Question of the Day

The first heart sound (S1 or the «lub» in «lub-dub») is caused by closure of the atrioventricular valves (AV valves, mitral and tricuspid) and opening of the pulmonic and aortic valves (semilunar valves). This is the end of diastole and start of systole.

The second sound (S2) is the closure of the aortic and pulmonic valves and opening of the AV valves. This is the end of systole and start of diastole. 

A third sound (S3) is the end of rapid ventricular filling and a fourth sound (S4) is atrial systole (atrial contraction).

You can often hear all 4 sounds in horses, but typically hear only S1 and S2 in cattle and small animals.