266 



PHYSIOLOGICAL CHEMISTRY. 



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

 substances. It has been generally believed that about 90 per cent 

 of the total nitrogen of the urine was present as urea. Recently, 

 however, Folin 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 accompanied 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 cause the excretion of total nitro- 



FIG. 85.; 



UREA. 





gen 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 nitrogenous ex- 

 cretions 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 the substance. Urea is also found in nearly all the fluids 

 and in many of the tissues and organs of mammals. The amount 



