Abdominal Vascular System -Anterior View
Veins are part of the circulatory system, along with the heart, arteries and capillaries. Veins are blood vessels that carry blood to the heart. Most veins carry deoxygenated blood from the tissues back to the heart. However, the pulmonary vein and the umbilical vein carry oxygenated blood. The pulmonary vein is carrying blood from the lungs to the heart, and is therefore carrying oxygenated blood.
Once oxygenated blood from the arteries delivers oxygen and nutrients to cells via the capillaries, the now deoxygenated blood enters the venules. Venules converge into larger veins as they collect more blood to be returned to the heart.
Veins differ from arteries in two significant ways. Veins operate at a lower blood pressure, since the blood has lost considerable pressure traveling through the arteries and capillaries. Therefore, veins are less muscular than arteries. Also, many veins must transport blood from the lower body up to the heart. This requires the blood to flow against gravity. To facilitate this upward flow, many veins have one-way valves that prevent blood from flowing back and pooling in the lower extremities.