potassium and chloride; combines with other ions to maintain acid-base balance; and is essential for impulse transmission of nerve and muscle fibers.

Hyponatremia is the most common of all electrolyte disorders. As serum sodium decreases, water in the ECF moves into the cells. There is less sodium available to move across an excitable membrane, which results in delayed membrane depolarization. Central nervous system (CNS) cells are most likely to be affected by these changes. Four different manifestations of hypona-tremia have been described based on the ratio of total body water (TBW) to total body sodium: hypovolemic hyponatremia, hypervolemic hyponatremia, euvolemic hyponatremia, and redistributive hyponatremia (Table 6).

• TABLE 6 Types and Causes of Hyponatremia
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