Heart, illustration. The heart is a hollow muscle that pumps blood around the body. The vena cava (top left) returns deoxygenated blood from the body to the heart's right atrium (chamber, upper left). The blood flows into the right ventricle (chamber, lower centre), which contracts to push the blood through the pulmonary artery (upper centre) to the lungs. From the lungs, newly oxygenated blood enters the left atrium (chamber, upper right) and flows to the left ventricle (chamber, lower right). The left ventricle contracts to push the blood to the aorta (arched, top centre) and around the body.