A 70 kg man receives 2 liters of isotonic (0.9%) saline intravenously. After equilibration, the primary change in body fluid compartments will be:

• A 2L distributed into the ECF only (approximately 500 mL intravascular, 1500 mL interstitial) ✓
• B 2L distributed equally between ICF and ECF (1L each)
• C 2L retained entirely in the intravascular compartment
• D 2L distributed 1.5L ICF and 0.5L ECF due to osmotic equilibration

Explanation

Isotonic saline has the same osmolality as body fluids (308 mOsm/L), so it does not shift water across cell membranes — it stays entirely in the ECF. The ECF is further divided between the intravascular compartment (plasma, ~25% of ECF) and interstitial compartment (~75% of ECF). Thus 2L of isotonic saline distributes approximately 500 mL to plasma and 1500 mL to interstitium. This is why isotonic saline is not an efficient plasma volume expander: only 1/4 stays intravascular. Colloids or hypertonic saline are more effective at expanding plasma volume per liter administered.

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

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