The American Journal of Physiology

The terms of the “modern” theory of renal function, as presented by Cushny (l), suggest an attractively simple explanation and definition of the water requirement. According to this theory, as glomerular filtrate passes along the renal tubules water and materials are reabsorbed from it by the tubule cells in such quantities as will sustain the normal composition of the blood plasma. If, as is usually the case, relatively more of water than of substances must be returned to the plasma, reabsorption is accomplished against a rising osmotic pressure in the tubular fluid. It is an easy surmise that, if circumstances require excretion of a large amount of substances with relatively little water, a level of osmotic pressure may be reached which will prevent further withdrawal of water from the tubular fluid, with the result that urine of the maximal possible concentration is produced. Obviously there is here the important implication that the minimal water requirement for excretion of the various substances in urine must be, per molecular amount, the same for each of them. This hypothesis agrees with Ambard’s conception of an identical “volume obligatoire” for the individual substances based on an approximate equivalence of the highest concentrations of urea, of glucose and of sodium chloride observed in urine. Ambard’s data were not extensive nor was their agreement of a convincing degree. Although subsequent investigations of this point have been numerous, information of a satisfactory precision is still lacking. It may be said however, in brief appraisement, of the evidence at hand that roughly the same values for maximal concentration in urine have been found for NaCl and NaHC03, and that this value probably also obtains for a mixture of these salts (2), that maximal concentrations determined for urea are in average somewhat higher than is found for NaCl (3), that urea and NaCl 571