RENAL SECRETION. 647 



mals are killed at various periods after such an injection, indigo carmine 

 may be traced from the blood into the interior of the epithelial cells and 

 from there into the interior of the uriniferous tubules. No trace of this 

 pigment passes through the glomeruli. 



This experiment demonstrates that even -when the blood pressure is 

 greatly reduced the epithelial cells of the kidney do not lose their 

 activity, but are still able to remove substances from the blood and 

 transfer them into the interior of the tubules. 



Certain substances which belong to the group of diuretics produce 

 a flow of urine without at all increasing the blood pressure ; such sub- 

 stances are urea, urates, etc. It follows that if the blood pressure has 

 not been increased, or, in fact, may even have been decreased, and yet 

 the flow of urine not be interfered with, some other portion of the 

 kidney besides the glomeruli is concerned in the separation of the water 

 together with the other constituents of the urine. 



It is capable of demonstration that urea passes from the blood into 

 the renal secretion through the activit}^ of the renal cells. 



In amphibious animals the kidney receives a supply of blood from 

 the renal artery and also from the renal portal system, which is formed 

 by a branch of the femoral vein which joins its fellow from the opposite 

 side to form the anterior abdominal vein. 



The renal artery alone supplies the glomerulus ; the renal portal 

 vein alone supplies the uriniferous tubules. If the renal artery be tied, 

 the blood is, of course, shut off from the glomeruli, and all filtration is 

 thus prevented. Urea, nevertheless, is still a constituent of the secre- 

 tion formed by such a kidney, and when urea is injected into the blood 

 it likewise causes a secretion of urine. 



On the other hand, substances which are presumably removed from 

 the blood by a process solely of filtration, viz., sugar, peptones, and vari- 

 ous salts, do not appear in the urine after the renal artery has been tied. 



It is thus evident that the secretion of urine is a double process, 

 partly a process of filtration, in which water and crystalline substances 

 are removed from the blood b}^ a process of transudation occurring in 

 the glomeruli. Everything, therefore, which increases blood pressure in 

 the renal arteries will lead to transudation and to an increase in the 

 watery constituents of the urine. 



The renal secretion is also an active secretory process in which the 

 epithelial cells lining the convoluted portions of the uriniferous tubules 

 are concerned in removing the specific constituents of the urine from the 

 blood, while perhaps completing the process of transformation of some of 

 the antecedents of urea into that substance. This subject will again be 

 referred to from this point of view when we consider the problems of 

 nutrition which occur in the animal body. 



