<P> The first heart sound, or S, forms the "lub" of "lub - dub" and is composed of components M (mitral valve closure) and T (tricuspid valve closure). Normally M precedes T slightly . It is caused by the closure of the atrioventricular valves, i.e. tricuspid and mitral (bicuspid), at the beginning of ventricular contraction, or systole . When the ventricles begin to contract, so do the papillary muscles in each ventricle . The papillary muscles are attached to the cusps or leaflets of the tricuspid and mitral valves via chordae tendineae (heart strings). When the papillary muscles contract, the chordae tendineae become tense and thereby prevent the backflow of blood into the lower pressure environment of the atria . The chordae tendineae act a bit like the strings on a parachute, and allow the leaflets of the valve to balloon up into the atria slightly, but not so much as to evert the cusp edges and allow back flow of blood . It is the pressure created from ventricular contraction that closes the valve, not the papillary muscles themselves . The contraction of the ventricle begins just prior to AV valves closing and prior to the semilunar valves opening . The sudden tensing of the chordae tendineae and the squeezing of the ventricles against closed semilunar valves, sends blood rushing back toward the atria, and the parachute - like valves catch the rush of blood in their leaflets causing the valve to snap shut . The S1 sound results from reverberation within the blood associated with the sudden block of flow reversal by the valves . The delay of T1 even more than normally causes the split S1 which is heard in a right bundle branch blockage . </P> <P> The second heart sound, or S, forms the "dub" of "lub - dub" and is composed of components A (aortic valve closure) and P (pulmonary valve closure). Normally A precedes P especially during inspiration where a split of S can be heard . It is caused by the closure of the semilunar valves (the aortic valve and pulmonary valve) at the end of ventricular systole and the beginning of ventricular diastole . As the left ventricle empties, its pressure falls below the pressure in the aorta . Aortic blood flow quickly reverses back toward the left ventricle, catching the pocket - like cusps of the aortic valve, and is stopped by aortic valve closure . Similarly, as the pressure in the right ventricle falls below the pressure in the pulmonary artery, the pulmonary valve closes . The S sound results from reverberation within the blood associated with the sudden block of flow reversal . </P> <P> Splitting of S2, also known as physiological split, normally occurs during inhalation because the decrease in intrathoracic pressure increases the time needed for pulmonary pressure to exceed that of the right ventricular pressure . A widely split S2 can be associated with several different cardiovascular conditions, including left bundle branch block, pulmonary stenosis, and atrial septal defect . </P> <P> The rarer extra heart sounds form gallop rhythms and are heard in both normal and abnormal situations . </P>

When is the second sound of the heartbeat produced