Heartbeat Sound Lub Dub [top]

A low, slightly prolonged sound. Think Luuub . What is happening: The atrioventricular (AV) valves are closing.

Contrary to popular belief, the "lub-dub" is not the sound of the heart muscle contracting. Instead, it is primarily caused by the and the resulting vibrations in the heart chambers and surrounding blood. nhlbi, nih (.gov) How the Heart Works - How the Heart Beats | NHLBI, NIH heartbeat sound lub dub