The superior vena cava is a large vein that carries deoxygenated blood from the upper body to the right atrium of the heart. It is located in the chest, just above the heart, and is responsible for returning blood from the head, neck, arms, and upper chest back to the heart. This vein plays a crucial role in the circulatory system by ensuring that oxygen-depleted blood is returned to the heart for reoxygenation.