384 



HUMAN PHYSIOLOGY 



3. Take a moderate breath, and force air (tidal air) through hmewater. 

 Notice what occurs. 



4. Force the last part of a deep expiration (reserve air) through lime- 

 water. Note result. What is one reason for deep breathing? 



Changes in Air in the Lungs. — Air is much warmer after leaving 

 the lungs than before it enters them. Expired air contains a con- 

 siderable amount of moisture, which it has taken up in the air 



sacs of the lungs. The presence of car- 

 bon dioxide may easily be detected in 

 expired air. Air such as we breathe 

 out of doors, contains, by volume : — 



Nitrogen 79.00 



Oxygen 20.90 



Carbon dioxide 04 



Tidal Air 

 30 cu. in. 



sResMtfM 



WO'O^ciG ^iW. 



^30 

 cu. in. 



Air expired from the lungs, con- 

 tains : — 



Nitrogen 79 



Oxj^gen 16 



Carbon dioxide 4+ 



In other words, there is a loss of be- 

 tween four and five per cent oxygen, 

 and nearly a corresponding gain in car- 

 bon dioxide, in expired air. There are 

 also some other organic substances pres- 

 ent. The volume of carbon dioxide 

 given off is always a little less than the 

 amount of oxj^gen taken in. This seems 

 to show that some ox^^gen unites with 

 some of the chemical elements in the body. 



Loss FROM THE LuNGs. — A man expires about 540 cubic inches of air 

 per minute; this would make a total of something over 770,000 cubic inches 

 in twenty-four hours. Of this air, about 4 per cent is carbon dioxide, the 

 amount varying with the amount and kind of work performed. There 

 would be at least 31,000 cubic inches of carbon dioxide expired in twenty- 

 four hours; this amount weighs nearly thirty ounces. The amount of 

 water evaporated by the lungs in twenty-four hours is estimated at half 

 a pint, so the lungs are a source of actual loss in body weight. 



Diagram showing the relative 

 amounts of tidal, comple- 

 mental, reserve, and residual 

 air. The brace shows the 

 average lung capacity for the 

 adult man. 



