Key Points

The kidneys maintain the normal constant osmolality of body fluids and normal constant body water content.

Because the hydraulic conductivity of most cell membranes is high, all body fluid compartments have approximately the same osmolality. The nonapeptide hormone vasopressin (or antidiuretic hormone, ADH) regulates the water permeability of the collecting duct and thus controls the osmolality of the urine. Osmoreceptors in the supraoptic and paraventricular nuclei of the hypothalamus respond rapidly to an increase in plasma osmolality (Posm) and increase the secretion of vasopressin from their nerve endings in the posterior hypothalamus. Thirst receptors in the hypothalamus also respond to increased Posm and lead to the sensation of thirst. In the absence of vasopressin, the luminal membrane of the collecting duct is water impermeable. Continued solute reabsorption without water can reduce the urine osmolality to 50 mOsm/kg H2O. In the presence of high vasopressin, the collecting duct is very water permeable and the urine osmolality approaches 1200 mOsm/kg H2O. Osmolal clearance (Cosm) is the clearance of all solutes calculated as (UF • Uosm)/Posm. Typically, it is in the range of 1.5-2.5 mL/min. Free water clearance (CH2O) is a measure of the

Essential Medical Physiology, Third Edition

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