Blood osmolality in healthy persons is maintained within narrow limits by a series of mechanisms that are described in detail in Chapter 25. Adjustments in water balance determine the constancy of blood osmolality, which is mediated by delicate alterations in thirst appreciation (with consequent promotion of drinking) plus the enormous capacity of the kidney to alter urine flow rates and urine osmolality in response to relatively small changes in the plasma vasopressin concentration (see Chap. 206). Thus, healthy humans are able to conserve their osmotic internal milieu despite extremes in climatic conditions, sustained severe exertion, or, to a certain degree, an inadequate supply of water.