The veins are vessel than carry bloody troward the heart. The superior vena cava, the inferior vena cava and the coronary vein are conected on the right atrium. The veins are vessel comprised of three layer called tunica adventitia, tunica media and tunica intima. More externay is the tunica adventitia composed of conective tissue. Some large caliber veins have small vessel in this layer, what we called vasa vasorum. The intermediary layer is called the tunica media, composed of connective tissuem and smooth muscle. Unlike the arterial layer, the vein tunica media is thin. The tunica intima is in direct contact with blood and has anticoagulanti and bactericide property. The vains have valve then help carry blood against gravity. Not every the vein carry venous blood. The pulmonary vein carry arterial blood back to the heart in the small circulation.