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In the cardiac cycle, what does the S2 heart sound correspond to?

A.

Atrial contraction

B.

Closure of the aortic and pulmonic valves

C.

Rapid filling of the ventricles

D.

Closure of the mitral and tricuspid valves

Answer and Explanation

The Correct Answer is B

A. Atrial contraction:
 Atrial contraction occurs just before ventricular systole and contributes minimally to heart sounds. The “lub-dub” of S1 and S2 does not correspond to atrial contraction.

B. Closure of the aortic and pulmonic valves:
 The S2 heart sound (“dub”) occurs during ventricular diastole when the aortic and pulmonic valves close. This sound indicates the end of systole and the beginning of ventricular relaxation. It is best heard at the base of the heart.

C. Rapid filling of the ventricles:
 Rapid ventricular filling occurs in early diastole and does not produce a normal heart sound. An abnormal sound (S3) can be associated with heart failure.

D. Closure of the mitral and tricuspid valves:
 The closure of the mitral and tricuspid valves produces the S1 sound (“lub”), which marks the beginning of ventricular systole. Confusing S1 and S2 can lead to misinterpretation of cardiac auscultation.


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Related Questions

Correct Answer is B

Explanation

A. Activation of chemoreceptors:
Chemoreceptors respond to oxygen, carbon dioxide, and pH levels in the blood, affecting ventilation rather than fluid retention.

B. Activation of the renin-angiotensin-aldosterone system:
RAAS increases blood pressure by promoting sodium and water reabsorption in the kidneys, increasing blood volume, and by causing vasoconstriction, increasing systemic vascular resistance. This is a major mechanism in long-term blood pressure regulation.

C. Baroreceptor reflex:
Baroreceptors detect acute changes in blood pressure and adjust heart rate and vascular tone but do not directly control sodium and water retention.

D. Release of antidiuretic hormone:
ADH increases water reabsorption in the kidneys, but its effect on sodium retention is indirect and less significant compared to RAAS.

Correct Answer is A

Explanation

A. To increase blood volume and systemic vascular resistance:
The RAAS system increases blood pressure by two main mechanisms: angiotensin II causes vasoconstriction, raising systemic vascular resistance, and aldosterone promotes sodium and water retention, increasing blood volume. Together, these actions restore perfusion pressure in hypotensive states.

B. To decrease heart rate and reduce cardiac output:
RAAS does not directly affect heart rate; it works primarily on vascular tone and fluid retention, not cardiac chronotropy.

C. To regulate oxygen levels in the blood:
Oxygen sensing is mainly controlled by chemoreceptors, not RAAS.

D. To control nerve impulses to the heart:
RAAS does not directly regulate cardiac conduction or nerve impulses; it acts on vascular smooth muscle and renal sodium retention.

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