How does it come back ?
Blood comes back to the heart via the veins. The small veins join together to form larger veins, which in turn join to form the two largest veins, the Superior (higher) Vena Cava and the Infer (lower) Vena Cava. The blood returns to the heart via these huge veins. This blood is poor in oxygen. The right atrium accepts this blood and sends it to the right ventricle, which sends it to the lungs for oxygenation (saturation of blood with oxygen).