does the first heart sound, lubb occur when the atrioventricular valves or the semilunar valves close?
The two major heart sounds are 'lub' (from the closure of AV-valves) and 'dub' (from the closure of aortic and pulmonary valves). Describe the sounds the heart makes. The first heart tone lub, or S1, is caused by the closure of the mitral and tricuspid atrioventricular valves at the beginning of ventricular systole.