GENERAL CHARACTERISTICS OF THE URINE. 213 



and particularly its alkalinity, constant, a loss of alkali is guarded 

 against by subjecting the various neutral salts to the specific activity 

 of the renal epithelial cells. As a result a portion of the alkali is 

 returned to the blood, and acid salts hence appear in the urine. 



In the herbivorous animals, on the other hand, in which a super- 

 abundance of alkaline salts is either directly ingested or is formed 

 within the body from salts of organic acids which have been taken 

 with the food, an alkaline urine is eliminated. In this case a 

 formation of acid salts and a return of alkali to the blood are 

 unnecessary. Similar conditions at times occur in man, and the 

 elimination of an alkaline urine, the alkalinity being due to a 

 fixed alkali, cannot hence be regarded as pathological. During the 

 process of digestion, indeed, when an additional amount of alkaline 

 salts finds its way into the blood in consequence of the formation 

 of hydrochloric acid, an increased alkalinity would result. This, 

 however, is prevented by the excretion of a urine which, if not 

 alkaline, is at least less acid. 



It has been stated above that the organic acids which are formed 

 during the nitrogenous metabolism of the body combine with the 

 alkaline carbonates of the lymph and blood-plasma to form neutral 

 salts. This statement requires modification in so far as it conveys 

 the idea that the acids in question are eliminated in the urine in 

 combination with fixed alkalies only. As a matter of fact, this is 

 true only in part, and a certain proportion of the acid is eliminated 

 in combination with ammonia. 



Generally speaking, the ammonium salts which are formed within 

 the body appear in the urine as urea, but aside from their impor- 

 tance in this respect they represent a reserve of alkali which is capa- 

 ble of preventing an undue diminution in the alkalinity of the blood 

 by vicariously taking the place of the fixed alkalies. This vicarious 

 action is normally also at work, but is then comparatively insig- 

 nificant in extent. If, however, a specially large demand is made 

 upon the alkalies of the body v as when mineral acids are ingested 

 for experimental purposes, the vicarious action of the ammonium 

 salts at once enters into play. Unless carried to extremes, the 

 alkalinity of the blood, in the carnivorous animals at least, remains 

 constant, but the elimination of urea is proportionately less, and the 

 deficit of nitrogen in this form appears as ammonia in combination 

 with acids. 



By gradually increasing the amount of acid it is thus possible to 

 bring about the almost complete disappearance of urea from the 

 urine. A point, however, is finally reached when the animal suc- 

 cumbs to acid intoxication, and then, and not before, may free acids 

 appear in the urine. Death in such cases results from suffocation, 

 as there is not sufficient alkali left in the lymph and plasma to com- 

 bine with the carbon dioxide in the tissues (see page 339). 



Conversely, it is possible to cause the ammonia to disappear from 

 the urine by the administration of a sufficiently large quantity of 



