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interstitial fluid and in the plasma is considered to be about equal.

Referring again to Figure 25-2, one can see that the extracellular fluid, including the plasma and the interstitial fluid, contains large amounts of sodium and chloride ions, reasonably large amounts of bicarbonate ions, but only small quantities of potassium, calcium, magnesium, phosphate, and organic acid ions.

The composition of extracellular fluid is carefully regulated by various mechanisms, but especially by the kidneys, as discussed later. This allows the cells to remain continually bathed in a fluid that contains the proper concentration of electrolytes and nutrients for optimal cell function.

Important Constituents of the Intracellular Fluid

The intracellular fluid is separated from the extracellular fluid by a cell membrane that is highly permeable to water but not to most of the electrolytes in the body.

In contrast to the extracellular fluid, the intracellular fluid contains only small quantities of sodium and chloride ions and almost no calcium ions. Instead, it contains large amounts of potassium and phosphate ions plus moderate quantities of magnesium and sulfate ions, all of which have low concentrations in the extracellular fluid. Also, cells contain large amounts of protein, almost four times as much as in the plasma.

Measurement of Fluid Volumes in the Different Body Fluid Compartments—The Indicator-Dilution Principle

The volume of a fluid compartment in the body can be measured by placing an indicator substance in the compartment, allowing it to disperse evenly throughout the compartment's fluid, and then analyzing the extent to which the substance becomes diluted. Figure 25-4 shows this "indicator-dilution" method of measuring the volume of a fluid compartment, which is based on the principle of conservation of mass. This means that the total mass of a substance after dispersion in the fluid compartment will be the same as the total mass injected into the compartment.

In the example shown in Figure 25-4, a small amount of dye or other substance contained in the syringe is injected into a chamber, and the substance is allowed to disperse throughout the chamber until it becomes mixed in equal concentrations in all areas. Then a sample of fluid containing the dispersed substance is removed and the concentration is analyzed chemically, photoelectrically, or by other means. If none of the substance leaks out of the compartment, the total mass of substance in the compartment (Volume B x Concentration B) will equal the total mass of the substance injected (Volume A x Concentration A). By simple rearrangement of the equa-

Dye Dilution Method Calculation
Indicator-dilution method for measuring fluid volumes.

tion, one can calculate the unknown volume of chamber B as

Volume B =

Volume A x Concentration A Concentration B

Note that all one needs to know for this calculation is (1) the total amount of substance injected into the chamber (the numerator of the equation) and (2) the concentration of the fluid in the chamber after the substance has been dispersed (the denominator). For example, if 1 milliliter of a solution containing 10mg/ml of dye is dispersed into chamber B and the final concentration in the chamber is 0.01 milligram for each milliliter of fluid, the unknown volume of the chamber can be calculated as follows:

0.01 mg/ml

This method can be used to measure the volume of virtually any compartment in the body as long as (1) the indicator disperses evenly throughout the compartment, (2) the indicator disperses only in the compartment that is being measured, and (3) the indicator is not metabolized or excreted. Several substances can be used to measure the volume of each of the different body fluids.

Determination of Volumes of Specific Body Fluid Compartments

Measurement of Total Body Water. Radioactive water (tritium, 3H2O) or heavy water (deuterium, 2H2O) can be used to measure total body water. These forms of water mix with the total body water within a few hours after being injected into the blood, and the dilution

Table 25-3

Measurement of Body Fluid Volumes volume can also be calculated if one knows the hematocrit (the fraction of the total blood volume composed of cells), using the following equation:

Volume

Indicators

Total body water

3H2O, 2H2O, antipyrine

Extracellular fluid

22Na, 125I-iothalamate, thiosulfate, inulin

Intracellular fluid

(Calculated as Total body water -

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Essentials of Human Physiology

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