A patient loses 3 liters of diarrheal fluid that is isotonic with plasma. Which change in body fluid compartments and plasma osmolality is most likely?

• A ECF decreases, ICF increases, plasma osmolality decreases
• B ECF unchanged, ICF decreases, plasma osmolality increases
• C ECF decreases, ICF unchanged, plasma osmolality unchanged ✓
• D ECF and ICF both decrease equally, plasma osmolality unchanged

Explanation

Isotonic fluid loss depletes the extracellular fluid without creating an osmotic gradient between ECF and ICF, so ICF volume remains unchanged and plasma osmolality stays normal. ECF includes plasma and interstitial fluid, both of which shrink. ICF increases only with hypotonic loss or hypertonic gain. Equal ECF and ICF loss occurs only with pure water loss. Plasma osmolality increases with hypertonic loss, not isotonic loss.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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