THE ESSENTIAL PHENOMENA OF RESPIRATION. 



15 



FIG. 4. 



BRONCHIAL 

 E8 



tions the air penetrates to the farthest vesicles of the 

 lungs. The walls of these vesicles are lined with innu- 

 merable little vessels, some of which bring in the blood 

 which has traversed the body (venous blood), while others 

 carry out the blood which here in the lungs becomes 

 charged with oxygen (arterial blood). The first are the 

 last ramifications of the pulmonary artery ; the second 

 unite to form vessels more 

 and more voluminous until, 

 on leaving the lungs, they 

 constitute the pulmonary 

 vein. 



Now that we know the 

 structure of the pulmonary 

 apparatus, it will be easy to 

 understand how respiration 

 takes place. It must never 

 be forgotten, however, that 

 respiration, properly speak- 

 ing, does not take place in the 

 lungs. These organs are 

 simply the locality in which 

 the exchange of gases takes 

 place, that is, the exchange 

 of pure air brought into the 

 vesicles for impure air 

 charged with carbonic acid 

 gas, which is brought by the 

 blood to the ramifications of the pulmonary artery. 



Let us first study how the air gets into the lungs, and 

 how it leaves the lungs after the exchange of gases, to 

 be again replaced by pure air. 



The conditions necessary for this entrance and exit 



The trachea (windpipe), bronchi, 

 and one of the lungs in section. 



