An ARTERY is a blood vessel that conducts blood away from the heart. All arteries have relatively thick walls that can withstand the high pressure of blood ejected from the heart.
A VEIN is a blood vessel that conducts blood toward the heart. Compared to arteries, veins are thin-walled vessels with large and irregular lumens because they are low-pressure vessels, larger veins are commonly equipped with valves that promote the unidirectional flow of blood toward the heart and prevent back-flow toward the capillaries caused by the inherent low blood pressure in veins as well as the pull of gravity.
A CAPILLARY is a microscopic channel that supplies blood to the tissues themselves. Exchange of gases and other substances occurs in the capillaries between the blood and the surrounding cells and their tissue fluid.