent backflow . </P> <P> Veins are present throughout the body as tubes that carry blood back to the heart . Veins are classified in a number of ways, including superficial vs. deep, pulmonary vs. systemic, and large vs. small . </P>

Why is it important for veins to have valves