When does the second heart sound (dup) occur?

The second heart sound, commonly referred to as "dup," occurs due to the closure of the semilunar (SL) valves—specifically, the aortic and pulmonary valves—at the end of ventricular systole. This closure signals the transition between the contraction phase of the heart and the relaxation phase, known as diastole.

During ventricular systole, the ventricles contract, forcing blood into the arteries through the open semilunar valves. Once the ventricles have finished contracting and the pressure inside them drops below the pressure in the aorta and pulmonary artery, the semilunar valves close, creating the second heart sound. This closure helps prevent backflow of blood into the ventricles, allowing them to fill with blood from the atria during diastole.

Understanding this timing and the mechanics of the heart's cycle is essential for recognizing how the heart functions as a pump, maintaining efficient blood circulation throughout the body.