| Feature | S1 ("Lub") | S2 ("Dub") | | :--- | :--- | :--- | | | Beginning of systole (contraction) | Beginning of diastole (relaxation) | | Valves Closing | Mitral & Tricuspid (AV valves) | Aortic & Pulmonic (Semilunar valves) | | Pitch | Lower | Higher | | Duration | Slightly longer | Slightly shorter | | Best Heard | At the cardiac apex (mitral area) | At the base (2nd intercostal space) | Introduction The rhythmic cycle of the human heartbeat
Under normal conditions, the "dub" may not be a single, pure sound. During inspiration (breathing in), increased venous return to the right heart delays the closure of the pulmonic valve. This causes the two components of S2 (aortic and pulmonic valves) to separate slightly, producing a physiological "splitting" of the dub—heard as "lub-du- dup " in young, healthy individuals.
Introduction The rhythmic cycle of the human heartbeat is most commonly identified by the onomatopoeic phrase "lub-dub." Far from being a simple noise, this two-part sound is a precise acoustic signature of cardiovascular mechanics. It represents the turbulent closing of the heart’s valves and is the primary indicator of basic cardiac function.