498 EXCRETION 



pressure of the liquids in contact with their attached and free sur- 

 faces. A token of the intensity of the metabolic effort required is 

 the marked increase in the absorption of oxygen which occurs 

 during diuresis, although it is not in proportion to the amount of 

 the diuresis. In one experiment the oxygen absorbed by a dog's 

 kidneys was n per cent, of what would have been used up by the 

 entire animal under normal conditions. There is no definite relation 

 between the oxygen taken in and the carbon dioxide given out at 

 any moment. 



What is the significance of the peculiar arrangement of the glomerular 

 bloodvessels, if the epithelium of the capsules has secretive powers like 

 that of ordinary glands ? It is difficult to believe that these unique 

 vascular tufts have not a near and important relation to the renal 

 function ; but it is by no means clear what that relation is . The secretion 

 of water, and even its rapid secretion, is not at all bound up with any 

 set arrangement of bloodvessels. Gland-cells all over the body secrete 

 water under the most varied conditions of blood -pressure, although a 

 comparatively high pressure is upon the whole favourable to a copious 

 outflow. 



But the kidney has other functions than mere excretion (p. 649). 

 And it may be that the simplest part of the latter process, 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. 571), 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 

 comparatively high blood-pressure a necessary condition of its acting 

 at all. And as a matter of fact water ceases to be s?creted 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 inter- 

 position 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 



