672 SECRETION OF UREA. [BOOK n. 



no accumulation in the blood of either bile acids or bile pigment. 

 With regard to the kidney and the most important constituent of 

 urine, namely urea, the case is different. If the kidneys in a 

 mammal be extirpated, or if the kidneys by disease or by ligature 

 of the ureters be so damaged as to be unable to carry on their 

 work, an accumulation takes place in blood, not as was once thought 

 of some antecedent of urea such as kreatin, but of urea itself. 

 In the case of birds and reptiles which excrete not urea but chiefly 

 uric acid the accumulation is one of uric acid. Obviously in 

 secreting urea the work of the epithelium of the tubules is largely 

 if not exclusively confined to simply picking the urea out of the 

 blood and pushing it so to speak into the lumina of the tubules. 

 We might perhaps say exclusively, for there is no evidence that 

 any urea at all is actually manufactured in the kidney. 



But even this mere picking up the urea is after all not a 

 simple process; the epithelial cell of the tubule is not a mere 

 passive sieve of peculiar structure especially adapted to strain off 

 the urea from the blood. As we have already seen, when urea or 

 uric acid is injected into the blood the result is not a mere 

 increase in the proportions of urea (or uric acid) present in the 

 urine which is being secreted. The injection leads to an increased 

 flow of urine, the whole activity of the cell is stirred up, and other 

 constituents, not at the moment like the urea existing in excess in 

 the blood, are discharged into the lumina of the tubules together 

 with the urea. 



How the urea, which is in this peculiar manner taken out of 

 the blood, comes to make its appearance in the blood is a problem 

 in which the kidney is not concerned and with which we shall 

 deal in treating of the metabolic events of the body generally. 



419. In the case of some other constituents of the urine we 

 have evidence that the cells do something more than simply pick 

 the constituent out of the blood. Hippuric acid, as we have seen, 

 occurs in small quantity in the urine of man, and in larger amount 

 in the urine of herbivora. Now hippuric acid may be formed by 

 the combination, with dehydration, of benzoic acid and glycin 

 (C 7 H 6 O 2 + C 2 H 5 N0 2 -H 2 = C 9 H 9 N0 3 ); and benzoic acid intro- 

 duced into the alimentary canal or injected into the blood, 

 reappears in large measure in the urine as hippuric acid. Some- 

 where in the body the benzoic acid meets with and combines 

 with glycin. And we have experimental proof that the com- 

 bination may and probably does take place in the kidney. 



If a circulation of blood be kept up through the blood vessels 

 of the kidney freshly removed from a living animal, and benzoic 

 acid and glycin be added to the blood as it is about to enter 

 into the kidney, hippuric acid will be found in the blood issuing 

 from the kidney, especially if the same blood be passed through 

 the kidney several times ; the blood used must be blood containing 

 oxyhsemoglobin, carbonic -oxide -haemoglobin not producing the 



