SECRETION. 195 



takes an active part in directing the stream of water from the blood to the 

 uriuiferous tubules, it is natural to ask by what mechanism this action is 

 effected. The problem is essentially similar to that already encountered in 

 explaining the flow of water in other glands (see p. 166). There is as yet no 

 satisfactory explanation given. It is to be supposed that"thls~pT6perty is 

 dependent upon some physical or chemical reaction of the substance of the 

 cell, and involves the existence of no form of energy not already known to us 

 in other ways ; but what the nature of these reactions is must be left for 

 future work. The extent of the activity seems to depend mainly on the 

 quantity of blood flowing through the glomeruli. The greater the quantity 

 of blood, the greater will be the quantity of water brought to the cells, and 

 the more complete also the supply of needful oxygen. In addition, substances, 

 such as the inorganic salts, which occur normally in the blood, or other sub- 

 stances which may be introduced therapeutical ly, may act as chemical irritants 

 to these cells, and thus increase their secretory activity. The normal stimulus 

 to the epithelial cells of the convoluted tubules, using the term convoluted to in- 

 clude the actively secreting parts, seems to be the presence of urea and related 

 substances in the blood (lymph). That the elimination of the urea is not a simple 

 act of diffusion seems to be clearly shown by the fact that its percentage in the 

 blood is much less than in the urine. In some way the urea is selected from 

 the blood and passed into the lumen of the tubule, and although we have 

 microscopic evidence that this process involves very active changes in the sub- 

 stance of the cells, there is no adequate theory of the nature of the force which 

 attracts the urea from the surrounding lymph. The whole process must be 

 rapidly effected by the cell, since there is normally no heaping up of urea in 

 the kidney-cells; the material is eliminated into the tubules as quickly as it 

 is received from the blood. The rate of elimination increases normally with 

 the increase in the urea in the blood, as would be expected upon the assump- 

 tion that the urea itself acts as the physiological stimulus to the epithelial 

 cells. 



The Blood-flow through the Kidneys. It will be seen from the dis- 

 cussion above that, other conditions remaining the same, the secretion of the 

 kidney varies with the quantity of blood flowing through it. It is therefore 

 important at this point to refer briefly to the nature and especially the regula- 

 tion of the blood-flow through this organ, although the same subject is referred 

 to in connection with the general description of vaso-motor regulation (see 

 Circulation). It has been shown by Landergren 1 and Tiegerstedt that the 

 kidney is a very vascular organ, at least when it is in strong functional activ- 

 ity such as may be produced by the action of diuretics. They estimate that 

 in a minute's time, under the action of diuretics, an amount of blood flows 

 through the kidney equal to the weight of the organ ; this is an amount from 

 four to nineteen times as great as occurs in the average supply of the other 

 organs in the systemic circulation. Taking both kidneys into account, their 

 figures show that (in strong diuresis) 5.6 per cent, of the total quantity of 

 1 Skandinavisehes Archiv fur Physiologic, 1892, Bd. iv. S. 241. 



