The Mechanism of Formation of Osmotically Concentrated Urine During the Antidiuretic State 1

The present study is concerned with the opera- tion by which the kidney, during antidiuresis, conserves water for the body by elaborating a urine osmotically more concentrated than the blood.It is appropriate to emphasize that in the mam- mals the osmotic pressure of the plasma and the interstitial fluid is one of the most closely guarded of all the homeostatic states, and that the kidney is chiefly responsible for the regulation of this os- motic pressure within narrow limits.Osmotic dilution of the plasma is normally offset by the excretion of a urine osmotically more dilute than the plasma, as in water diuresis, while an increase in osmotic concentration of the plasma is offset by the excretion of urine osmotically more concen- trated than the plasma.The present paper is concerned only with the mechanism of the excretion of a hypertonic urine.That no osmotic concentration is achieved in the separation of the glomerular filtrate is established, to the satisfaction of all investigators, by the well known micropuncture studies of Richards and his coworkers in the Amphibia, and Walker, Oliver, and their coworkers in the guinea pig, rat and opossum.These studies have also demonstrated, at least under the recorded experimental conditions, that the urine remains isosmotic with the plasma, ,or nearly so, throughout the length of the proximal segment, at a time when the bladder urine may be concentrated osmotically to a considerable extent.Unfortunately, such icropuncture studies as are