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A vein is a blood vessel that carries blood toward the heart. Compared with arteries, veins have thinner walls and larger, more irregular lumens. Because they are low-pressure vessels, larger veins are equipped with valves that ensure the unidirectional flow of blood toward the heart and prevent backflow toward the capillaries. This valve system helps counteract both the low pressure within veins and the effects of gravity. An artery is a blood vessel that carries blood away from the heart. Arteries have relatively thick walls that can withstand the high pressure of blood pumped from the heart. Arteries closest to the heart have the thickest walls, with a high concentration of elastic fibers across all three of their layers (tunics). *LICENSES AND ATTRIBUTIONS CC licensed content, Shared previously, Structure and Function of Blood Vessels, Authored by: OpenStax College. License: CC BY: Attribution. License Terms: Download for free at http://cnx.org/contents/WNsszrPZ@4/Structure-and-Function-of-Bloo*
