OBSERVATIONS ON THE CONCENTRATING AND DILUTING MECHANISMS OF THE DISEASED KIDNEY*

Abstract

The concentrating and diluting mechanisms of the normal kidney contribute in an impressive manner to the defense of water economy. In health, a given amount of solute may be excreted in a widely varying volume of water which is determined under most circumstances by the water needs of the organism. This ability to regulate the osmolality of the urine accounts for a range of urine concentrations which extends from marked hypotonicity to marked hypertonicity.1 In the course of advancing renal disease, the total amount of solute excretion per 24 hours may remain comparable to values achieved in health. It is a clinical observation of long standing, however, that the patient's ability to vary the volume of water in which this solute is contained becomes progressively limited. Hence, the range over which urine tonicity may be extended becomes more and more restricted. Typically, this restriction is manifested initially by a decrease in the maximum attainable urine osmolality, followed somewhat later by a progressive inability to decrease the tonicity of the urine. Ultimately, urine tonicity deviates little in either direction from the concurrent plasma osmolality, and the stage of so-called "permanent isosthenuria" or "fixed specific gravity" supervenes.