Disorders of Water Balance: Hypernatremia
Hypernatremia is defined as a serum or plasma [Na+] > 145 mEq/L and hyperosmolality (serum osmolality > 295 mOsm/kg H2O). Serum [Na+] is determined by total body Na+, K+, and water content: Serum [Na+] = (Nae + +Ke +)/TBW, where Nae + and Ke + are total exchangeable quantities of these cations, and TBW is total body water. Therefore, hypernatremia can develop by a deficit in total body water and/or a gain of Na+. In a healthy individual, an increase in serum [Na+] and associated hyperosmolality create thirst, and water intake lowers serum [Na+] to a normal level (Chap. 11). Hypernatremia develops when patients cannot experience or respond to thirst, and have no access to water.
KeywordsWater balance disorders Hypernatremia Hyperosmolality Nephrogenic diabetes insipidus
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