Heart Sounds During Cardiac Cycle
Four heart sounds are produced during a cardiac cycle:
First heart sound (S1)
Second heart sound (S2)
Third heart sound (S3)
Fourth heart sound (S4)
First and second heart sounds...
are classical or major or physiological heart sounds.
can be heard normally by stethoscope.
Third and fourth heart sounds...
cannot be heard with a stethoscope under normal conditions.
First Heart Sound (S1)
Coincides with closure of atrioventricular valves.
During diastole, the atrioventricular valves are open
At the beginning of the systole, the contraction of ventricles causes sudden backflow of blood at atrioventricular valves
↓ Valves to close and bulge towards the atria
Chordae tendineae abruptly stop this back-bulging motion
The blood that was trying to go back into the atria is bounded back into the ventricles
Sets up vibrations in blood, valves, and the ventricular wall
The vibrations travel through the adjacent tissues
Heard as sound
Note: Although it's a common belief, the sound is not produced due to slapping together of leaflets of valve because the blood in between the leaflet cushions the slapping.
Resembles spoken word 'Lub'
Duration: 0.14 sec
Second Heart Sound (S2)
Coincides with closure of semilunar valves
is almost similar to first heart sound.
At the end of the systole, the backflow of blood from major arteries to ventricles
↓ Closes the semilunar valves and makes them bulge into the ventricles
But the elastic stretch of the valves recoils the blood back into the arteries
This produces vibration in blood, valves, and wall of large arteries
The vibrations are transmitted to adjacent tissues
heard as second heart sound
Resembles spoken word 'Dub'
Duration: 0.11 sec
Third Heart Sound (S3)
Heard during early diastole (rapid ventricular filling).
Produced by rapid inrushing of blood from atria into ventricles.
Hear physiologically in:
In older people its indicative of pathology.
Protodiastolic Gallop or Ventricular gallop
When the third heart sound gets added to the physiological S1 and S2
Resultant three sound sequence 'S1-S2-S3' sound like a galloping horse
Its called protodiastolic gallop or ventricular gallop
Fourth Heart Sound (S4)
Heard late during diastole (atrial contraction)
Pathological conditions in which ventricular compliance is decreased
↓ i.e. ventricles do not dilate easily
Atria contract more forcefully to fill ventricles
Produces fourth heart sound
Presystolic Gallop or Atrial Gallop
When fourth heart sound is added to the physiological S1 and S2
The resultant sequence 'S4-S1-S2' also sound like a galloping horse
Its called presystolic gallop or atrial gallop