DIURETICS 225 



colloids, which amount to 8 per cent, of the plasma, are 

 retained in the blood. 



The dextrose, which amounts to 0-1 per cent., filters 

 through the glomerulus, but, being within its threshold, 

 it all passes back through the tubules. Were it to exceed 

 the threshold, as in diabetes, the excess would pass into 

 the urine, while the previous amount would continue to 

 be reabsorbed. The same apphes to sodium and chloride. 

 As regards the uric acid, Cushny reminds us that this (in 

 mammals, at any rate) is not an end product of metabohsm 

 in the same sense as urea, but that there is always an 

 attempt on the part of the body to convert uric acid into 

 urea. Uric acid therefore possesses a low threshold. It 

 is incompletely excreted, but any excess' in the blood 

 affects the urine, not the reabsorbed fluid. Potassium 

 behaves in a similar manner. As for urea and sulphate, 

 their fate is simple. They are never reabsorbed. 



It will thus be seen that however the composition of the 

 blood may vary, the substances which pass through the 

 glomeruh are always returned to the blood in amounts up 

 to their threshold values, while excess passes over to the 

 urine. The composition of the reabsorbed fluid is constant. 

 If, for example, the blood is more dilute, a more dilute 

 glomerular filtrate is formed. But the composition of the 

 reabsorbed fluid being unaltered, the result is that the 

 dilution only affects the urine. 



An objection which might at first sight be urged against 

 this theory is the large amount of reabsorption of water 

 which is entailed, one litre of urine corresponding to 

 67 htres of plasma. But when it is remembered that the 

 daily flow of blood through each kidney is estimated at 

 900 htres, this objection falls to the ground. 



DIURETICS 



These may be divided into two groups : — 



1. Substances usually present in blood. 



2. Foreign substances. 

 15 



