As the heart beats, it pumps blood through a system of blood vessels, called the circulatory system. The vessels are flexible, hollow tubes that carry blood to every part of the body.
- Arteries carry oxygen-rich blood away from the heart.
- Veins return oxygen-poor blood back to the heart.
Veins are flexible, hollow tubes with flaps inside called valves. When your muscles contract, the valves open and allow blood to move through the veins. When your muscles relax, the valves close, keeping blood flowing in one direction through the veins. Veins become larger and larger as they get closer to the heart. The superior vena cava is the large vein that brings blood from the head and arms to the heart, and the inferior vena cava brings blood from the abdomen and legs into the heart.
If the valves inside your veins become damaged as a result of venous disease, the valves may not close completely, allowing blood to leak backward or flow in both directions.