4c8 A MANUAL OF PHYSIOLOGY 



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 circula- 

 tion, where the escape of anything valuable means probably its total 

 loss, the glomerular epithelium may be endowed with a general power 

 of resistance to transudation, which renders a comparatively 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 more 

 highly organized parts of the renal tubules are shielded from an ex- 

 cessive 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 proper 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 epi- 

 thelium surrounded by capillaries of the second order, and therefore 

 with a smaller blood-pressure than exists in the capillaries of most 

 glands, while the same is true of bile, another proteid-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 l^ss 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 cedematous. Whether the redema is due to re- 

 absorption 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 secreted at a 

 pressure of 60 mm. of mercury from the low-pressure blood 

 of the second set of renal capillaries as from the high- 

 pressure blood of the glomeruli. 



