An economy of water in renal function referable to urea.

The results of a study in this laboratory of t.he quantit.ies of water required for the removal in urine of urea and of several salts were published several years ago (1). The plan of t,he experiments was extremely simple. The animals used were rats. Crea or salt was added to a basal maintenance diet in progressive increments and the int.ake of the added substance and the accompanying quantit,y of water drunk by t.he animals were measured. The water intake was assumed to be applied entirely to the removal of t,he ingested materials in the urine, the additional factors of water expenditure, which were relatively very small in the presence of the large quantities of material claiming excretion in urine, being disregarded. The results were presented as concent#ration values for the ingested substance referred to the volume of the water intake and were regarded as approximately describing the actual concentration in the urine of the subst.ance or subst.ances under investigation. The data thus obt.ained showed clearly that the water requirements for the removal in urine of equivalent osmolar quantities of t.he several salt-s studied is closely identical, and that, much less water is required for the removal of a corresponding quantity of urea. It was also found that, when mixt.ures of sa1t.s are fed, their individual water requirements remain additive. When, however, mixt.ures of urea and salt are fed, it was found that much less water is required for the removal of the two substances together in urine than is prescribed by their water requirements as separately determined. This curious finding indicating an economy of water in t*he excret#ion in urine of mixtures of materials containing urea was regarded as deserving further description by a more accurate method of study. To this end experiments were undertaken in which the data were obtained directly from t-he urine. The results of these experiments are presenttied in this paper.