842 PHYSIOLOGY OF DIGESTION AND SECRETION. 



the total nitrogen eliminated in the urine, without regard to its 

 specific form. This determination is made usually by the method 

 of Kjeldahl. The total weight of nitrogen multipUed by 6.25 

 gives the amount of protein broken down, since nitrogen forms, on 

 the average, 16 per cent, of the weight of the protein molecule. In 

 an average-sized man the total nitrogen eliminated in a day varies, 

 let us say, between 14 and 18 gms., which would correspond to 88 

 and 117 gms. of protein. It is often necessary to distinguish 

 between the forms in which this nitrogen is eliminated, and in 

 analyses of the urine for qualitative purposes, that is, to throw 

 light on the kind of metabolism taking place in the body, the fol- 

 lowing division or partition of the nitrogen is made according to 

 special methods of analysis:* 



1. The urea nitrogen, that is, the amount of nitrogen excreted 

 in the form of urea. The amount of nitrogen eliminated in this 

 form varies with the amount of protein food in the diet, as is ex- 

 plained below. Under normal conditions it constitutes 80 per 

 cent, or more of the total nitrogen. 



2. The amiyionia nitrogen, the nitrogen excreted in the form of 

 ammonia salts which liberate free ammonia on the addition of a 

 fixed alkali. Under normal conditions it constitutes from 4 to 5 

 per cent, of the total nitrogen. The amount of ammonia nitrogen 

 may be much increased under certain pathological conditions in 

 which there is a tendency toward acidosis, that is to say, toward 

 the production of a dangerous excess of acids in the body which 

 unless neutralized would alter the reaction of the blood. The 

 ammonia serves to neutralize these acids. As is described below 

 in connection with the discussion of the origin of urea, ammonia 

 is formed constantly in the body in the hydrolysis of the protein 

 molecule and in the process of deaminization of the amino-acids 

 arising from this hydrolysis. For the most part the ammonia 

 thus formed combines with some of the carbonic acid arising from 

 the oxidative processes to form ammonium carbonate, which is 

 then converted to urea and secreted. But it may be used to 

 neutralize other acids as well, and in this respect constitutes one 

 of the important means by which the normal reaction of the body 

 Hquids is safeguarded. 



3. The creatinin nitrogen, the nitrogen excreted in the form of 

 creatinin. An average estimate for the human urine is 3.6 per 

 cent, of the total nitrogen. This product has also a special meta- 

 boUc significance, which is discussed below. 



4. The purin nitrogen, the nitrogen excreted in the form of 

 purin compounds (uric acid, xanthin, hypoxanthin) . These 



* Folin, "American Physiological Journal," 13, 45, 1905. 



