31 - 35 Physiology SBAs for the Primary FRCA

31 - 35 Physiology SBAs for the Primary FRCA
Photo by Unseen Studio / Unsplash

Question 31

You are discussing with your consultant how to measure the various fluid compartments of the body. She starts asking you what tracer you would use for each fluid compartment.

Which of the following is incorrectly paired?

  • Blood - isotopic red blood cells
  • Extracellular fluid - inulin
  • Plasma - Evan's blue dye and tracer-labelled albumin
  • Total body water - isotopic water
  • Transcellular fluid - mannitol


  • Transcellular fluid - mannitol

We generally assume that around 60-70% of the human body is water.

This is distributed among a variety of compartments, in two categories:

  • Intracellular
  • Extracellular

The intracellular fluid compartment is around double the extracellular, and the extracellular compartment is further divided into:

  • Interstitial fluid
  • Plasma fluid
  • Transcellular fluid (CSF, synovial fluid etc)

To measure the volume of a given compartment you use the following equation:

Volume = dose of tracer / concentration of tracer

For this to work, you need to use a tracer that you know is going to stick to the compartment of interest.

All of the above tracers are correctly linked to their relevant fluid compartment, apart from mannitol, which is another method of measuring ECF.