Unoxygenated blood transmitted from the body in veins and carried back to the heart.
Blood found in the veins. Except in the pulmonary vein, venous blood is poor in oxygen and rich in carbon dioxide, which is carried to the right side of the heart for transport to the lungs, where it is given off. (The pulmonary vein carries freshly oxygenated blood from the lungs to the left atrium of the heart.)
Blood circulating in veins. In systemic veins it is dark and poorly oxygenated; in the pulmonary veins it is fully oxygenated.