Expansion
Deuterium for total body water, inulin for extracellular fluid, albumin for plasma
Expansion
All use the dilution principle: volume = amount given divided by concentration achieved, after correcting for any excreted.
| Compartment | Indicator |
|---|---|
| Total body water | Deuterium oxide, tritiated water, antipyrine |
| Extracellular fluid | Inulin, mannitol, sucrose, radiolabelled sulphate |
| Plasma volume | Evans blue dye, radiolabelled albumin |
| Blood volume | Radiolabelled red cells, or plasma volume divided by (1 minus haematocrit) |
| Intracellular fluid | Not measurable directly; calculated by subtraction |
The requirements of an ideal indicator explain the choices: it must distribute evenly through the compartment, not cross out of it, be non-toxic, not be metabolised rapidly, and be easy to measure.
Interstitial volume is likewise calculated, as extracellular fluid minus plasma volume.
A practical note: the haematocrit correction for blood volume uses a factor of about 0.91, since whole-body haematocrit is slightly lower than the venous value because of plasma skimming in small vessels.