A patient with severe hypernatraemia (serum Na+ 168 mEq/L) is being corrected with IV hypotonic fluid. Which compartmental shift predominates?
- A Water moves from extracellular to intracellular compartment as ECF osmolarity falls ✓
- B Water moves from intracellular to extracellular compartment as osmolarity is corrected
- C Sodium shifts intracellularly to equalise concentrations
- D The interstitial compartment expands at the expense of the plasma compartment
Correct answer: A. Water moves from extracellular to intracellular compartment as ECF osmolarity falls
Explanation
Hypernatraemia means elevated ECF osmolarity; cells are shrunken because water has left them osmotically. When hypotonic fluid is administered, ECF osmolarity falls; osmotic equilibrium drives water from the ECF into cells. If correction is too rapid, cerebral cells reabsorb water faster than osmolytes can be cleared, risking cerebral oedema. This is why hypernatraemia should be corrected slowly (reduce serum Na+ by no more than 10 mEq/L per 24 hours). Na+ and other solutes do not shift freely across cell membranes to equalise concentrations.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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Written and medically reviewed by the StethoPrep medical team.