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