In humans, oxygen-rich blood returns from the lungs to the left atrium, then flows into the left ventricle.When the left ventricle contracts, it pumps oxygenated blood into the aorta, which distributes it to the entire body.Right side → pumps deoxygenated blood to lungsLeft side → pumps oxygenated blood to bodySo, the oxygen-rich blood is pumped out by the contraction of the left ventricle.