systemic vein, one of a number of veins that drain deoxygenated blood from most of the body, as opposed to the pulmonary circulation. Systemic veins arise in tiny plexuses that receive blood from capillaries and converge into trunks that increase in size as they pass toward the heart. They are larger and more numerous than the arteries, have thinner walls, and collapse when they are empty. Groups of systemic veins include the coronary veins, the superior vena cava and its tributaries in the upper body, and the inferior vena cava and its tributaries in the lower body.
