CHAP. HI.] ELIMINATION OF WASTE PRODUCTS. 665 



the kidney, that is with a greater flow of blood through it, and 

 diminished or arrested flow of urine is coincident with shrinking 

 of the kidney, that is with a diminished flow of blood through it. 



A striking instance of this is afforded by the experiment of 

 dividing in the dog the spinal cord below the medulla. The 

 blood-pressure then, as we know, falls rapidly, owing to the 

 removal of constrictor impulses from the small arteries and the 

 great diminution of peripheral resistance which follows upon so 

 many small arteries becoming dilated ; and though the renal 

 arteries probably share in the general relaxation yet, owing to the 

 fall of pressure in the aorta conjoined as this is by a corresponding 

 rise of pressure in the vena cava, the flow of blood through the 

 kidney is largely diminished. We find that after the operation 

 the secretion of urine is greatly diminished ; indeed, in most cases, 

 the flow from the end of a cannula is almost arrested. In fact we 

 may almost make the general assertion that, when in the dog the 

 blood-pressure falls to about 30 mm. Hg or less, the secretion of 

 urine is for the time stopped. These and other results support 

 the view stated above that the secretion of urine is in quite a 

 special way dependent on the flow of blood through the kidney ; 

 and we may further conclude that the secretion which is so 

 particularly influenced by the flow of blood is that special kind of 

 secretion, allied to filtration, which takes place through the 

 glomeruli, and not the more ordinary kind of secretion by means 

 of the epithelium of the tubuli uriniferi. But before we proceed 

 to discuss how the increased flow of blood increases the glomerular 

 flow of urine, we must turn to consider the functions of the 

 epithelium of the tubuli. 



Secretion by the Renal Epithelium. 



416. The glomerular mechanism is after all a small portion 

 only of the whole kidney, and the epithelium over a large part of 

 the course of the tubuli uriniferi bears most distinctly the characters 

 of an active secreting epithelium. These facts would lead us a 

 priori to suppose that the flow of urine is in part the result of an 

 active secretion comparable to that of the salivary or other glands 

 which we have already studied. And we have experimental and 

 other evidence that such is the case. 



In the first place a flow of urine may be artificially excited 

 even when the natural flow has been arrested by diminution of 

 blood-pressure. Thus if, when the urine has ceased to flow in 

 consequence of a section of the medulla oblongata, certain sub- 

 stances, such as urea, urates, sodium acetate, and the like, be 

 injected into the blood, a more or less copious secretion is at once 

 set. up. This secretion is, or at least may be, unaccompanied by 

 any rise of general blood-pressure sufficient to account for the 

 increased secretion as the mere result of an increased flow of 



