THE MECHANISM OF FORMATION OF OSMOTICALLY CONCENTRATED URINE DURING THE ANTIDIURETIC STATE 1

Abstract

The urinary concentrating process was examined in 54 tests on 31 normal subjects in the antidiuretic state by the induction of osmotic (mannitol) diuresis at urine flows ranging from 5 to 40 ml./min. Antidiuresis was induced either by abstinence from fluids for 12 to 14 hrs. (hydropenia), by hydropenia supplemented by Pitressin, or by the admn. of Pitressin to hydrated subjects. The degree of osmotic concn. of the urine was determined by application of the cryoscopic method to plasma and urine, and the calculation of the quantity of water (in ml./min. and here designated as TCH2O) required to restore the urine to an isosmotic state with respect to the glomerular filtrate. The conclusion is reached that the concentrating mechanism operates essentially by the removal of a constant max. quantity of solute-free water irrespective of the urine flow so long as the latter somewhat exceeds this max. reabsorptive rate. It is believed that unknown factors operate systematically in some individuals or under certain circumstances to cause slight, regular increases or decreases in TcH2O with increasing diuresis. This concentrating operation appears to be generally identical whether the anti-diuretic state is induced by endogenous antidiuretic hormone, by Pitressin in physiological doses, or both.