The sounds of the heartbeat heard through a stethoscope are made by the valves closing due to blood pressure.The two sounds of the heartbeat are described as lub-dub.The first 'lub' is made when the blood is forced against the atrio ventricular valves as the ventricles contract.The second sound comes as a backflow of blood closes the semilunar valves in the aorta and pulmonary artery as the ventricles relax.