518 



THE EXCRETION OF URINE 



water and perhaps of some of the dye from the dilute glomerular filtrate, 

 as to accept Heidenhain's hypothesis. 



In the following table taken from Cushny the movements of the con- 

 stituents of the plasma may be followed through the kidney. The ulti- 

 mate destination of each is indicated in the enclosures. 



(From Cushny.2) 



It will be noted that the. dextrose alone is completely absorbed, and 

 that the urea and the sulphate are not absorbed at all from the glom- 

 erular filtrate. The other salts are partly absorbed. 



As already mentioned, Barcroft and Straub have shown that the 

 diuresis which results from the injection of saline into the blood is not 

 accompanied by any increase in the oxygen consumption of the kidney. 

 This observation, coupled with the fact that the total amount of chloride, 

 urea, and sulphate which is excreted during saline diuresis, is greater than 

 under normal conditions indicates that the excretion of these salts is 

 not due to any vital secretory power of the kidney, but rather to factors 

 that are extrarenal in origin. 



The diuresis produced by adding urea or sodium sulphate to the blood, 

 on the other hand, is accompanied by an increase in the oxygen con- 

 sumption of the kidney. This increase can not be due to active elimina- 

 tion of these salts by the tubules, the work of which requires oxygen, 

 for no increase in oxygen consumption accompanies the increased ex- 

 cretion of the same salts under saline diuresis. Sulphate and urea are 

 nonthreshold substances, and are not absorbed by the tubules. The 

 explanation of the oxygen consumption is probably that the osmotic 

 pressure which these bodies in the glomerular filtrate exert makes it 

 necessary for the epithelium to oppose a greater absorbing force to con- 

 centrate the urine, and hence a greater expenditure of energy is requird. 



Diuretics. The action of the xanthine compounds caffeine, theo- 

 bromirie and theophylline in the production of diuresis is unexplained. 



