EXCRETION 277 



is not understood, but it may be due in part to the 

 kidney acting more slowly when diseased. 



Solid substances excreted by the kidney tend to carry 

 a certain amount of water with them. The polyuria of 

 diabetes mellitus is a good example of the excretion of 

 sugar causing such a flow of water. Some diuretics 

 probably owe their power of increasing the volume of 

 the urine to the same mechanism. 



Acidity. The acid reaction of the urine is due to the 

 presence of acid phosphates and organic acids. Much 

 discussion took place at one time as to how it is that the 

 kidney can separate an acid urine from an alkaline fluid 

 such as the blood. It must be remembered, however, 

 that, although the blood is alkaline to litmus, its chief 

 salts, acid phosphates, and bicarbonates, are technically 

 acid. The former acid is eliminated by the kidney, the 

 latter (C0 2 ) by the lung ; hence the preponderance of 

 acid in the renal excretion. The mineral constituents 

 of an animal diet yield an acid, and those of a vegetable 

 diet an alkaline ash, which is the reason why the urine 

 of carnivora is highly acid, whilst that of herbivora is 

 alkaline. 



Advantage may be taken of this effect of diet in 

 modifying the reaction of the urine in cases of disease. 



The total amount of acid eliminated by the kidney is 

 much greater during the day than during the night, but 

 owing to the concentration of the night urine its per- 

 centage acidity is greater than that of the day. It is 

 for this reason that uric acid is more apt to separate out 

 in the urine secreted by night, a fact which must be 

 borne in mind when one is treating cases of gravel. 



