SECRETION 355 



the blood, the quantity of liquid to be dealt with by the cells of the kidney is 

 the same, whether it comes to the cells from the blood or from the lumen of 

 the tubules. 



Perhaps the following a priori considerations may assist the argument. If 

 there is no absorption of water by the tubules, it is necessary to assume that not 

 only are urea and similar metabolites secreted by the tubule cells, but also 

 useful substances like glucose and sodium chloride. Now it is difficult to under- 

 stand why a wasteful, or at any rate useless, process should have been produced 

 in the course of evolution. If we confine our attention to the higher land 

 animals, it might seem an inefficient process to filter off water and solutes from 

 the blood, only to be in great part reabsorbed. The ancestral excretory organs, 

 however, were probably merely filters, like the glomerulus, and the process was 

 a satisfactory one, since there was no need to preserve either water or salts on 

 account of their abundance in the ocean. As regards organic, diffusible food 

 materials which would escape with the filtrate, they might be kept back in great 

 part by adsorption on colloidal surfaces in the cells of the organism. In the course 

 of evolution on land, the saving of water and salts became more and more 

 advantageous, so that the power of reabsorption began to show itself, while the 

 necessity of so large a volume of filtrate was decreased by the development 

 of active secretory cells for elimination of the waste products. 



We must not forget, moreover, that the filtration process involves no 

 expenditure of energy on the part of the kidney itself, however large the amount 

 filtered. The energy comes from the heart and is but a small fraction of that 

 used in other ways. 



Ribbert (1883) believes that he has positive evidence of the absorption of water by the 

 tubules in the results of removing the medulla of the rabbit's kidney, which takes away the 

 greater part of the tubules. It was found that a much more dilute urine was excreted. But 

 the kidney is a very sensitive organ, and the procedure a somewhat violent one, so that too 

 much stress must not be laid on these experiments. 



Absorption of water, in the mammal entirely performed in the renal tubules 

 themselves, appears to take place also in the cloaca, or posterior part of the 

 alimentary canal, in the bird. Sharpe (1912) finds that the urine is a clear liquid in 

 the ureter and only attains its well-known semi-solid nature after leaving the ureter. 



If we grant the process of glomerular filtration, and the evidence for this is 

 overwhelming, the fact that the urine contains a larger percentage of sodium 

 chloride than the blood does, is an indirect proof of absorption of water. For, 

 as we shall see later, there is every reason to believe that no secretion of sodium 

 chloride occurs in the tubules. 



So far, then, we may state what appears to be the most probable view thus : 

 The glomeruli filter from the blood sufficient fluid to contain the whole of the 

 sodium chloride excreted and probably more; part of the water together with 

 a part of the valuable solutes, such as sodium chloride, glucose and amino-acids, 

 is reabsorbed in the tubules. But this process would not sufficiently get rid of the 

 urea produced by the organism, unless a great excess of sodium chloride were 

 also excreted ; accordingly, urea and similar substances are actively secreted by 

 the tubule cells, and turned into the fluid as it passes over them. We have then 

 to see further what evidence there is that these two processes of secretion by the 

 tubules and of absorption of certain solutes by them actually take place. 



Secretion by Tubules. Bauer (1898) injected intravenously a solution of uric acid 

 in piperazine ; considerable diuresis resulted the kidneys, on microscopic examina- 

 tion twenty to sixty minutes afterwards, showed no uric acid in the glomeruli, 

 probably because the filtrate was too dilute. But in the lumen of the convoluted 

 tubules there were considerable masses of uric acid deposit. This, however, might 

 have been produced by absorption of water. That it was not so, and was probably 

 due to secretion by the cells, was shown by the presence of uric acid particles 

 within them. The cells of the medulla were free from uric acid, which was only 

 present in the lumen of the tubules in this situation. It seems impossible to 

 believe that so much could have been filtered off by the glomeruli in the time, even 

 if we reject the evidence of the deposit inside the cells. 



