Q.'Lub' & 'dub' are the sounds of

Press a stethoscope to someone's chest and you hear a steady, repeating "lub-dub, lub-dub." It is tempting to imagine this is the muscle thumping, but the two sounds are actually made by the heart's valves snapping shut. Knowing where each sound comes from is what this idea is about.

The heart has valves that keep blood moving in one direction only, and closing a valve to block backflow makes an audible sound. During each cardiac cycle two clear sounds are produced:

• The first heart sound, heard as "lub", accompanies the closing of the tricuspid and bicuspid valves — the valves between the atria and the ventricles. These shut when the ventricles begin to contract and blood tries to flow backward toward the atria.
• The second heart sound, heard as "dub", accompanies the closing of the semilunar valves — the valves guarding the openings into the pulmonary artery and the aorta. These shut when the ventricles relax and their pressure falls, preventing blood from draining back into the ventricles.

So the familiar "lub-dub" is really the story of one cardiac cycle told in sound: the atrioventricular valves closing as the ventricles squeeze, then the semilunar valves closing as the ventricles relax.