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vein
(vān)
a vessel through which blood passes from various organs or parts back to the heart, in the systemic circulation carrying blood
that has given up most of its oxygen. Veins, like arteries, have three coats, an inner coat (tunica intima), middle coat (tunica media), and outer coat (tunica externa); however, in veins these are less thick and collapse when the vessel is cut. Many veins, especially superficial ones, have
valves formed of reduplication of their lining membrane.
The three coats of a vein: (A), tunica intima (endothelium); (B), tunica media; (C), tunica externa.
Copyright 2007. An Elsevier publication. All rights reserved.
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