Large veins are blood vessels that carry deoxygenated blood back to the heart. They are part of the circulatory system and are essential for maintaining proper blood flow. The largest vein in the body is the superior vena cava, which collects blood from the upper body, while the inferior vena cava gathers blood from the lower body. These veins have thinner walls compared to arteries and often contain valves that prevent blood from flowing backward. Large veins are crucial for returning blood to the heart, where it can be re-oxygenated and pumped back out to the body.