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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.


  • Two-thirds (67%) of total body water (TBW) is distributed intracellularly (ICF), and one-third (33%) is contained in the extracellular space.

  • Addition of an isotonic solution to the extracellular fluid (ECF) does not change intracellular volume. Adding a hypertonic solution to the ECF decreases cell volume, whereas adding a hypotonic solution increases it. Table 75–1 summarizes the composition of commonly used IV solutions and their respective 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.

TABLE 75–1abcComposition of Common IV Solutions

HYPONATREMIA (SERUM SODIUM <135 mEq/L [<135 mmol/L])


  • Results from an excess of extracellular water relative to sodium because of impaired water excretion.

  • Causes of nonosmotic release of arginine vasopressin (AVP), commonly known as antidiuretic hormone, include hypovolemia; decreased effective circulating volume as seen in patients with congestive heart failure (CHF); nephrosis; cirrhosis; and syndrome of inappropriate antidiuretic hormone (SIADH).

  • Depending on serum osmolality, hyponatremia is classified as isotonic, hypertonic, or hypotonic (Fig. 75–1).

  • Hypotonic hyponatremia, the most common form of hyponatremia, can be further classified as hypovolemic, euvolemic, or hypervolemic.

  • Hypovolemic hypotonic hyponatremia is associated with a loss of ECF volume and sodium, with the loss of more sodium than water. It is seen in patients with diarrhea or in those taking thiazide diuretics.

  • Euvolemic hyponatremia is associated with a normal or slightly decreased ECF sodium content and increased TBW and ECF volume. It is most commonly the result ...

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