THE SECRETION OF URINE. 417 



will produce considerable effects in the kidney itself without appreciably 

 modifying the general blood-pressure. 



351. If, while the kidney is in the oncometer, and the various experi- 

 ments on section and stimulation of nerves and the like are being carried 

 on, a canula be tied in the ureter, the secretion of urine may be watched at 

 the same time. It will then be seen that the flow of urine through the end 

 of the canuhi is not equable, and does not either increase or decrease in an 

 even manner. On the contrary, it will frequently be found that a sort of 

 gush of urine takes place, several drops following each other in rapid suc- 

 cession, followed by a cessation of flow ; and if the ureter be watched it 

 will be seen that the gushes of urine are synchronous with waves of 

 peristaltic contraction sweeping down the ureter. Obviously the urine 

 collects, to a certain extent, in the pelvis of the kidney, and is driven 

 thence by muscular action from time to time ; to this point we shall return 

 later on. 



Making every allowance, however, for these irregularities of flow, we may 

 take the rate of flow from the end of the canula as a measure of the rate 

 of secretion ; and it is found that as a general rule increased flow of urine 

 .is coincident with swelling of 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 expriment 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 general 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 dimin- 

 ished. We find that after the operation the secretion of urine is greatly 

 diminished ; indeed, in most cases the flow from the end of a canula 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. 



352. 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 d 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 ex- 

 perimental 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 



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