CHAPTER 9 



THE RESPIRATORY SYSTEM 



The energy for the carrying out of Hfe processes is largely derived 

 from the oxidation of the simple sugar, glucose. This reaction is in- 

 dicated by the following simplified equation : 



CeH,=Oe+60. > 6CO. + 6H=0 + free energy 



The only two substances involved in this equation are the sugar, in 

 this case glucose, and oxygen. In the chapter on digestion, the manner 

 in which food materials were made available to the cells was explained ; 

 the present chapter is concerned with the manner in which the oxygen 

 is made available and the carbon dioxide is removed. 



The circulatory system in both these cases actually delivers the glu- 

 cose and the oxygen to the sites where they are needed. The prepara- 

 tion of the food for this use was the function of the digestive system ; 

 similarly the function of the respiratory system is to make the essential 

 oxygen available to the circulating medium. 



No matter how complex the accessory structures may be, all 

 respiratory devices have three essential requirements : there must be 

 a thin, moist living membrane with oxygen on one side and a cir- 

 culatory medium on the other. Throughout the different animal groups, 

 various respiratory devices have been developed, but all have this same 

 basic structure. 



So long as an animal is very small, no elaborate respiratory devices 

 are needed. Diffusion of oxygen into the body cell or cells occurs 

 through the cell membrane. This is the condition among the protozoans 

 and among the less complex metazoans such as the sponges and some of 

 the worms. As the body bulk increases, simple diffusion is not suffi- 

 cient to supply oxygen to the inner cells. In larger animals, various 

 special structures increase surface for respiratory exchange in limited 

 areas. Accompanying these developments, the circulatory mechanism 

 increases in complexity and efficiency. In the earthworm, a fairly simple 

 development of this type can be seen. Here the moist skin serves as 



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