What's sound of the heart

Most people can hear two heart sounds per cycle - a quiet "lub" and a louder "dub". The "lub" sound is the contraction of the two atria, which results in the ventricles being filled with blood. The "dub" sound is the contraction of the two ventricles, which results in blood being forced into the arterial system. However, a trained physician can hear four sounds - the two described above, and two more that represent the sound of the valves opening back up after the contraction is over.