Heart section. View of a human heart sectioned to reveal the ventricles and valves. The heart is the organ responsible for pumping blood around the body. Here, the heart's lower chambers, the ventricles are shown. When the right ventricle (triangular, at left) fills with blood, its walls contract to force blood through the pulmonary artery and to the lungs. When walls of the left ventricle (round, at right) contract, blood is forced into the aorta and on to the body. Between the ventricles is the muscular ventricular septum (thick wall). Between the ventricles and the atria (upper chambers, removed) are valves (white) which ensure the direction of blood flow.