<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 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>
where does s1 occur in the cardiac cycle
where does s1 occur in the cardiac cycle
at the beginning of ventricular contraction , or systole
at the beginning of ventricular contraction , or systole