550 THE EXCRETION OF URINE 



Among the experiments that have been offered in support of the absorption of fluid 

 and salts by the tubules, are those in which the pressure of the urine in the tubules is 

 slightly increased by partial closure of the ureter (Cushny). In these experiments the 

 ureter of one kidney is partly closed with a clamp and the excretion obtained from 

 this kidney is compared with that of the opposite normal kidney. Considerable ob- 

 struction of the ureter results in a decrease in the amounts of water, chloride and 

 urea excreted, but the urea content is decreased relatively less than is the chloride 

 and water content. These results can be explained on the basis that a pressure that 

 is sufficient to oppose the head of pressure producing filtration in the giomerulus will 

 reduce the amount of the glomerular filtration, and accordingly the time allowed for 

 the passage of this filtrate along the tubules is increased and absorption becomes more 

 complete. Since urea is probably not absorbed at all and chloride is, the discrepancy 

 in the effects on the excretion of urea and chlorine in the partially obstructed kidney 

 can be explained. When the obstruction of the ureter is only slight, however, opposite 

 results to that just mentioned are obtained (Brodie and Cullis). This observation is 

 difficult to harmonize with the reabsorption hypothesis. 



When very large amounts of water are taken by mouth, it often happens that the 

 urine excreted has a concentration of salts less than that present in the fluid of the 

 blood. Some investigators believe that such a condition is possible only on the as- 

 sumption that water is actively excreted, but a more plausible explanation based on 

 the modern theory is that the water that is absorbed from the alimentary tract reaches 

 the kidney as a dilute saline solution, and is rapidly filtered off in a form somewhat 

 more dilute than the optimal solution which blood plasma must have for the well-being 

 of the tissues. The tubules reabsorb the amounts of water and of substances, such as 

 chlorides and sugar, that are necessary to restore the plasma to the optimal concentra- 

 tion, but they do not reabsorb the nonthreshold substances, such as sulphates and urea. 



Many attempts have been made, by destroying the capsules or the 

 tubules by means of poisons or by operation, to determine directly or 

 indirectly the question of the function of the tubules. In such experi- 

 ments, however, the number of factors involved confuses the issue and 

 makes the results practically valueless so far as determining the normal 

 function of the tubules. Progress is however to be expected in the 

 near future since Richards and \Yeani have succeeded in developing a 

 method by which sufficient jfiltraU 1 can be removed from the capsule of 

 Bowman in living frogs, to make inicrocheinical analyses possible. 



Most important results supporting the reabsorption hypothesis have 

 already been obtained in the case of glucose and chlorides. Thus, when 

 glucose was injected subcutaneously some appeared in the capsular 

 filtrate but none in the bladder urine; likewise chloride was found in 

 the filtrate but none in the bladder. 19 



