The "Lub" is the sound of the exit doors closing just as the heart tries to pump.
| Sound | Name | Valve Action | Phase | Pitch | Clinical Significance | | :--- | :--- | :--- | :--- | :--- | :--- | | | S1 | AV Valves (Mitral & Tricuspid) close | Systole (Contraction) | Low, Long | Indicates the start of the pumping phase. | | Dub | S2 | Semilunar Valves (Aortic & Pulmonary) close | Diastole (Relaxation) | High, Short | Indicates the start of the filling phase. | what produces the lub dub heart sounds