THE RATE OF RESPIRATION 183 



Oxidation of the food then takes place, releasing energy for cell 

 work and forming the wastes, carbon dioxide, water, and urea. 

 The carbon dioxide is given off to the blood. Respiration is of 

 two types, external respiration and internal respiration. External 

 respiration is concerned with inspiration and expiration. Internal 

 respiration involves the exchange of air betw^een the blood and 

 tissues. 



The rate of respiration. The average rate of respiration for an 

 adult is twelve to sixteen per minute. This rate is partly deter- 

 mined by the amount of carbon dioxide in blood. If this amount 

 rises above a certain percentage, the nerve centers controlling res- 

 piration are stimulated and this results in deeper breathing at an 

 increased rate. Thus more oxygen is obtained and the carbon 

 dioxide is removed more rapidly. This explains the second wind 

 of athletes. Due to violent exercise, a great deal of oxidation 

 takes place, causing the accumulation of carbon dioxide in the 

 blood. Then the nervous response follows, increasing the respira- 

 tory rate and giving the athlete more oxygen for releasing more 

 energy. The rate of respiration may, also, be influenced by strong 

 emotion and by old age. In respiration, the lungs are never 

 emptied of air ; only about one tenth of the air is normally 

 changed with each respiratory movement. This is called tidal air. 



Problem. Modifications of breathing. 



I. Analyze the type of respiration, and next to each write whether it is 

 an inspiration or an expiration in each of the following : yawning, sighing, 

 sneezing, coughing, hiccuping, sobbing. 



II. Explain how the control of the di^pliragm will probably suppress any 

 one of them. 



The air we breathe. Inspired air contains about 20.96 per cent 

 oxygen, 79 per cent nitrogen, and 0.04 per cent carbon dioxide. 

 Expired air contains about 16.4 per cent oxygen, 79 per cent ni- 

 trogen, and 4.1 per cent carbon dioxide. The same air can be 

 breathed many times before the oxygen is entirely exhausted. 



