A 70 kg man is given 1 litre of isotonic saline (0.9% NaCl, osmolality 308 mOsm/kg). Immediately after equilibration, the predicted distribution of this volume across body fluid compartments is:

• A Entire 1 L remains in extracellular fluid; ~250 mL enters plasma, ~750 mL stays in interstitium ✓
• B 1 L distributes across total body water: ~420 mL ICF, ~580 mL ECF
• C 1 L distributes into ECF only with approximately 333 mL plasma and 667 mL interstitium
• D 1 L distributes as 250 mL intravascular, 500 mL interstitial, and 250 mL intracellular

Explanation

Isotonic saline expands only the extracellular fluid compartment because it is isosmotic with plasma — it does not change plasma osmolality and therefore provides no osmotic gradient to drive water into or out of cells. The ECF comprises approximately 20 L (plasma ~3 L = 15%, interstitium ~12 L = 60% of ECF). After isotonic volume addition, it distributes within the ECF proportionately: ~25% into plasma (~250 mL) and ~75% into the interstitium (~750 mL). No water shifts into the intracellular compartment (option B/D would apply to hypotonic fluid). Option C gives the same answer but with slightly different numbers; 1/4 to plasma, 3/4 to interstitium is the standard teaching.

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

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