THE SECRETION OF URINE 1147 



loids of the blood which are diffusible and are not closely combined with 

 its colloidal constituents. Loewi has shown that in this connection a contrast 

 is to be drawn between the behaviour of substances, such as urea or sodium 

 chloride, and certain other constituents of the blood such as phosphates or 

 sugar. Any increase in the rate at which the glomerular secretion takes 

 place must cause a corresponding increase in, the total amount of the solid 

 diffusible constituents of the blood-plasma which are turned out within a 

 given time. Thus every diuresis increases the total output of chlorides and of 

 urea. On the other hand, a diuresis caused, for example, by drinking large 

 quantities of water does not increase the total output of phosphates in a 

 given time, nor does it increase the very small amount of sugar which is 

 normally excreted by the kidneys. If, however, sodium phosphate be 

 previously injected into the blood, then any diuresis increases the output 

 of this salt. The same thing holds for sugar. If an excess of free uncom- 

 bined sugar be present in the blood, either in consequence of intravenous 

 injection of this substance or as a result of previous extirpation of the pan- 

 creas, any form of diuresis will increase the rate at which it is turned out 

 by the kidneys. Loewi concludes that phosphates, which must be present 

 in minimal quantities in the glomerular transudate, are for the most part 

 secreted by the activity of the cells of the convoluted tubules. Under abnormal 

 conditions, e.g. as after administration of phloridzin, the cells of the kidneys 

 can be excited to a similar activity with regard to sugar. After phloridzin 

 injection the urine contains considerable quantities of sugar, but the rate at 

 which the sugar is secreted is not affected in any way by raising the rate of 

 urinary secretion, e.g. by the injection of such substances as sodium sulphate, 

 which increases the rapidity of the glomerular process of transudation. 



ABSORPTION BY THE RENAL TUBULES. The experiments of Ribbert, 

 mentioned above, in which removal of the medullary portion of the kidney 

 led to the formation of an increased quantity of a more watery urine, points 

 to the possession by the tubules of a power of absorbing water. We have 

 other evidence that this power of resorption is not confined to water, but 

 may affect also the dissolved constituents of the glomerular transudate. It 

 was pointed out by Meyer that, if two salts, such as sodium sulphate and 

 sodium chloride, were present at the same time in the glomerular transudate, 

 any process of resorption should affect chiefly the more diffusible salt, namely, 

 sodium chloride. Such a differential resorption would account for the much 

 greater diuretic power of sodium sulphate as compared with sodium chloride. 

 In certain experiments Cushny produced a diuresis by the injection of 

 equal parts of equivalent NaCl and Na 2 S0 4 solutions into the veins of a 

 rabbit. An increased flow of urine was produced which lasted two hours 

 and a half. The chlorides of the urine rose with the diuresis and reached 

 their maximum at the height of the urinary flow. They then sank, and in 

 some experiments had practically disappeared from the urine towards the 

 end of the observation. The concentration of the sulphates, however, con- 

 tinued to rise in the urine to the end of the experiment. Thus in the first of 

 two identical experiments, when the rabbit was killed at the height of the 



