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vein
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.
==Structure==
In general, veins function to return deoxygenated blood to the heart, and are essentially tubes that collapse when their lumens are not filled with blood. The thick outermost layer of a vein is made of connective tissue, called tunica adventitia or tunica externa. There is a middle layer bands of smooth muscle called tunica media, which are, in general, thin, as veins do not function primarily in a contractile manner. The interior is lined with endothelial cells called tunica intima. The precise location of veins varies much more from person to person than that of arteries. Veins often display a lot of anatomical variation compared with arteries within a species and between species.
Most veins are equipped with valves to prevent regurgitation (reverse blood flow). The valves were described by Jacques Dubois, but their true function was later discovered by William Harvey.〔Power, D'Arcy, ''William Harvey'', in ''Masters of Medicine' series, pub. T. Fisher Unwin, London, 1897〕
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