In haemorrhagic shock (class III, ~30–40% blood volume loss), which of the following compensatory responses occurs?
- A Increased vagal tone causing bradycardia to conserve myocardial O2 — the primary compensatory mechanism
- B Baroreceptor-mediated sympathetic activation causing tachycardia, vasoconstriction, increased contractility; ADH and RAAS activation cause fluid retention; redistribution of blood from non-essential to essential organs ✓
- C Renin secretion increases immediately within seconds — the fastest compensatory response
- D Reduced renal blood flow suppresses erythropoietin, causing compensatory polycythemia within 24 hours
Explanation
Class III hemorrhagic shock triggers multiple compensatory responses in sequence: (1) Immediate (<seconds): baroreceptor-mediated sympathetic surge → tachycardia, increased contractility, arteriolar and venous constriction → maintained MAP; (2) Minutes: ADH (vasopressin) release from posterior pituitary; RAAS activated (renin from reduced renal perfusion → angiotensin II → aldosterone → Na+/water retention); (3) Hours: fluid shift from interstitium to plasma (transcapillary refill). Erythropoietin-driven erythropoiesis takes days. Bradycardia is NOT a compensatory response (indicates decompensation or neurogenic shock).
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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