A 70 kg man is given 2 L of hypotonic (0.45% NaCl) IV fluid rapidly. Which of the following CORRECTLY describes the shift in body fluid compartments?

• A Expands ECF only; ICF remains unchanged because NaCl does not cross cell membranes
• B Expands ICF only; hypotonic fluid preferentially distributes intracellularly
• C Expands ECF predominantly; osmotic equilibrium is maintained by ADH release preventing water entry into cells
• D Expands both ECF and ICF; ECF osmolality falls, water shifts into ICF; cells swell ✓

Explanation

Half-normal (0.45%) NaCl is hypotonic relative to plasma (154 mOsm/L vs ~285 mOsm/L for plasma). The 2 L distributes throughout total body water after equilibration. The addition of free water component lowers ECF osmolality, creating an osmotic gradient driving water into cells (ICF), causing cellular swelling. Approximately one-third of infused volume stays in ECF and two-thirds shifts to ICF (like free water distribution). NaCl does cross cell membranes slowly, but the primary immediate effect is osmotic water movement. ADH does not prevent intracellular water entry — that is determined by osmotic gradients, not ADH.

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

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