Tritiated water (3H2O) injected IV equilibrates with which body fluid compartment for measurement?
- A Plasma volume only
- B Extracellular fluid volume
- C Total body water ✓
- D Intracellular fluid volume
Correct answer: C. Total body water
Explanation
The indicator-dilution principle requires a substance to distribute freely throughout the compartment being measured. Water distributes freely across all membranes — cell membranes, capillary walls — and thus equilibrates with total body water (TBW, ~60% of body weight in men, ~50% in women). Plasma volume is measured with Evans blue or radiolabelled albumin (confined to vasculature); ECF is measured with mannitol, inulin, or radiosulphate (distributes through ECF but not ICF). Intracellular volume is calculated as TBW minus ECF.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.