THE SECRETION OF URINE 481 



urinary tubules, and ligation of this vein affects but little the amount 

 of urine secreted. On the other hand, ligation of the renal arteries 

 which supply the glomeruli causes a cessation of the urinary flow. As 

 a, small amount of secretion may then be excited by the injection of 

 a diuretic, it is concluded that the tubules can secrete some water. 



Th3 Secretory Function of the Tubule. If the kidney tubules are 

 damaged by a poison, and a solution of sodium chloride added to 

 the blood, the excess of chloride quickly passes into the urine. On 

 the other hand, if urea is added to the blood, it is not excreted. 



This experiment indicates that the chief secretory function of the 

 tubules is to add the waste nitrogenous products to the fluid separated 

 from the glomerulus. Confirmatory evidence has been obtained by 

 injecting into the blood of an animal an organic dye, such as 

 indigo-carmine, which is secreted in the urine. The site of secretion 

 can only be ascertained after stopping the glomerular secretion of 

 water. This is effected by the fall of blood-pressure which follows a 

 section of the spinal cord. Under these circumstances, the convoluted 

 portions of the tubules of the kidney are found filled with pigment 

 granules. In the bird, after ligation of the ureter, there follows a 

 deposition of urates (which correspond to the urea of the mammal) 

 within the cells of the tubules. Uric acid and its salts can be stained 

 with silver nitrate, and demonstrated within the cells of the tubules, 

 the stain being developed by a solution of hydroquinone. 



Vacuoles akin to excretory vacuoles have been described in the 

 cells of the convoluted tubules. These gradually grow in size, and 

 eventually void their watery and granular contents into the lumen 

 of the tubule. In thirsty animals fed on dry food the cells fill up 

 most of the lumen, and are full of granules. After diuresis, the cell 

 are shrunken and the lumen is wide. 



Resorption by the Renal Tubules. The evidence so far adduced 

 in favour of resorption is far from conclusive. It is claimed that, 

 after removal of the medulla of the kidney, there tends to be secreted 

 a urine which is much more watery. It is also claimed that such a 

 function is indicated by notable differences in the rate of secretion 

 of two salts when injected into the blood in equal amounts of their 

 equivalent solutions e.g., of NaCl and Na 2 S0 4 . The difference is 

 equally well explained by a selective secretory activity. The kidney 

 has the special function of turning out from the blood foreign salts, 

 and of keeping constant the concentration of normal salts in the 

 blood. 



" Pigment casts " have been found in the collecting tubules after 

 the injection of carmine into the circulation, and this may point to 

 a coneentration of the urine, possibly in the second convoluted 

 tubules. Only a little carmine is to be found in the first convoluted 

 tubules, for the glomerular secretion of water washes it on as it is 

 secreted there. This is the best evidence so far adduced in favour of 

 resorption within the urinary tubule. 



The concentration of urea in the blood is 0-5 to 0-6 per inille, 



31 



