Question

The first sound (S$_1$) 'lubb' is caused by blood turbulence associated with:

• A. Opening of atrioventricular valves
• B. Closing of atrioventricular valves
• C. Opening of semilur valves
• D. Closing of semilur valves

Hint:

$S_1$ ('Lubb') = Closure of Atrioventricular (AV) valves at start of Ventricular Systole.
$S_2$ ('Dubb') = Closure of Semilur (SL) valves at start of Ventricular Diastole.
Valves opening does NOT produce normal heart sounds!

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks to identify the physiological mechanical event in the cardiac cycle responsible for generating the first heart sound ($S_1$, commonly voiced as 'lubb').

Step 2: Key Formula or Approach:
Heart sounds are produced by blood turbulence resulting from the sudden closure of heart valves during the cardiac cycle:
• First Heart Sound ($S_1$, 'Lubb'): Closure of Atrioventricular (AV) valves (Tricuspid and Mitral/Bicuspid valves).

• Second Heart Sound ($S_2$, 'Dubb'): Closure of Semilur (SL) valves (Aortic and Pulmory valves).

Step 3: Detailed Explation:

• Cardiac Cycle Timing of $S_1$: The first heart sound occurs at the onset of ventricular systole (isovolumetric contraction phase).

• Mechanical Event: As ventricles begin to contract, intraventricular pressure rapidly rises above atrial pressure. This pressure differential forces the atrioventricular (AV) valves (mitral and tricuspid) to sp shut sharply.

• Sound Generation: The sudden closure of the AV valves causes blood turbulence and vibrations in the ventricular walls and surrounding major blood vessels, creating the characteristic low-pitched, longer-duration $S_1$ 'lubb' sound.

• Comparison with $S_2$: The second heart sound ($S_2$, 'dubb') occurs at the beginning of ventricular diastole due to the closure of semilur valves as intraventricular pressure falls below arterial pressure.

Step 4: Fil Answer:
The first heart sound ($S_1$) 'lubb' is caused by turbulence created by the closure of atrioventricular valves.