Skip to Main Content

++

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 comprised of intravascular fluid and extravascular, or interstitial, fluid. Body fluid compartment proportions for an adult are diagrammed in Figure 17-1. Figure 17-2 presents the individual characteristics of each compartment. Three fundamental homeostatic equilibriums govern the behavior of fluids: the osmotic equilibrium, the electric equilibrium, and the acid-base equilibrium.

++
FIGURE 17-1.

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

Graphic Jump Location
++
FIGURE 17-2.

Chemical composition of body fluid compartments. [Reproduced with permission from Brunicardi FC, Andersen DK, Billiar TR, et al (eds): Schwartz's Principles of Surgery, 10th ed. McGraw-Hill, Inc., 2015. Fig 3-2, p. 67.]

Graphic Jump Location
++

The key point is that sodium is much more concentrated in the ECF (~140 mEq/L) than in the ICF (~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.

++
Table Graphic Jump Location
TABLE 17-1Electrolyte Concentrations of Fluids (mEq/L)
++
Table Graphic Jump Location
TABLE 17-2Definition of Terms

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.