Human Anatomy/Angiology/Veins
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Veins are by definition vessels that bring back to the heart, either from the capillaries within the systemic circuit or from the capillaries in the lungs (pulmonary circuit). Veins consist of three layers; the tunica adventitia (or tunica externa) is the outer layer, the tunica media is the middle layer, and the tunica intima is the inner layer. Large veins such as those in the legs contain bicuspid (two leaflets) valves that prevent the back flow of blood. The valves increase in number the further away from the heart.