The "dub" is the sound of these exit valves snapping shut. It is a shorter, sharper, and higher-pitched sound than the "lub." It marks the moment the heart is empty and beginning to refill with blood for the next beat.

The human heart is one of the hardest-working organs in the body, beating approximately 100,000 times a day. For most of us, the sound of our own heartbeat is a background rhythm we rarely notice unless we are running or frightened. When a doctor places a stethoscope against a chest, they are listening for a specific, rhythmic pattern: the familiar "lub-dub."