What is the first heart sound primarily caused by?

The first heart sound, known as S1, is primarily caused by the closure of the atrioventricular (AV) valves, specifically the mitral and tricuspid valves. This sound occurs at the onset of ventricular contraction, or systole, when the ventricles contract and the pressure within them rises, forcing the AV valves to close to prevent backflow into the atria. The closure of these valves creates a distinct sound due to the vibration of the valve leaflets, the chordae tendineae, and the surrounding cardiac tissue.

S1 marks the beginning of the pumping phase of the heart and is an essential component of the heart's auscultation findings. Understanding the origin of S1 helps in recognizing normal heart sounds and identifying potential abnormalities in heart function, as changes in the intensity or quality of this sound may indicate valve dysfunction or other cardiac conditions.