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  • Fluid and electrolyte homeostasis is maintained by feedback mechanisms, hormones, and many organ systems, and is necessary for the body’s normal physiologic functions. Disorders of sodium and water, calcium, phosphorus, potassium, and magnesium homeostasis are addressed separately in this chapter.


  • Total body water (TBW) ranges from 45% to 60% of body weight depending on sex and age and is distributed primarily into two compartments: intracellularly (ICF; two-thirds [67%] of TBW), and one-third (33%) is contained in the extracellular space.

  • Effective osmoles are solutes that cannot freely cross cell membranes, such as sodium and potassium. Addition of an isotonic solution to the extracellular fluid (ECF) does not change ICF volume because there is no change in effective ECF osmolality. Adding a hypertonic solution to the ECF decreases ICF volume, whereas adding a hypotonic solution increases it. Table 76-1 summarizes the composition of commonly used IV solutions and their expected distribution into the ECF and ICF compartments.

  • Hypernatremia and hyponatremia can be associated with conditions of high, low, or normal ECF sodium and volume. Both conditions are most commonly the result of abnormalities of water metabolism. It’s important to understand the difference between dehydration (loss of TBW producing hypertonicity) and hypovolemia (volume depletion due to a symptomatic deficit in ECF volume).

TABLE 76-1Composition of Common IV Solutions

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