The proteins, on being metabolized in the body, yield varying amounts of arginine, which may undergo hydrolysis into ornithine and urea. In this way a small part of the nitrogen of protein may reach the urea stage through a series of direct cleavages. It is altogether probable, however, that much the greater part of the urea eliminated arises as follows: The protein in catabolism is split to amino acids, which are deaminized (as in the conversion of alanine to methyl glyoxal above mentioned), the nitrogen of the amino group being split out as ammonia, which with the carbonic acid constantly being produced in metabolism forms ammonium carbonate.* Loss of one molecule of water yields ammonium carbamate, which in turn on loss of one molecule of water yields urea.

(NH4)2CO3 -> NH4CO2NH2 + H2O NH4CO2NH2 ->CO(NH2)2 + H2O

Ammonium chloride or sulphate evidently cannot be changed to urea in this way; and experiments show that if hydrochloric or sulphuric acid is introduced into the blood, it is eliminated by the kidneys largely as ammonium salt, and the quantity of urea is correspondingly decreased. In diseased conditions of the liver the organic salts of ammonia (which normally should be burned to carbonate and then converted as above) may also pass through and be eliminated without being changed to urea. In health and on a full protein diet (say about 100 grams protein per day) from 82 to 88 per cent of the total nitrogen excreted by the kidneys is usually in the form of urea. On a low protein diet this percentage is lower.

* If ammonium salts of organic acids are first formed, the complete oxidation of the organic acid radicle will bring this ammonia also into the form of carbonate. K