HOW DO WE BREATHE? 



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then, if work is done in the cells, oxygen must get to 

 all parts of the body in order to release energy there. 

 To get oxygen there, it is first necessary to get it in- 

 side the body. Here is where the process of breathing 

 comes in. 



How Do We Breathe? Study carefully the diagram 

 below. You will notice that the air passage leads from 

 the mouth down into the chest, where it divides into 

 two branches and finally breaks up into small branches 

 which end in a mass of tiny air sacs in the lungs. The 

 lungs are really spongy masses of air sacs and connecting 

 tubes. The walls of these little clusters of sacs are lined 

 with blood vessels, and 

 when air passes into 

 them from the outside 

 when we take a breath, 

 oxygen gets through 

 these thin walls of the 

 blood vessels into the 

 blood. While this is 

 happening, another gas, 

 carbon dioxide, passes 

 out from the blood into 

 the air in the little 

 sacs. Thus we see an 

 exchange of gases takes 

 place in the lungs. But 

 this does not get the 

 air into the cells ; that 

 is accounted for by the 

 circulation of b,lood, 

 which, as we shall see 

 later, carries the oxygen to all parts of the body by 

 means of the red corpuscles and unloads it where it will 

 be used in the cells. 



The breathing apparatus of man. Read the 



text and explain how and where oxygen might 



get into the blood. 



