RESPIRATION 



163 



As the lower end of the trachea enters the chest, or thorax, it 

 divides into right and left branches, the bronchial tubes, which 

 thereupon directly enter the lungs, each of which is a bag of elastic, 

 spongy tissue. Within the lungs, the bronchial tubes divide into 

 smaller and again smaller branches, until finally they form micro- 

 scopic twigs, each ending in one or more tiny air sacs, or alveoli. 

 Thus there are many thousands of alveoli in the human lungs, 



Alveolus 



Right bronchial tube 

 Connective tissue 



Trachea 



Body wall 

 Pleura 



Pleura 



covering 



lung 



Pleural 

 cavity 



Right lung cut open 



Space occupied 

 by heart, etc. 



Diaphragm 



Left lung intact 



Fig. 118. — Diagram of a vertical section through the human thorax, 



showing lungs and associated structures. 



everyone in direct communication with the outer air. Further- 

 more, each alveolus is profusely supplied with tiny thin-walled 

 blood vessels, or capillaries, through which flows blood sent 

 by the heart and soon to return to the heart so that it may be 

 distributed to every part of the body. It is while the blood is in 

 the capillaries of the alveoli that it gives up to the air in the alveoli 

 carbon dioxide, water, and heat taken from the tissues, and at the 

 same time receives oxygen. This is effected through the delicate 

 walls of the capillaries and alveoli. So the alveoli are really the 

 effective surface of the lungs. (Fig. 118.) 



