124 GENERAL ZOOLOGY 



Carbon dioxide, whether derived from carbohydrates, fats, 

 or proteins, makes its way by the blood to the lungs, and 

 there passes through the moist membrane and is exhaled. 

 The carbon dioxide which is given off through the respira- 

 tory organs is produced within the living protoplasm. 

 Lungs, gills, or other respiratory surfaces are regions where 

 the blood supply from all parts of the body is able to get 

 rid of its carbon dioxide. Between the entrance of the oxy- 

 gen into the body and the discharge of carbon dioxide, the 

 essential part of respiration takes place. This is the actual 

 union of oxygen with stored food material within the proto- 

 plasm or with the protoplasm itself. 



Metabolism. In the foregoing paragraphs we have shown 

 how food material through the process of digestion and 

 assimilation becomes made over into the substance of a 

 living animal. The term anabolism covers all these changes 

 from the time the food enters the body until it has become 

 a part of the living animal either as living protoplasm or 

 as stored food material. All the constructive or building-up 

 activities of the animal body, namely, growth, repair, and 

 storage, are the products of anabolism. 



Living bodies are constantly changing. One of the most 

 characteristic changes is the destruction of protoplasm and 

 stored material to furnish the energy characteristic of living 

 things. These destructive changes make up the process 

 called katabolism. Respiration and the getting rid of other 

 waste materials which have been part of the living animal 

 (excretion, see page 125) are the chief manifestations of 

 katabolism. 



All the physiological and chemical changes which occur 

 between the time when food is taken into the body and when 

 it is used or broken down to liberate energy are collectively 

 called metabolism. Metabolism is thus the sum of all the 

 changes involved in anabolism plus those of katabolism. 



