516 THE EXCRETION OF URINE 



are present is entirely -different from that in the blood plasma or in the 

 glomerular filtrate. This is shown in the following figures, which give an 

 average normal value for the urea, uric acid, chlorine, and glucose in 100 

 c.c. of protein-free blood plasma and 100 c.c. of urine. In the third col- 

 umn is given the change in concentration which has occurred in the 

 kidney. 



Here the blood plasma fluid contained but 0.033 per cent of urea, and 

 the urine 2 per cent. Accordingly, 6 liters of glomerular filtrate would 



f 2 

 be required to furnish 100 c.c. of urine, \- - = 6000). Six liters of 



I 0.33 



glomerular filtrate would contain 6.6 grams of sugar, 0.132 grams of 

 uric acid, and 24.6 grams of chlorine. But 100 c.c. of urine contains no 

 glucose, 0.05 grams of uric acid and 0.6 grams of chlorine. According 

 to the modern theory, these figures indicate that during the passage of the 

 urine through the tubules 5900 c.c. of water, 6.6 grams of sugar, 24 grams 

 of chlorine and 0.067 grams of uric acid would have to be absorbed by 

 the renal epithelium in the production of 100 c.c. of urine containing 

 the concentration given above. 



Among the most convincing experiments that can be offered in sup- 

 port 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. 

 In general, obstruction of the ureter results in a decrease in the amounts 

 of water, chloride and urea excreted. But, curiously, the urea content is 

 decreased relatively less than is the chloride and water content. These 

 results can be explained on the basis that any pressure acting 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 filtrate along the tubules is increased and absorp- 

 tion 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 very large amounts of water are taken by mouth, it often hap- 



