466 A MANUAL OF PHYSIOLOGY 



the elimination of water and salts, is largely thrown upon the 

 Malpighian corpuscles, as a physiologically cheaper machine than 

 the epithelium of the tubules, which is left free for more complex 

 labours. These may include not only the separation of nitrogenous 

 metabolites, but also synthetic processes possibly concerned in the 

 regulation of protein metabolism. One characteristic synthesis, 

 the union of benzoic acid and glycin to hippuric acid, has already 

 been referred to. As will be shown later (p. 508), it takes place 

 mainly, in some animals perhaps exclusively, in the kidney. The 

 epithelium of the glomerulus, being a less highly organized and less 

 delicately selective mechanism than that of the convoluted tubules, 

 may more easily respond to increase of blood-pressure by increased 

 secretion. At the same time, placed as it is at the last flood-gate 

 of the circulation, where the escape of anything valuable means 

 its total loss, the glomerular epithelium may be endowed with a 

 general power of resistance to transudation, which renders a com- 

 paratively high blood-pressure a necessary condition of its acting 

 at all. And as a matter of fact water ceases to be secreted by the 

 kidney long before the blood-pressure in the glomeruli can have 

 fallen below that which suffices for the highest activity of the liver. 

 Perhaps, however, the high minimum pressure required (30 to 40 mm. 

 of mercury in the dog) is merely the necessary consequence of the 

 long and difficult path which most of the blood going through the 

 kidney has to take, and that a sufficient blood-flow cannot be kept 

 up with less. It may be, too, that the comparatively small surface 

 of the glomeruli, restricted in order to leave room for the more 

 highly organized parts of the renal mechanism, entails the more 

 intense and concentrated activity, which the high blood-pressure 

 renders possible, and the simplicity of work and organization renders 

 harmless. 



An obvious result, and perhaps an important one, of the peculiar 

 arrangement of the bloodvessels of the kidney is that the renal 

 tubules proper are shielded from an excessive blood-pressure by the 

 interposition of the glomeruli as a block. This may be either 

 because the epithelium of the tubules would not perform its work 

 so well under a high blood-pressure, or because there would be a 

 danger of substances which ought to be retained being cast out into 

 the urine. In this connection it is interesting to note that the specific 

 constituents of urine are separated by epithelium surrounded by 

 capillaries of the sscond order, and therefore with a smaller blood- 

 pressure than exists in the capillaries of most glands, while the 

 same is true of bile, another (practically) protein -free secretion. 



The maximum secretory pressure in the kidney, as shown 

 by a manometer tied into the divided ureter, is about 60 mm. 

 of mercury in the dog, or less than half that of saliva. If the 

 escape of the urine is opposed by a greater pressure than this, 

 or if the ureter is tied, the kidney becomes oedematous. Whether 

 the oedema is due to reabsorption of urine or to the pouring 

 out of lymph owing to the pressure of the dilated tubules on 

 the veins has not been definitely settled. It has been already 

 pointed out that there is no necessary relation between the 

 blood-pressure in the capillaries of a gland and its secretory 

 pressure ; and, so far as this goes, water might just as well be 



