The arterial baroreflex arc rapidly corrects a sudden drop in blood pressure. Following a hemorrhage of 500 mL, carotid sinus baroreceptors fire less frequently. Trace the reflex response accurately.

• A Reduced baroreceptor firing → decreased inhibitory input to RVLM → RVLM activation → increased sympathetic outflow → tachycardia, vasoconstriction, increased contractility ✓
• B Reduced baroreceptor firing → increased NTS inhibition of RVLM → RVLM activation → increased sympathetic outflow → tachycardia, vasoconstriction, increased contractility
• C Reduced baroreceptor firing → direct RVLM inhibition → parasympathetic activation → bradycardia and vasodilation to preserve brain flow
• D Reduced baroreceptor firing → ADH release from posterior pituitary → immediate arterial vasoconstriction offsetting blood pressure drop

Explanation

Baroreceptors tonically inhibit the rostral ventrolateral medulla (RVLM) via the nucleus tractus solitarius (NTS). When baroreceptor firing decreases (as with hypotension), this inhibitory tone on RVLM is relieved; RVLM neurons become more active and increase sympathetic outflow to the heart (tachycardia, positive inotropy) and blood vessels (vasoconstriction), restoring arterial pressure. Simultaneously, cardiac vagal tone decreases, further accelerating heart rate.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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