CHAPTER 1 8 



THE BREATHING SYSTEM 



In the previous chapter we saw how the tern performs the job of distributing the 

 energy-rich food is broken down and ab- oxygen, thus working hand in hand with the 

 sorbed into the blood stream, ready for dis- breathing system to solve this complex 

 tribution to all of the cells of the body problem of oxygen supply, 

 where it is used for growth, repair, and The definition of respiration, as generally 

 energy release through burning. In order accepted today, is not breathing but the 

 that burning can occur, oxygen must be actual burning of the food in the cells, re- 

 present, because all energy obtained by a leasing energy and the waste product, car- 

 living organism is through oxidation. Each bon dioxide. Breathing, on the other hand, 

 cell, then, must not only have a supply of is merely part of the mechanism by which 

 fuel and building materials but it must also cells obtain oxygen. Lungs, gills, and other 

 have oxygen to burn the fuel. The function structures are designed to remove oxygen 

 of the breathing system in animals is to pro- from the air or water and to pass it on to 

 vide a means of getting oxygen out of the some sort of transportation system by which 

 air and into the vicinity of the cell so that it can reach every individual cell. It is im- 

 it can be utilized. Another function is the portant that the distinction between breath- 



elimination of COo. In animals with consid- 

 erable bulk, where the cells are a great dis- 

 tance from the surface, the circulatory sys- 



ing and respiration be kept in mind during 

 the following discussion. 



All animals from amoeba to man respire 



475 



