XXIII. EXCRETION. 



In the preceding chapters we have traced in detail the 

 processes by means of which food and oxygen are obtained 

 and distributed to the cells of our tissues. We have also 

 seen that the function of some of this food is to make 

 new protoplasm, while other food substances free energy 

 by uniting chemically with oxygen in the process called 

 oxidation, and thus give to the protoplasm its varied 

 power of action, secretion, contraction, or the like. The 

 making of food into new protoplasm we call assimilation, 

 while the production of energy we call oxidation. We 

 give the name of constructive metabolism or anabolism to 

 all the processes which lead up to the production of activity 

 on the part of the cells. AVhen oxidation takes place two 

 results foUow, namely, the activity of the cell through the 

 energy produced, and the production of wastes such as 

 carbon dioxide and urea. This breaking up of the cell 

 through oxidation is called destructive metabolism or 

 katabolism. Anabolism and katabolism, which together 

 make up metabolism, are going on continually as long as 

 the cells remain active. We have seen in our study of 

 respiration how the cells receive their oxygen. We have 

 also noted that the blood carries the carbon dioxide back 

 to the lungs, and that these organs remove it from the 

 body. How are the other wastes removed from the cells? 

 To answer this question we must study another set of 

 organs whose main function is the removal from the body 

 of waste products of the cell. All organs concerned in 



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