TRANSPORT OF MATERIALS 139 



developed in the cells lining the tubules of the kidney a capacity 

 for reabsorbing water and its valuable solutes, leaving untouched 

 the waste products. In other words, a solution is reabsorbed 

 containing all the normal constituents of the blood plasma, except 

 urea and such substances as are not wanted. Of course, the liquid 

 which is reabsorbed does not contain proteins, because they are 

 absent from the glomerular filtrate. Now, it has been pointed out 

 by Professor Cushny that all the various phenomena connected 

 with the formation of urine can be most simply and easily explained 

 if we suppose that the solution reabsorbed by the tubule cells, and 

 passed back to the blood, is that to which the mechanism of these 

 cells, together with the other cells of the body, is adjusted. That 

 is, a solution which contains not merely the normal salts and food 

 materials of the blood, but also in the exact concentration which 

 they possess in the blood. Such a fluid then is invariably absorbed, 

 whatever the composition of the glomerular filtrate passing over 

 the cells. If the filtrate contains a notable excess of some solute, 

 even of a valuable constituent such as sugar, it may happen that a 

 part of it is lost by failure to be absorbed. Moreover, the rate at 

 which absorption is possible is limited, so that if filtration is very 

 rapid, there may not be time for absorption to take place as 

 perfectly as under normal conditions. 



In order to see how the process works, let us take the case in 

 which the blood has been diluted by drinking a large quantity of 

 water. In the first place, the colloids will be present in lower con- 

 centration than normal, hence the available filtration pressure is 

 raised. The filtrate will, however, be more dilute than normal, 

 while the tubules absorb from it a solution of the normal concen- 

 tration. The result is that the concentration of the blood is more 

 or less quickly raised again. 



Suppose next that the blood has become concentrated by loss 

 of water from the skin. It is obvious that the filtration will be 

 slower, but a certain amount of water must be lost in order to keep 

 in solution the excretory products. The liquid absorbed is of 

 normal concentration, so that the blood does not become so con- 

 centrated as it would otherwise, while the urine may be of very 

 high concentration. 



Although the glucose and amino-acids are practically com- 

 pletely reabsorbed, there is always a loss of sodium chloride, which 

 is replaced by fresh supplies in the food. And, as already 

 remarked, if abnormal amounts of the former materials are present 

 in the blood, we find them in the urine. If the liver is disordered, 

 so that the deamination of amino-acids is interfered with, we find 

 these acids in the urine. 



Since the filtration pressure in the glomeruli is the factor con- 



