It has already been shown that the nitrogen of the protein of food enters the circulation chiefly, if not wholly, as amino acids and is taken up as amino acids by the various body tissues. The amino acids thus obtained by the tissues from the food serve as material for the building up of body proteins; but in the breaking down of body proteins there is doubtless a liberation of amino acids of the same kinds. Amino acids from either source are subject to deaminization in the tissues, and in so far as α-amino groups are concerned the process doubtless consists chiefly in the splitting out of the nitrogen as ammonia, most of which is later changed to urea. Nitrogen in other forms than α-amino acids may be expected to undergo a somewhat different metabolism, and it is well known that the urine always contains other nitrogen compounds in addition to ammonium salts and urea.

Much light has been thrown upon the chemistry of protein metabolism by the study of the quantitative relations existing among the different forms of nitrogen in the urine under different conditions. For our present purpose it will be sufficient to consider only the more important of the nitrogen compounds of the urine and the relations which they are believed to bear to the processes of normal metabolism.