A third heart sound occurs early in diastole. In young people and athletes it is a normal phenomenon. In older individuals it indicates the presence of congestive heart failure.
The third heart sound is caused by a sudden deceleration of blood flow into the left ventricle from the left atrium. In the anatomy video you will see a thin-walled, dilated left ventricle with generalized decreased vigor of contraction.
In the presence of a third heart sound (S3) the first heart sound is decreased in intensity while the second heart sound is increased in intensity.
The third heart sound is a low frequency sound best heard with the bell of the stethoscope pressed lightly on the skin of the chest.