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The lub-dub sound is generated by the opening and closing of the heart's valves, specifically the atrioventricular (AV) valves and the semilunar valves. The AV valves, comprising the mitral and tricuspid valves, separate the atria and ventricles, while the semilunar valves, consisting of the aortic and pulmonary valves, separate the ventricles and the aorta and pulmonary artery, respectively.
The longer pause after the S2 and before the next S1 is when the heart muscle relaxes and the chambers fill with oxygen-rich blood. lub dub sounds of the heart
The very brief pause between the S1 and S2 is when the heart is actively squeezing. The lub-dub sound is generated by the opening