Related Questions     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.  Learn more about vein with a free
trial on  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