A patient loses 2 litres of isotonic saline through a drain. Which body fluid compartment is exclusively depleted, and what is the expected effect on plasma osmolality?
- A Extracellular fluid only; plasma osmolality remains unchanged at ~290 mOsm/kg ✓
- B Intracellular and extracellular fluid equally; plasma osmolality falls
- C Plasma volume only; interstitial fluid is unaffected because proteins prevent redistribution
- D Intracellular fluid only; extracellular osmolality rises, drawing water out of cells
Explanation
Isotonic saline (0.9% NaCl, ~308 mOsm/kg) has approximately the same tonicity as extracellular fluid (ECF). Loss of isotonic fluid depletes only the ECF compartment (plasma + interstitial fluid) without altering plasma osmolality; therefore no osmotic gradient is created across cell membranes and intracellular fluid volume is unchanged. ADH secretion is not triggered because osmolality is unchanged. Isotonic volume depletion is detected by baroreceptors, leading to renin-angiotensin-aldosterone activation. Option C is incorrect because plasma and interstitial fluid readily equilibrate. Options B and D describe responses to hypotonic or hypertonic losses respectively.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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Written and medically reviewed by the StethoPrep medical team.