

THE SECRETION OF URINE 479 



either locally, as by ligature of the renal artery, or generally by division 

 of the spinal cord in the neck, then the secretion ceases. When the 

 arterial pressure in the renal artery is diminished, as by stimulation 

 of the spinal cord without previous section of the renal nerves, or of 

 the splanchnic nerve, or by haemorrhage, then the flow of urine is 

 diminished. 



Such experiments undoubtedly show that, the flow of urine is 

 increased, when the renal arterial pressure is high, and diminished 

 when it is low. The results may, however, be correlated with the flow 

 of blood through the kidney. When the flow of blood is increased or 

 diminished, so also is the secretion of urine. Against the nitration 

 hypothesis is the fact that ligature of the renal vein, which certainly 

 raises the pressure in the renal capillaries to the full arterial pressure, 

 not only causes no increased flow of urine, but stops it altogether. 

 Again, when the renal artery is ligated for a few minutes, and the liga 

 ture then removed, the flow of urine does not immediately begin 

 again for an hour or so, and, when it does, the character of the urine 

 is pathological it contains albumin. 



On the filtration hypothesis, it is assumed that F (the filtration 

 pressure) = P (the kidney arterial pressure) ~p (the glomerular 

 pressure). F = P p. 



This being the case, then, in the condition of P (p + p/) where pt 

 indicates a slight obstruction to the flow in the ureter, and therefore 

 a rise of glomerular pressure, and a consequent diminution of the 

 filtration pressure there should be a diminished flow of urine. This, 

 however, has been shown experimentally not to be the case. Indeed, 

 under these conditions, there is an increased flow of urine, and if 

 phloridzin is administered to the animal, there is an increased ex- 

 cretion of sugar as the result of this obstruction. The obstruction 

 stimulates the kidney to secrete. 



Further, it may be pointed out that, according to the views ex- 

 pressed in the section on capillary pressure, it does not seem possible 

 for a high filtration pressure to exist within the glomeruli. The 

 pulsatile force is transmitted to, and expands, all parts of the kidney. 



The histological examination of the kidney shows no evidence 

 of membranes so arranged as to allow filtration from the capillaries 

 into the capsules. There is nothing to keep the membrane separating 

 blood and urine open and rigid as a filtration membrane. The capsule 

 and tubules are surrounded by a membrane, but this is so arranged 

 as to limit their expansion and allow the passage from capillary to 

 lumen of tubule by osmotic or other forces set up within the tubule 

 by the active secretion of the renal cells. The structural arrange- 

 ments point to a pull of fluid from capillary to tubule, not to a mechani- 

 cal push produced by blood-pressure. 



The process of secretion at the glomerulus must be just as much 

 an act of cell activity as is the formation of the corresponding waste 

 fluid in unicellular organisms, or in worms which have their nephridia 

 bathed in blood-sinuses conditions which clearly negative the 

 filtration hypothesis. 



