THE BREATHING SYSTEM 



485 



LUMO 



TISSUE 



Fig. 18-9. This is a schematic representation of how respiration occurs in the alveoli (external) and in the tissues 



(internal). An explanation is found in the text. 



parts of the body. Receptors lie in the walls 

 of the larynx and pharynx which are sensi- 

 tive to foreign bodies or harmful gases. For 

 example, when a particle of food gets into 

 the larynx, breathing ceases at once to pre- 

 vent the passage of the particle farther into 

 the delicate respiratory tract. This is fol- 

 lowed by a closing of the epiglottis until 

 considerable pressure is built up in the 

 chest cavity. It then lets go explosively in 

 the form of a cough as already described. 

 Likewise, if irritating gases such as chlorine 

 are drawn into the pharynx, breathing 

 ceases instantly with the closing of the air 

 passage by the epiglottis. Here is another 

 safeguard to prevent injury to the extremely 

 sensitive and delicate lining of the respira- 

 tory tract. Severe pain and sudden chilling 

 will also inhibit the respiratory center. One 

 also "catches his breath" because of the 

 action of these nerves on the respiratory 

 center. Thus we see how this mechanism is 

 tuned to the job of protecting the important 

 breathing organ so that its function may go 

 on uninterrupted throughout life. 



GASEOUS EXCHANGE 



The exchange of COo and Oo in the 

 alveoli of the lungs is called external res- 

 piration, in contrast to the exchange at the 

 tissue cells which is referred to as internal 

 respiration. By measuring the amount of 

 oxygen and carbon dioxide in expired air 

 as compared to that of the atmosphere, it is 

 easy to demonstrate that some exchange has 

 taken place, as shown in the following 

 figures: 



Inspired Expired Differ- 



Air Air ence 



Oxygen 20.0% 14.5% 5.5% 



Carbon dioxide . . 0.04% 5.5% 5.5% 

 Nitrogen 79.0% 80.0% 1.0% 



Water and other gases have not been taken 

 into account in the above approximate fig- 

 ures. It is noted that atmospheric air has 

 lost approximately 5 per cent of its oxygen 

 and has picked up approximately 5 per cent 

 carbon dioxide in passing through the 

 alveoli of the lungs. Blood passing near the 

 alveoli has lost most, but not all, of its car- 



