A patient receives 3 litres of isotonic (0.9%) saline intravenously. How will this expand the body fluid compartments?

• A Equally expands ICF and ECF — 1.5 L each
• B Expands only ECF (both plasma and interstitial fluid); ICF is unchanged ✓
• C Expands plasma volume only, not interstitial
• D Expands ICF only, as sodium is the main intracellular cation

Explanation

Isotonic saline (0.9% NaCl, osmolality ~308 mOsm/kg) does not change plasma osmolality; therefore, no osmotic gradient is created across cell membranes and ICF volume is unchanged. The infused volume distributes within the ECF compartment only — approximately 75% to interstitial fluid and 25% to plasma. This is why isotonic saline is used for ECF volume expansion but not for ICF depletion (that requires hypotonic solutions).

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

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