THE RESPIRATORY SYSTEM. 67 



organs is much quickened by exercise. So the rapid- 

 ity of the flow of the blood through the lungs de- 

 pends very much on the degree of our bodily activity. 

 When we lie in bed, for instance, the circulation goes 

 on very steadily, and our breathing is performed 

 very moderately and quietly. But as soon as we 

 arise and move about, both the circulation and respi- 

 ration are quickened. The more vigorous the activ- 

 ity of the body the more air is drawn into the lungs 

 to purify the greater quantity of flowing blood. The 

 chest muscles act more strongly and every cell in the 

 lungs is inflated. All this tends to produce pure 

 blood and active lungs, and, consequently, good 

 health. 



The quality or purity of the air which 



t^Bre'ath. * we breathe is quite as important as the 

 quantity which we inhale. We have 

 learned that the oxygen, which is one of the elements 

 or gases which compose the air, is taken from the 

 lung-cells to unite with the blood. This alone would 

 make the breath which is given out impure because 

 of its having been robbed of its oxygen. But the air 

 which is forced out from the lungs is made much 

 more impure by the gases which come from the im- 

 pure blood of the body. These gases escape from 

 the lungs at every breath. It is plain that if we 

 breathe in a close room or in a confined body of air, 

 every breath adds to the degree of impurity of the 

 air, so that the latter becomes more and more unfit 

 to breathe. It becomes unfit to sustain life, not only 



