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 (Cuslmy). 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 glomerulus will 

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

 the passage of this nitrate 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 hap- 

 pens 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 assumption that water is actively ex- 

 creted, 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 nec- 

 essary to restore the plasma to the optimal concentration, 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. 



Other experimenters have attempted to show absorption in the tubules by injecting 

 diffusible substances, such as chemicals and dyes, into the ureter under what they 

 deemed sufficient pressure to force the solution into the tubules, and by an examination 

 of the blood or the tissues to determine whether or not the injected substances had been 

 absorbed. The results obtained by this method are not convincing, probably chiefly 



