372 PHYSIOLOGICAL CHEMISTRY 



NH 2 

 UREA, C = 0. 



NH 2 



Urea is the principal end-product of the metabolism of protein 

 substances. It was formerly believed that about 90 per cent of the 

 total nitrogen of the urine was present as urea. Folin, however, has 

 shown that the distribution of the nitrogen of the urine among 

 urea and the other nitrogen-containing bodies present depends entirely 

 upon the absolute amount of the total nitrogen excreted. He found 

 that a decrease in the total nitrogen excretion was always accom- 

 panied by a decrease in the percentage of the total nitrogen excreted 

 as urea, and that after so regulating the diet of a normal person as to 



FIG. 1 1 8. UREA. 



cause the excretion of total nitrogen to be reduced to 3-4 grams in 24 

 hours, only about 60 per cent of this nitrogen appeared in the urine as urea. 

 His experiments also seem to show urea to be the only one of the nitro- 

 genous excretions which is relatively as well as absolutely decreased as a 

 result of decreasing the amount of protein metabolized. This same 

 investigator reports a hospital case in which only 14.7 per cent of the 

 total nitrogen was present as urea and about 40 per cent was present as 

 ammonia. Morner had previously reported a case in which but 4.4 

 per cent of the total nitrogen of the urine was present as urea, and 26.7 

 per cent was present as ammonia. 



Urea occurs most abundantly in the urine of man and carnivora 

 and in somewhat smaller amount in the urine of herbivora; the urine 

 of fishes, amphibians, and certain birds also contains a small amount of 



