FEED NUTRIENTS IN THE ANIMAL BODY 81 



(3) they may serve as a source of body fat ; and (4) they 

 may serve as a source of ])ody glycogen. 



The protein tissues of the body, especially the more active 

 ones, are continually being broken down or worn out and 

 must be repaired in order to maintain the life of the animal. 

 It was stated that in digestion, the proteins are split up into 

 the simple amino acids of which they are composed and are 

 absorbed in this form. The absorbed amino acids are carried 

 by the blood to the various protein tissues of the body, such 

 as the muscular tissue, the connective tissue, etc. Inasmuch 

 as all proteins are not composed of the same amino acids, 

 each tissue probably selects from the blood only those amino 

 acids which can be used for its repair. 



It has already been shown that growth consists largely 

 of an increase of the protein tissues of the body. Thus the 

 tissues take up from the blood those amino acids which 

 may be used for growth and build them up into the tissue 

 proteins. Also they may be used for foetal development 

 or milk production. If there is any surplus of amino acids 

 in the blood above the needs of the protein tissues for repair 

 and growth, it may be oxidized for the liberation of energy. 

 The principal end products of the oxidation of the amino 

 acids are water, carbon dioxide, urea, ammonia, and uric 

 acid. The carbon dioxide is excreted largely through the 

 lungs, and the other products are excreted through the 

 urine, mainly, and through the perspiration, to a slight 

 extent. 



Any surplus of amino acids above the demands of the body 

 for repair and growth which is not used for fuel purposes 

 probably may be converted into fat and glycogen and stored 

 as such in the body, although this has never been proven 



