When the ventricles contract to pump blood out to the body and lungs, the valves between the upper chambers (atria) and lower chambers (ventricles) snap shut to prevent blood from flowing backward. That sudden closure and the vibration of the valves and surrounding blood create the low-pitched "lub" sound.
Here’s the content you requested, structured for clarity: the lub sound of the heart is caused by
The closing of the atrioventricular (AV) valves — specifically the tricuspid valve (right side) and the mitral valve (left side) — at the beginning of ventricular contraction (systole). When the ventricles contract to pump blood out