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. In contrast to veins,
arteries carry blood away from the heart. Veins are less muscular than arteries and are often closer to the skin. There are valves in most veins to prevent
backflow.