vein, blood vessel that returns blood to the heart. Except for the pulmonary vein, which carries oxygenated blood from the lungs to the heart, veins carry deoxygenated blood. The oxygen-depleted blood passes from the capillaries to the venules (small veins). The venules feed into larger veins, which eventually merge into the superior and inferior vena cavae, large vessels that consolidate the blood flow from the head, neck, and arms and from the trunk and legs, respectively (see also circulatory system). The vena cavae direct the blood back into the heart. The walls of a vein are formed of three layers like the walls of an artery. However, these layers are thinner and less muscular and collapse when empty. With such notable exceptions as the portal system, most veins contain valves, formed by pouches in their inner coats, that keep the blood from flowing backward. Valves are most numerous in the veins of the extremities, and are absent in the smallest veins. Veins are subject to inflammation, dilatation or enlargement (as in a varicose vein), rupture, and blockage by blood clots (thrombosis).

varicose vein, superficial vessel that is abnormally lengthened, twisted, or dilated, seen most often on the legs and thighs. Varicose veins develop spontaneously, and are usually attributed to a hereditary weakness of the vein; the valves in the vein that keep the blood circulating upward toward the heart are usually incompetent. Increased pressure from long standing or exertion, or internal factors such as pregnancy, or lessened support by the tissues surrounding the veins that occurs with aging and obesity causes the weakened veins to dilate. Mild varicosities often cause no discomfort. Persons with more severe cases may develop swelling of the legs, ankles, and feet, and local eczema or ulcers. Mild varicosities may be treated with rest, elevation of the legs, and the use of elastic bandages or stockings. In severe cases surgical treatment may be necessary. Traditional surgery involves tying off and removing a vein segment. Varicose veins may now also be treated without removing them through an endoscopic surgical procedure that uses the heat produced by radio waves or a laser to seal off the veins. Varicose veins that occur around the rectum are called hemorrhoids, and those that form in the scrotum are called varicoceles.

Vessel that carries blood to the heart. Except for the pulmonary veins, veins bear deoxygenated blood from capillaries, which converge into threadlike venules and then veins, finally emptying into the venae cavae (see cardiovascular system; vena cava). Blood moves through veins by contraction of the surrounding muscles. Backflow is prevented by valves in most veins' inner layer (tunica intima), which lacks the elastic membrane lining of arteries. The thin middle layer (tunica media) is mostly collagen fibres, and the thick outer layer (tunica adventitia) is mostly connective tissue. Seealso circulation; varicose vein.

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or varix

Twisted vein distended with blood. Varix also covers arteries and lymphatic vessels (see lymphatic system). Varicose veins occur mostly in the legs, when malfunctioning valves let blood pool in veins near the skin. Causes include hereditary valve and vein wall weakness and internal or external pressure on veins. Varices are common in pregnancy, suggesting that hormone abnormalities play a role. Symptoms include a heavy feeling, with leg cramps and swelling after standing a long time. Complications include skin ulcers and thrombosis. Treatment involves strong support hose, injection therapy, or surgery. Varices in the esophagus, which often occur in liver disease, can ulcerate and bleed. Seealso hemorrhoid.

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