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The total body water (TBW), which accounts for approximately 60% of total body weight, can be divided into intracellular and extracellular fluid (ICF and ECF, respectively) compartments. The ECF is comprised of intravascular and interstitial (extravascular) fluid. Body fluid compartment proportions for an adult are diagrammed in Figure 21-1. Typical concentrations of the most physiologically important anions and cations of the fluid compartments, as well as their concentrations in commonly used IV fluids, are listed in Table 21-1.

Figure 21-1.

Relation of fluid compartment to body weight and each other. ECF = extracellular fluid; ICF = intracellular fluid; IF = interstitial (extravascular) fluid; IVF = intravascular fluid; TBW = total body water.

Table 21-1 Electrolyte Concentrations of Fluids (mEq/L)


Concentrations are often expressed in moles, equivalents, or osmoles per liter. These terms are defined in Table 21-2.

Table 21-2 Definition of Terms

1 Eq of [Na+] is equal to 23 grams/1, or 23 grams, whereas 1 Eq of [Ca2+] is equal to 40 grams/2, or 20 grams. For example, 1 mol of NaCl dissolves to produce 1 mol of [Na+], and 1 mol of [Cl], for a total of 2 mol of osmotically active particles. Therefore, 1 osm of NaCl is 0.5 mol of NaCl.

The serum osmolality can be measured directly by determining the freezing point of the serum, because 1 mol of solute will lower the freezing point of 1 kg of water by 1.86°C (3.35°F). Osmolarity can be estimated by adding the measured [Na+], [Cl...

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