A 70 kg man receives 3 L of isotonic (0.9%) saline intravenously over 4 hours. Which compartment(s) will be primarily expanded, and by how much?
- A ECF expands by 3 L; ICF unchanged; with 2.25 L going to interstitium and 0.75 L to plasma ✓
- B Intracellular fluid compartment expands by 3 L; ECF unchanged
- C ECF expands by 3 L; ICF unchanged; all 3 L distributed equally between plasma and interstitium
- D Total body water expands by 3 L distributed equally between ICF (1 L) and ECF (2 L)
Correct answer: A. ECF expands by 3 L; ICF unchanged; with 2.25 L going to interstitium and 0.75 L to plasma
Explanation
Isotonic saline has an osmolality matching plasma (308 mOsm/L) so it does not alter osmolality, creating no driving force for water to cross cell membranes — therefore all 3 L stays in the ECF. Within ECF, plasma is 25% and interstitium is 75% of ECF by volume; 3 L × 0.25 = 0.75 L added to plasma and 3 × 0.75 = 2.25 L to interstitium. This is why large volumes of isotonic saline cause peripheral edema with relatively modest plasma volume expansion — only 25% reaches the intravascular space.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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