382 PHYSIOLOGY CHAP. 



The urine gives an acid reaction with the flesh diet and mixed 

 diet, as well as on a diet of bread. It is easily seen from the 

 above data that in all three tables the content of chlorine and 

 sulphuric acid is more than sufficient to combine with the whole 

 of the alkali in the form of chlorides and sulphates. 



It is convenient in considering the total organic and inorganic 

 substances found in the urine to classify them into different 

 groups based on the approximate criteria of their characteristic 

 affinities, chemical constitution, and origin. Hoppe-Seyler and 

 Halliburton distinguish the following groups of substances in the 

 urine : 



(a) Nitrogenous Substances of the Fatty Series. Among these 

 are urea, uric acid, allantoin, xanthine, guanine, creatine, creatinine, 

 sulphocyanic acid. 



(b) Non-nitrogenous Substances of the Fatty Series. These 

 include the fatty acids of the series C n H 2n 2 , oxalic acid, lactic 

 acid, glycerophosphoric acid, small quantities of carbohydrates. 



(c) Compounds of the Aromatic Series. These include the 

 ethereal sulphates formed from phenol, cresol, pyrocatechol, indoxyl, 

 hippuric acid, the aromatic oxy-acids. 



(d) Organic Substances not exactly determined. This group 

 includes the pigments and chromogens of urine, the enzymes 

 (especially pepsin), the mucin and a small amount of proteins. 



(e) Inorganic Salts. These are sodium and potassium chloride, 

 potassium sulphate, sodium, calcium and magnesium sulphate, 

 silicic acid, ammonium compounds and calcium carbonate. 



(/) Gases, i.e. nitrogen and carbonic acid. 



To these substances normally present in human urine, many 

 others are added in abnormal conditions and special diseases, i.e. 

 serum-albumin and other proteins, haemoglobin, and methaenio- 

 globin, bile pigments and salts, leucine and tyrosine, glucose 

 and lactose, glycuronic acid, fat, lecithin, cholesterol and cystine. 

 To these we must add a number of other substances derived 

 from special foods and drugs. Lastly, there are organised 

 cells, e.g. epithelia of kidneys anc[ bladder, urinary casts, blood 

 corpuscles. 



We must briefly run through the characteristics and origin of 

 the principal substances comprised in these groups. 



II. Among the nitrogenous urinary constituents of the fatty 

 series, Urea is the most important, and is the end-product of protein 

 katabolism. Chemically considered it is a carbamide, i.e. carbonic 

 acid, in which the two hydroxyl groups are substituted by two 

 (amino) groups (NH 2 ), and its formula is CO(NH 2 ) 2 . It was 

 first obtained artificially by Wohler in 1828 by heating ammonium 

 cyanate, which is the isomer of urea (NH 4 )'OCN. 



By heating with water at 140 C., by boiling with acids or 

 alkalies, or by the action of the enzymes secreted by the bacilli of 



