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FLUIDS AND SODIUM

PATHOPHYSIOLOGY

Total body water (TBW), which accounts for approximately 60% of total body weight, can be divided into intracellular fluid (ICF) and extracellular fluid (ECF) compartments. The ECF is composed of intravascular fluid and extravascular, or interstitial, fluid. Three fundamental homeostatic equilibriums govern the behavior of fluids: the osmotic equilibrium, the electric equilibrium, and the acid-base equilibrium.

The key point is that sodium is much more concentrated in the ECF (approximately 140 mEq/L) than in the ICF (approximately 10 mEq/L), but is equal in both compartments of the ECF because the capillary membrane between intravascular fluid and interstitial fluid is permeable to water and electrolytes. In contrast, the cell membrane is permeable to water but not to electrolytes, which are moved through ionic pumps against gradient to keep the intracellular sodium concentration constant around 10 mEq/L and potassium at 150 mEq/L. Table 17-1 lists the electrolyte concentration of body fluids and the most commonly used therapeutic solutions. Table 17-2 defines commonly used terms that describe measures or characteristics of electrolytes and/or disorders.1

TABLE 17-1Electrolyte Concentrations of Fluids
TABLE 17-2Definition of Terms

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