846 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. Its special metabolic significance 

 is described below. 



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 

 products give an index of the amount of nucleic acid metabolized 

 in the body, and the nitrogen thus eliminated may be given an 

 average value of 1 to 2 per cent, of the total nitrogen. 



5. The Amino-acid Nitrogen, f Some of the nitrogen is excreted 

 in the form of amino-acids, either free or combined. An example 

 of the combined form is the compound hippuric acid, which 

 consists of benzoic acid in combination with amino-acetic acid 

 (glycin). 



6. Unknown Nitrogen. Certain nitrogenous substances are 

 excreted in the urine whose structure and physiological significance 

 are as yet undetermined. Among these substances are the so- 

 called oxyproteic acids which contain both nitrogen and sulphur. 

 They are derived presumably from the metabolism of proteins, but 

 the special significance to be attributed to them cannot be stated. 



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



t Henriques and Sorenson, "Zeitschrift fur physiol. Chemie," 64, 120, 1910. 



