There are two final cellular elements of the heart that are essential to its function. First, there are valves between the chambers to ensure that blood does not flow backward. The cells of the atria—the collecting chambers—contract first sending blood into the ventricles. The valves between the atria and ventricles close, making a flapping noise: Lub, the first heart sound. And after that, the cells of the ventricle contract in a similarly coordinated manner. The outlet valves of the ventricles close: Dub, the second heart sound. Lub-Dub, Lub-Dub. The sound of a citizenry in lockstep, working
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