Which heart sound is associated with the closing of the AV valves at the start of ventricular systole?

• A. S1 (first heart sound)
• B. S2 (second heart sound)
• C. S3
• D. S4

Answer: (A)

The first heart sound is produced by the closure of the atrioventricular valves as the ventricles begin to contract. When the ventricles generate enough pressure to exceed the atrial pressure, the mitral and tricuspid valves snap shut to prevent backflow into the atria. This sudden valve closure and the resulting vibrations create the audible “lub” at the start of ventricular systole, and it typically coincides with the upstroke of the QRS complex on the ECG. By contrast, the second heart sound comes from the closure of the semilunar valves at the end of systole, while S3 and S4 are extra sounds related to ventricular filling and atrial contraction, respectively.