Question
The first sound (S$_1$) 'lubb' is caused by blood turbulence associated with:
This question was previously asked in
CUET PG 2026 Agri-Business Management Question Paper (25-Mar-2026) (Shift 2)
The correct answer is
Closing of atrioventricular valves
First Heart Sound S1 Lubb Causes
The first heart sound, referred to as S1 and often described as 'lubb', originates from the events occurring at the beginning of ventricular systole (the heart's contraction phase).
Mechanism of S1 Production
S1 is primarily caused by the:
- Closing of the atrioventricular (AV) valves: This includes the mitral valve (between the left atrium and left ventricle) and the tricuspid valve (between the right atrium and right ventricle). The closure of these valves prevents the backflow of blood into the atria as the ventricles begin to contract. The resulting vibrations and turbulence generate the 'lubb' sound.
Analysis of Options
- Opening of atrioventricular valves: This event occurs during diastole (heart relaxation) and is not the cause of S1.
- Closing of atrioventricular valves: This correctly explains the origin of S1 ('lubb').
- Opening of semilunar valves: These valves (aortic and pulmonary) open after the AV valves close and ventricular pressure exceeds that in the aorta and pulmonary artery. Their opening does not cause S1.
- Closing of semilunar valves: The closure of the aortic and pulmonary valves marks the end of ventricular systole and causes the second heart sound (S2), 'dub'.
Therefore, the turbulence associated with the closing of the atrioventricular valves is the source of the first heart sound (S1) 'lubb'.
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