General physiological principles: a free MRCS Part A question

The indicator-dilution principle gives a compartment volume from the dose of marker divided by its equilibrium concentration, provided the marker distributes only within that compartment. Inulin (like mannitol, sucrose, thiosulphate or radiolabelled bromide) crosses capillary walls freely but cannot enter cells, so it measures the extracellular fluid. Deuterium oxide and antipyrine, like tritiated water, cross all membranes and measure total body water. Evans blue dye and radiolabelled albumin bind to or are plasma proteins that remain within the vasculature and so measure plasma volume. The intracellular volume cannot be measured directly and is calculated as total body water minus extracellular fluid; interstitial volume is extracellular fluid minus plasma volume. Reference: Ganong 26e.