FOOD AND DIGESTION 69 



the blood is reduced and carbon dioxide is not carried 

 away promptly. This explains the cyanosis and the 

 labored breathing. 



As has been stated, protein may be broken up in the 

 body to yield energy, but this is extravagant for two 

 reasons; first, because protein food is actually much 

 more expensive and, second, that it probably causes more 

 wear and tear than carbohydrate on the body when used 

 in excessive amounts. The most important role of protein 

 is to serve as tissue-building material, a role for which 

 it is indispensable, carbohydrate and fat being unable 

 to replace protein for this purpose. 



Under normal conditions, all, or nearly all, of the 

 protein is absorbed in the form of what is known as 

 "ammo acids." These are organic acids containing the 

 NH or amid group, and are known as the "building 

 stones" of the proteins. Under the combined action of 

 pepsin, trypsin, and erepsin, this decomposition takes 

 place, and the amino acids enter the blood vessels of 

 the intestinal villi and are carried to the liver in the 

 portal vein. It is probable that the liver does not effect 

 any change in them at this time, but allows them to 

 pass through the tissues where they may be utilized. 

 Wherever there is need of building material, the "build- 

 ing stones" are taken from the blood and utilized to 

 construct the particular kind of protoplasm needed. 



Even with the ordinary amounts of protein in the 

 diet, only a part is used for tissue building or repair; a 

 considerable portion being burned to yield energy. The 

 end products of this breaking up of protein are not so 

 simple as are those representing the final results of car- 

 bohydrate and fat decomposition. Under the action of 

 tissue enzymes, the nitrogen is split off and is carried to 

 the liver. Here it is converted into "urea," which 



