328 KBSPIEATION 



the composition by volume of expired air is shown in the 

 following table : 



Expired Air. 



Oxygen 16.02% 



Nitrogen 79.00% 



Carbon dioxide 4.38% 



Water 0.60% 



A comparison of this table with that for inspired air shows 

 that in the lungs the air loses some 4.94% of oxygen, and 

 takes on some 4.34% of carbon dioxide and 0.6% of water. 

 The proportion of nitrogen remains imchanged. Expressed 

 in terms of a day's breathing, the body obtains for the tis- 

 sues, in a day, about two pounds of oxygen, and loses in 

 that time some two pounds of carbon dioxide and a half a 

 pound of water. 



Expired air also contains traces of ill-smelling, organic 

 compounds. These are what give the bad smell to a poorly 

 ventilated room that has been occupied for some time by 

 people. They render the air dangerous and poisonous to 

 breathe. A bad digestion also adds to the ill-smelling 

 compounds which, emerging from the gullet, mix with the 

 expired air and make a fetid breath, while decaying teeth 

 add still other odoriferous compounds. 



Blood changes in the lungs. — The tables above indi- 

 cate only the changes which take place in the inspired 

 and expired air. They do not explain their purpose. 

 To do this we must recall the structure of the lungs. 

 It will be remembered that the inspired air ultimately 

 reaches the air sacs and that these sacs are lined with 

 thin walled capillaries. The blood, which has been cir- 

 culating through the body tissues, comes to these capillar 



