The arterial baroreceptor reflex arc involves stretch receptors in the carotid sinus and aortic arch. In a patient who stands up rapidly (orthostasis), the sequence of baroreceptor responses is:

• A Decreased baroreceptor firing → reduced inhibition of vasomotor centre → increased sympathetic output → tachycardia and vasoconstriction ✓
• B Increased baroreceptor firing → increased vagal tone → increased HR
• C Reduced arterial pressure → increased baroreceptor firing → bradycardia
• D Increased baroreceptor firing → increased sympathetic output → vasoconstriction

Explanation

On standing, venous pooling in the legs reduces venous return and cardiac output, lowering mean arterial pressure. Reduced arterial wall stretch decreases baroreceptor afferent firing (via CN IX from carotid sinus and CN X from aortic arch), reducing tonic inhibition of the vasomotor centre (NTS pathway). The result is increased sympathetic outflow (tachycardia, vasoconstriction, venoconstriction) and decreased vagal tone — the normal orthostatic response that maintains blood pressure upon standing.

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

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