Vessel that carries blood to the heart.
Except for the pulmonary veins, veins bear deoxygenated blood from 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 {{link=collagen">collagen fibres, and the thick outer layer (tunica adventitia) is mostly connective tissue . See also circulation ; varicose vein .