a vessel through which blood passes from various organs or parts back to the heart; all veins except the pulmonary veins carry blood low in oxygen. Like arteries, veins have three coats, an inner, middle, and outer, but the coats are not so thick, and they collapse when the vessel is cut. Many veins have valves formed of reduplications of their lining membrane, which prevent the backward flow of blood away from the heart. Called also vena[TA].

  The three coats of a vein: (A), tunica intima (endothelium); (B), tunica media; (C), tunica externa.


accompanying vein, vena comitans.
accompanying vein of hypoglossal nerve, vena comitans nervi hypoglossi.
afferent veins, veins that carry blood to an organ.
allantoic veins, paired vessels arising from the dorsal aorta that accompany the allantois; they enter the body stalk of the early embryo with the allantois and later form the umbilical veins.
anastomotic vein, inferior, vena anastomotica inferior.
anastomotic vein, superior, vena anastomotica superior.
angular vein, vena angularis.
antebrachial vein, median, vena mediana antebrachii.