Extracellular Fluid Volume

To determine the volume of the ECF compartment, a substance is used that can readily permeate capillary walls but not cell membranes (Fig. 5B). The most commonly employed substance for this purpose is a polyfructoside, inulin, which has a molecular weight of approximately 5500 and a radius of approximately 15 A, or 1.5 nm. This substance is inert and nontoxic. (Another important use of inulin is discussed in Chapter 26.)

The only minor complication that arises in determining the size of the ECF compartment (illustrated in Fig. 5B), stems from the fact that substances that can readily permeate the capillary wall are readily filtered by the renal glomeruli and may be excreted; this is particularly true for inulin, which, after filtration, is excreted completely. Thus, in applying the principle of conservation of mass, one must take into account the amount of the substance lost from the body in the course of the measurement.

Example: An individual's urinary bladder is cathe-terized and drained, then 2.4 g of inulin is injected intravenously; 6 hours later, a venous sample is withdrawn and found to have a plasma inulin concentration of 0.1 mg/mL. During those 6 hours, 200 mL of urine was produced having an inulin concentration of 5 mg/ mL. Thus, 1.0 g of the original 2.4 g of inulin injected was lost. It follows that the ECF volume is given by:

In this case, therefore,

of water. Fortunately, however, only approximately 2 hours are needed for them to achieve a uniform distribution in the TBW and, during that period, the amount of water lost by the body is less than 1% of the TBW.

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Get Rid of Gallstones Naturally

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