CHAPTER XI. 

 SPECIAL METABOLISM. 



But we must now return to the more theoretical aspects of our 

 subject. We will proceed to trace out very briefly the interme- 

 diary stages in metabolism through which proteins, fats and car- 

 bohydrates have to pass in order to yield the energy required 

 to drive the animal machine and to supply material with which 

 to repair the broken-down tissues. 



Metabolism of Proteins. We must follow the amino bodies 

 after their absorption into the blood until they ultimately reap- 

 pear, the nitrogen among the nitrogenous constituents of urine 

 and the carbon as part of the carbon dioxide of expired air. In 

 order to do this it is necessary for us to become familiar with 

 the nature and source of the urinary substances ivhich contain. 

 nitrogen, and to consider some of the most important chemical 

 relationships of these substances, so that we may understand how 

 they become formed in the body. The substances in question are : 

 urea, ammonia, creatinin, the purin bodies, and undetermined 

 nitrogenous substances. Urea and ammonia may be considered 

 together. 



UREA AND AMMONIA.- There is no doubt that it is as ammonia 

 that the nitrogen of the amino bodies is set free in the organism. 

 The free ammonia would, however, be highly poisonous, so that 

 it immediately becomes combined with acid substances to form 

 harmless neutral salts. The acid which is ordinarily used for 

 this purpose is carbonic, of which there is always plenty in the 

 blood and tissue juices. The ammonium carbonate thus formed 

 becomes changed into urea by removal of the elements of water 

 from the molecule, thus: 



OH ONH 4 NH, NH, 



/ / / / 



2NH., + CO = CO H.,0 = CO H,0 = ( 1 () 



\ \ \ \ 



OH ONH 4 ONI I, MI 



Ammonia Carbonic Ammonium Ammonium I'rcu 



acid carbonate carbamate 



108 



