852 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 multiplied 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 ammonia 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, 

 while not altering perceptibly the actual hydrogen-ion concentra- 

 tion of the blood, does reduce the amount of alkali reserve that 

 serves to protect the reaction. The ammonia serves to neutralize 

 these acids. As is described below in connection with the dis- 

 cussion 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 v the normal 

 reaction of the body liquids 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 

 metabolic 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. 



