Measuring the volumes of the fluid compartments see Table 51 1 Dilution method

a. A known amount of a substance is given whose volume of distribution is the body fluid compartment of interest.

- For example:

(1) Mannitol is a marker substance for ECF because it is a large molecule that cannot cross cell membranes and is therefore excluded from the ICF.

(2) Evans blue is a marker for plasma volume because it is a dye that binds to serum albumin and is therefore confined to the plasma compartment.

b. The substance is allowed to equilibrate.

c. The concentration of the substance is measured in plasma, and the volume of distribution is calculated as follows:

Tr , Amount

Concentration where:

Volume = volume of distribution or volume of the body fluid compartment (L) Amount = amount of substance present (mg) Concentration = concentration in plasma (mg/L)

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