The production of the first heart sound is because of which of the following?

Appeared in: AIIMS Bhopal NO - 2018 (Shift-1st)

Explanation

The first heart sound (S1), known as "lub," is caused by the closure of the atrioventricular (AV) valves: the mitral and tricuspid valves.
This event occurs at the beginning of ventricular systole, when the pressure in the ventricles exceeds the pressure in the atria, forcing the AV valves to shut.
The sound is produced by the vibrations of the taut valve leaflets, the chordae tendineae, and the ventricular walls as blood surges against the closed valves.
S1 is best heard at the apex of the heart (mitral area) and the lower left sternal border (tricuspid area).

Option A: The rushing of blood into the ventricles during the rapid filling phase of diastole is typically silent in healthy adults.
Option C: The closure of the semilunar (aortic and pulmonary) valves produces the second heart sound (S2), or "dub".
Option D: The contraction of the atria (atrial systole) pushes the final volume of blood into the ventricles just before ventricular contraction.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Physiology of Heart Sounds (S1) as background academic context rather than a clinical decision trigger.
Auscultating S1 and S2 is a fundamental nursing skill for assessing cardiac rate, rhythm, and function.
The intensity of S1 can provide clues about valve health and cardiac conditions. A loud S1 may indicate mitral stenosis, while a soft S1 can suggest impaired ventricular contraction or a long PR interval on the ECG.
Identifying abnormal sounds like splits, murmurs, or extra sounds (S3, S4) in relation to S1 and S2 is critical for detecting valvular disease, heart failure, and other cardiovascular pathologies.

How to Approach the Question

First, identify the core concept of the question, which is the physiological cause of the 'first heart sound'.
Recall the events of the cardiac cycle. The cycle consists of two main phases: systole (contraction) and diastole (relaxation).
Associate the heart sounds 'lub' (S1) and 'dub' (S2) with specific valve closure events.
Remember that the cycle begins with ventricular contraction (systole), which must be preceded by the closure of the valves that let blood into the ventricles. This is the 'lub' or S1 sound.
Evaluate each option against this physiological sequence. The closure of the atrioventricular valves (mitral and tricuspid) matches the timing and cause of S1.
Eliminate the other options by identifying what they cause: closure of semilunar valves causes S2, atrial contraction can cause S4, and rapid ventricular filling can cause S3.

Concept Tested & Keywords

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E6 Physiology Guyton 4SAE Part 2A p. 12-13

E6 Physiology Ganong 27e Vol 2 Part 1 p. 86-88

Practise the full AIIMS Bhopal NO - 2018 (Shift-1st)

Attempt every question from this paper in a timed mock, then review the full solution for each one.