What causes the 'lub' sound in a heartbeat?

The 'lub' sound in a heartbeat, often referred to as the first heart sound (S1), is primarily caused by the closing of the mitral and tricuspid valves at the onset of ventricular systole. This mechanical closure occurs when the ventricles contract and pressure within them rises, preventing blood from flowing back into the atria. As the valves close, they create vibrations that lead to the characteristic 'lub' sound.

This sound is a crucial part of the heart's functioning, as it signifies that the heart is effectively pumping blood into the arteries. Understanding this physiological mechanism is essential for recognizing normal heart function and identifying potential cardiac issues during clinical assessments.