top of page

๐Ÿ“š > Physiology > Cardiovascular System > Heart as a Pump_>

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.

  • Mechanism:

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

โ†“ 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

  • Mechanism:

  • is almost similar to first heart sound.

At the end of the systole, the backflow of blood from major arteries to ventricles

โ†“ 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:

  • Children

  • Young Adults.

  • 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)

  • Mechanism:

Pathological conditions in which ventricular compliance is decreased

โ†“ 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


Next >>

Arteries and Veins > โ–ถ๏ธ Physiological Splitting of Second Heart Sound
bottom of page