Measurement of total body water (TBW) using deuterium oxide (D2O) dilution gives a result of 42L in a 70 kg man. If Evans blue dye dilution gives a plasma volume of 3.5L and hematocrit is 45%, the interstitial fluid volume is approximately:
- A 8.5 L
- B 10.5 L ✓
- C 11.5 L
- D 14 L
Correct answer: B. 10.5 L
Explanation
Step 1: Blood volume = Plasma volume / (1 - Hematocrit) = 3.5 / 0.55 = 6.36 L. Step 2: ECF volume (measured by inulin/thiosulfate dilution; estimated here) ≈ TBW × 1/3 = 42 × 0.333 = 14 L (or 20% of body weight). Step 3: Interstitial fluid = ECF - Plasma volume = 14 - 3.5 = 10.5 L. ICF = TBW - ECF = 42 - 14 = 28 L. The interstitial compartment (~10.5L) is thus approximately 3x the plasma volume. Note: Transcellular fluid (~1-2L) is a minor ECF sub-compartment not always measured.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.