176 



RESPIRATION 



Effects of severe exercise. For those who are weak 

 and habitually inactive and unused to hard work, this 

 feeling of respiratory distress during unusual and severe 

 exercise may be associated with much strain and conse- 

 quent injury to the heart because of its increased labor. 

 The lungs themselves are but rarely injured in this way, 

 although the distress is felt to be respiratory, because 



Deep 

 breathing 



Moderate 

 breathing 



Quiet 

 breathing 



Fresh air 



. Air left 

 in lungs 



Pio. 104. Diagram showing relative amounts of fresh and stale air in lungs at 

 different depths of breathing. 



of the close relation between the nervous control of circu- 

 lation and respiration. 



Adjustment of respiration and circulation to 

 exercise. When muscles work, the blood coming from 

 them is, as we have seen, richer in waste products, 

 especially carbon dioxide, and poorer in oxygen and 

 food materials. This change in the blood is directly 

 proportional to the work done by the muscles. The 

 amount of blood thus affected depends on the size and 



