[Experimental studies on the specific weight and osmotic activity of nephrologically important solutions]

The concentrating power of kidney is examined with respect to the excretion of osmotically active particles in terminal urine. Results of osmometric measurements on 14 solutions of physiological significance reveal that with increasing concentration there is some asymptotic lowering of the osmotic activity as a result of increasing physicochemical interactions. The specific gravity, however, increases in constant proportion. From these in vitro studies it can be concluded that by measurement of the osmolality in terminal urine, the complex renal elimination is reflected in approximation only. The interactions which can also be detected in "chemically pure" solutions, influence the osmolality even in primary urine, and even stronger during concentration to terminal urine. Nevertheless, osmometry of terminal urine represents the elimination capacity of kidney essentially better than other concentration measures like molality or specific gravity of urine.