Sweat

The composition of transcellular fluid differs from both plasma and interstitial fluid since it is controlled by the secretory cells.

Measurement of Fluid Compartment Volumes

Compartment volumes are estimated by dilutional techniques. In these methods, an indicator dye that is freely distributed (but contained) within the compartment being estimated, is injected into the compartment. The mass of indicator used and the concentration in the fluid are measured. Then the size of the compartment can be determined using the formula:

In reality, this method calculates the volume of distribution of the injected indicator. The mass of indicator must be corrected for excretion and metabolism during the time allowed for distribution.

Indicator methods used to estimate the volume of various compartments are summarized in Figure FL.4.

MEASUREMENT OF FLUID COMPARTMENT VOLUMES

Compartment

Indicator

Comments

Total body water (TBW)

Antipyrine D2O

Tendency to underestimate uniform distribution

Extracellular fluid (ECF)

Radio isotopes of Na+, Br+, Cl-

These enter cells and so overestimate

Saccharides (mannitol, inulin)

Incomplete distribution and so underestimate

Plasma volume (Vpl)

Radio isotope 131l albumin

Total blood volume (VBL) may be derived from VPL and haematocrit (Hct)

Red cell volume (VRBC)

Red cells tagged with radio isotope 51Cr

Measures fraction of red blood cells tagged to determine VRBC measures concentration of tagged cells to determine VBL

Intracellular fluid (ICF)

Derived from: ICF = TBW - ECF

Interstitial fluid volume (Vint)

Derived from: Vint = ECF - Vpl

Figure FL.4

Figure FL.4

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