Vein

A vessel that returns blood towards the heart from the various organs and tissues of the body. The walls of veins, like those of arteries, consist of a smooth inner lining, a muscular middle layer, and a fibrous outer covering. However, blood pressure in veins is lower than in arteries, and the walls of veins are thinner, less elastic, less muscular, and weaker than those of arteries. The linings of many veins contain folds, which act as valves, ensuring that blood flows only towards the heart. Blood is helped on its way through the veins by pressure on the vessel walls from the contraction of surrounding muscles. (See also circulatory system.)