FORMATION OF UREA 203 



Whether urea is produced directly in the tissues, or whether 

 only certain substances antecedent to it are there formed, it 

 cannot be doubted that it is the chief final product of nitrogen- 

 ous ingesta and nitrogenous dissimilation. It is practically 

 the only way in which the nitrogen of proteid foods can escape 

 from the body. It exists not only in the blood but in the lymph, 

 vitreous humor, sweat, milk, saliva, etc. It has been stated that 

 the taking of large quantities of liquids lowers the specific gravity 

 of the urine by diluting it; this is true, but the actual amount of 

 urea is increased somewhat by such a procedure. It is not sur- 

 prising that the quantity of urea is largely increased when much 

 nitrogenous food is taken, and that it is greatly decreased by an 

 exclusively vegetable diet. Anything, like exercise, which will 

 increase actual tissue metabolism, will increase the output of urea, 

 while anything retarding tissue metabolism, like alcohol, will 

 decrease the output. The average amount of urea for 24 hours 

 is 350 to 450 grains. 



Formation of Urea. Seeing that urea is the typical end product 

 of the physiological oxidation of the proteids, it becomes of 

 interest to determine, if possible, where urea formation takes 

 place. It is known that the liver is very active in producing 

 this substance; but it is not alone by this organ that urea is 

 formed. At the present time the prevailing opinion is that, for 

 the most part, the proteids under destructive metabolism in the 

 tissues do not reach the urea stage of transformation, but are 

 converted into ammonia compounds (which differ very slightly 

 from the urea in chemical composition), and these compounds 

 are conveyed by the blood to the liver, where the slight change 

 necessary to make them urea is effected under the influence of 

 this organ. Ammonium carbamate seems the typical compound, 

 but ammonium carbonate and others are probably likewise con- 

 verted. Artificial circulation of these compounds through the 

 liver gives rise to urea; removal of the liver increases the am- 

 monia compounds and decreases the urea in the urine; ammonia 



