Veins are blood vessels that carry deoxygenated blood back to the heart. They are part of the circulatory system, which also includes arteries and capillaries. Veins have thinner walls than arteries and often contain valves that prevent blood from flowing backward. This is important for maintaining proper blood circulation, especially in the limbs where blood must travel against gravity. The largest vein in the body is the vena cava, which returns blood from the body to the heart. Veins can be found throughout the body, including in the arms, legs, and organs. Healthy veins are essential for overall health, as they help ensure that blood circulates efficiently.