A 70 kg man has a total body water (TBW) of 42 L. He loses 3 L of pure water (insensible loss). What happens to his intracellular and extracellular fluid volumes?
- A All 3 L lost from extracellular compartment only
- B 2 L lost from ICF, 1 L from ECF proportionally
- C ICF loses 2 L, ECF loses 1 L — proportional to their normal volumes (2:1 ratio) ✓
- D ECF loses 3 L, ICF volume unchanged
Correct answer: C. ICF loses 2 L, ECF loses 1 L — proportional to their normal volumes (2:1 ratio)
Explanation
Pure water loss is distributed across TBW in proportion to compartment volumes. Normal TBW = 42 L: ICF = 28 L (2/3), ECF = 14 L (1/3). After 3 L pure water loss, new TBW = 39 L. Water distributes proportionally: ICF loses 2/3 × 3 = 2 L (new ICF = 26 L) and ECF loses 1/3 × 3 = 1 L (new ECF = 13 L). This is because water moves freely across cell membranes and the loss causes hyperosmolality, drawing water osmotically from both compartments.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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Written and medically reviewed by the StethoPrep medical team.