Complex sentence: The normal heart rhythm contains two audible heart sounds called S 1 and S 2 that give the well-known "lub-dub" rhythm; they are caused by the closing of valves in the heart.
Split into simpler sentences: The normal heart rhythm contains two audible sounds, called S 1 and S 2, giving the well-known "lub-dub" rhythm. They are caused by the closing of valves in the heart.