RESPIRATION AND EXCRETION 



383 



To 

 pulmonary 



vevn 



divides into two bronchi. The bronchi within the lungs break up 

 into a great number of smaller tubes, the bronchial tubes, which 

 divide somewhat like the small branches of a tree. This branch- 

 ing increases the surface of the air tubes within the lungs. The 

 bronchial tubes, indeed all the 

 air passages, are lined with 

 ciliated cells. The cilia of 

 these cells are constantly in 

 motion, beating with a quick 

 stroke toward the outer end 

 of the tube, that is, toward 

 the mouth. Hence, if any 

 foreign material should get 

 into the windpipe or bronchial 

 tubes, it will be expelled by 

 the action of the cilia. It is 

 by means of cilia that phlegm 

 is raised from the throat. 

 Such action is of great im- 

 portance, as it prevents the 

 filling of the air passages with 

 foreign matter. The bronchi 

 end in very minute air sacs called alveoli, little pouches having 

 elastic walls, into which air is taken when we inspire or take a 

 deep breath. In the walls of the alveoli are numerous capil- 

 laries, the ends of arteries which pass from the heart into the 

 lung. It is through the very thin walls of the alveoli that an inter- 

 change of gases takes place which results in the blood giving up part 

 of its load of carbon dioxide, and taking up oxygen in its place. 



The Pleura. The lungs are covered with a thin elastic membrane, 

 the pleura. This forms a bag in which the lungs are hung. Between the 

 walls of the bag and the lungs is a space filled with lymph. By this means 

 the lungs are prevented from rubbing against the walls of the chest. 



Breathing. In every full breath there are two distinct movements, 

 inspiration (taking air in) and expiration (forcing air out). In man an 

 inspiration is produced by the contraction of the muscles between the 

 ribs together with the contraction of the diaphragm, the muscular wall 

 just below the heart and lungs ; this results in pulling down the diaphragm 

 and pulling upward and outward of the ribs, thus making the space within 



Diagram to show what the blood loses and 

 gains in one of the air sacs of the lungs. 



