THE SECRETION OF THE URINE 505 



example, even in the most dilute urine, the concentration of urea 

 will in general be much greater than in the blood. It is of interest 

 in connection with the work performed by the kidney that when 

 the flow of urine is increased by diuretics like caffeine or sodium 

 sulphate, which cause the secretion of urine with a very different 

 crystalloid composition from that of the plasma, more oxygen is 

 used up, whereas the diuresis caused by the injection of Ringer's solu- 

 tion where the urine and plasma do not differ materially in the amount 

 or kind of the non-protein constituents, is accomplished practically 

 without change in the oxygen consumption of the kidney. 



Significance of the Glomeruli. 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 blood- 

 vessels. 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 perhaps other functions than excretion 

 (Chapter XI.). 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. 580), 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 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 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 



