During hemorrhagic shock, the vasomotor center activates sympathetic outflow. Which cardiovascular reflex provides the most powerful short-term defense against a 30% blood volume loss?

• A Chemoreceptor reflex — hypoxia activates carotid bodies causing vasoconstriction as the primary hemodynamic response
• B Arterial baroreceptor reflex (carotid sinus and aortic arch) — reduced stretch causes decreased baroreceptor firing, disinhibiting the vasomotor center, causing marked sympathetic activation within seconds ✓
• C CNS ischemic response (Cushing reflex) — brain ischemia activates the vasomotor center as the primary fast response
• D Stress relaxation — vascular compliance immediately reduces venous pressure, restoring cardiac output

Explanation

The arterial baroreceptor reflex (carotid sinus/aortic arch) operates within a latency of <1 second and is the primary rapid defense against blood pressure changes. With hemorrhage, reduced arterial distension decreases baroreceptor afferent firing in CN IX (Hering's nerve) and CN X (aortic depressor nerve) to the NTS. NTS reduces inhibitory output to the vasomotor center (RVLM), increasing sympathetic outflow → tachycardia, increased contractility, and vasoconstriction. The CNS ischemic (Cushing) response is an emergency 'last resort' reflex activated only at MAP <40-50 mmHg; chemoreceptor cardiovascular effects are secondary to the ventilatory response.

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

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