The first heart sound (S1) represents mitral and tricuspid valve closure as the ventricular pressure exceeds atrial pressure at the beginning of systole.
The second heart sound (S2) represents aortic and pulmonary valve closure at the end of systole/beginning of diastole. The second heart sound is physiologically split because the aortic valve normally closes before the pulmonic valve. It is normal for the split of S2 to widen during inspiration and narrow during expiration.
The third heart sound (S3) represents rapid ventricular filling during early diastole. The third heart sound can be a normal finding in children and young athletes. In older adults S3 is associated with heart failure.
The fourth heart sound (S4) occurs during late diastole, just before S1. It represents the vibration of a stiff ventricle as it’s forced to receive blood during atrial contraction.