Chronic Hyperosmolality

Abstract

AN ELEVATION of the serum sodium concentration above 145 mEq/liter or the serum osmolality above 290 mOsm/liter of water indicates insufficient water for the amount of solute in the body. As serum sodium concentration is regulated by the thirst-neurohypophyseal-renal axis, hypernatremia always suggests dysfunction of one or more components of this regulatory system. Hypernatremia may be caused by fluid deprivation or by excess solute intake. Normally these circumstances result in intense thirst, increased antidiuretic hormone (ADH) production, and the elaboration of a small quantity of highly concentrated urine. In both man and lower animals an increase of only 1% or 2% in the effective solute concentration of the serum is sufficient to stimulate maximal release of ADH,¹ which acts on the renal tubules to produce maximal reabsorption of water and maximal water conservation. When, in response to the thirst induced by hypernatremia, water is ingested, the resulting dilution of