Inasmuch as capillaries and most plasma cell membranes are highly permeable to water, the osmolarity of the plasma, ISF, and ICF compartments are equal and have a value of approximately 295 mOsm/L. The pH of the ECF compartment is approximately 7.4, whereas, because of metabolic activities, that of the intracellular

There is one simple principle underlying all approaches to measuring the sizes of the fluid compartments—namely, the principle of conservation of matter. As illustrated in Fig. 4, if a certain quantity (Q) of some substance is injected into a fluid compartment having a volume (V), then, when an equilibrium (or steady-state) distribution is achieved, the concentration of that substance in the container will be Q/V.

TABLE 1 Important Components of Extracellular and Intracellular Fluid Compartments

Osmolarity (mOsm/L) 295 295

aThese are reasonably approximate values for the plasma and interstitial fluid compartments. As noted in the text, because of differences in protein concentration, these two compartments display small differences in ion concentration.

bThese are approximate values for many cell types.

If we know Q and can determine the value of Q/V, it is a simple matter to determine V, the volume of distribution.

Many substances can be employed to determine the sizes of the body fluid compartments. The essential characteristics of all are that they (1) are nontoxic at the concentrations employed, (2) are neither synthesized nor metabolized, and (3) do not bring about shifts in fluid distribution among the compartments.

Plasma Volume

The simplest of all of the body fluid compartments to measure is the plasma volume (PV). A substance is used that does not readily permeate capillary walls so that,

Extracellular3 Intracellular

FIGURE 4 Principle underlying all methods for measuring the volumes of the body fluid compartments.

In this case, therefore,

350,000 100 cpm/L

Given these data, one can also determine the total blood volume employing a measure known as the hematocrit, which is defined as the fraction of the total blood volume comprised of cells, mainly red blood cells. In this case, the hematocrit (Hct) is

The total blood volume (TBV) is given by:

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