The union of veins from legs and trunks form the
inferior vena cava
The inferior vena cava, also referred to as the posterior vena cava, is a large vein that carries de-oxygenated blood from the lower body to the heart. De-oxygenated blood means most of the oxygen has diffused into tissues. Deoxygenated blood, therefore, the appear darker. The inferior vena cava empties into the right atrium of the heart.