A 70 kg man is infused with 2 litres of isotonic saline (0.9% NaCl, osmolality 308 mOsm/kg). How does this affect his body fluid compartments?

• A Expands only the extracellular fluid (ECF) compartment; because saline osmolality approximates ECF, there is no osmotic gradient to drive fluid into cells, so intracellular fluid (ICF) volume is unchanged ✓
• B Distributes equally between ECF and ICF in proportion to their normal fractional volumes (1/3 ECF, 2/3 ICF), so ~667 mL enters ECF and ~1333 mL enters ICF
• C Expands ECF and contracts ICF because NaCl raises ECF osmolality, drawing water out of cells
• D Remains entirely within the plasma compartment because NaCl does not cross the capillary endothelium

Explanation

Isotonic saline (osmolality ~308 mOsm/kg, close to plasma 285–295 mOsm/kg) distributes throughout the entire ECF compartment—both plasma (25%) and interstitial fluid (75%)—but does not enter cells because there is no effective osmotic gradient across cell membranes. ICF volume remains unchanged. In a 70 kg man, ECF is approximately 14 L; after 2 L isotonic saline, ECF becomes ~16 L with the same osmolality. This contrasts with hypotonic saline (expands both ECF and ICF) and hypertonic saline (expands ECF, contracts ICF). NaCl does cross capillary walls freely (unlike colloids); option D is incorrect.

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

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