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[L. vena, vein]
A vessel carrying blood toward the heart. Most veins originate in capillaries and drain into increasingly larger veins until their blood is delivered to the right atrium of the heart. Portal veins also originate in capillaries, but their branches decrease in size to pass through another set of capillaries before joining more typical veins on their way toward the heart. For all veins, the precursor veins that empty into a secondary vein are called tributaries of the secondary vein.
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The deep veins typically accompany arteries, and artery and vein have the same name, e.g., radial artery and radial vein. In contrast, the superficial or subcutaneous veins typically travel alone. The deep and superficial veins have frequent interconnections, i.e., anastomoses; in general, veins have more anastomoses than do arteries. Throughout the body, the pattern of veins is more variable than is the pattern of arteries.

The walls of most veins have three layers: an endothelial lining, which folds at intervals into leaflets that act as one-way valves, a thin middle layer often containing smooth muscle, and a thin outer layer composed of fibrous connective tissue. In contrast, the veins of the brain have no valves, and the largest venous channels in the brain are not veins but rather endothelial-lined spaces in the dura mater called 'dural sinuses' or 'venous sinuses'; dural sinuses receive blood from cerebral veins and deliver blood to other dural sinuses or to the internal jugular vein.

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