In a research subject, inulin dilution yields an extracellular fluid volume of 14 L and Evans blue dye dilution yields a plasma volume of 3 L. What is the subject's estimated interstitial fluid volume?
- A 3 L
- B 7 L
- C 11 L ✓
- D 14 L
Explanation
Interstitial fluid volume cannot be measured directly because no indicator distributes solely into it. It is obtained by subtraction: extracellular fluid volume measured with inulin, which diffuses freely across capillary walls, minus plasma volume measured with Evans blue dye, which binds albumin and stays intravascular. Fourteen litres minus three litres gives eleven litres, consistent with the standard partition of ECF into roughly one quarter plasma and three quarters interstitial fluid. Option B would follow from erroneously subtracting plasma volume from intracellular fluid.
Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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