The Rate of Urea Excretion. Iii. the Effect of Changes in Blood Urea Concentration on the Rate of Urea Excretion. by T. Addis and C. K. Watanabe

On one side of the urea-secreting cells of the kidney there is blood and on the other side there is urine. The rate of urea excretion is the rate at which these cells transfer urea from blood to urine. Under these circumstances it seems probable that changes in the concentration of urea in the ,blood will have an effect on the rate of urea excretion. If the concentration of urea in the blood were the only factor governing the rate of excretion, it would be possible to obtain a direct measure of its effect by determinations of the rate at different levels of blood concentration. But we have already shown’ that the rate may vary when the blood concentration remains constant,, and that there may be considerable differences in the rate, even when the concentration of urea in the urine as well as in the blood is constant. It is therefore certain that other factors besides the urea concentration of the fluids on either side of the kidney cells exercise a determining influence on the rate of excretion. And unless these other factors can be kept constant, any observed rate of excretion may be higher or lower than that which would have resulted had the blood concentration been the only operative factor. We have not succeeded in framing experimental conditions under which we could be sure that this necessary constancy was attained. This of course was t,o be expected, since some factors which seem likely to influence the rate of work of the kidney