Veins are blood vessels that carry deoxygenated blood back to the heart. They are an essential part of the circulatory system, working alongside arteries, which transport oxygen-rich blood away from the heart. Veins have thinner walls than arteries and often contain valves that prevent blood from flowing backward, ensuring it moves in the right direction. The largest vein in the body is the vena cava, which collects blood from the upper and lower parts of the body and returns it to the heart. Healthy veins are crucial for maintaining good circulation and overall health, helping to deliver nutrients and remove waste from the body's tissues.