The “lub” sound (S1) of the heart is caused by the closure of the atrioventricular valves (tricuspid and bicuspid/mitral valves) at the beginning of ventricular systole (contraction). This prevents blood from flowing back into the atria.
The “dub” sound (S2) is caused by the closure of the semilunar valves (pulmonary and aortic valves).
List I | List II | ||
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A | Mesozoic Era | I | Lower invertebrates |
B | Proterozoic Era | II | Fish & Amphibia |
C | Cenozoic Era | III | Birds & Reptiles |
D | Paleozoic Era | IV | Mammals |