Which two valves closing cause the first heart sound (Lub)?

The first heart sound, often referred to as "Lub," is primarily produced by the closure of the mitral valve and the tricuspid valve at the start of ventricular contraction, or systole.

During the cardiac cycle, blood flows into the ventricles during diastole, and as the ventricles fill and then contract, the pressure within them rises. This rising pressure forces the mitral and tricuspid valves to close to prevent backflow of blood into the atria. The closure of these valves produces the sound associated with the "Lub."

The aortic and pulmonary valves are responsible for the second heart sound, known as "Dub," which occurs when these valves close at the end of ventricular contraction. Understanding this sequence is critical for interpreting heart sounds and their association with the cardiac cycle. Recognizing the role of the mitral and tricuspid valves in generating the first heart sound is essential for grasping basic cardiovascular physiology and how heart sounds relate to the mechanical actions of the heart.