Diaphragm 



228 How a Complex A?ii?nal Uses Food unit iv 



cartilage. Their walls contain, besides 

 the epithelial lining, rings of involuntary 

 muscle. In an attack of asthma these 

 muscles contract; this results in partially 

 shutting off the air supply. 



The lungs — a vast membrane. A study 

 of lamb or beef lungs, as described in 

 Exercise 3, will help you to understand 

 the structure of lungs. The branch from 

 the windpipe disappears inside of the 

 lung. Here, as you know, it divides into 

 branches, bronchial tubes, taking on 

 the appearance of a tree without leaves. 

 The branching is clearly diagramed in 

 Figure 226. The bronchial tubes are not 

 lung tissue. The lung tissue itself is found 

 where the leaves of the tree would be. 

 Each fine twig opens out into an air sac 

 whose walls are made of a single layer of 

 flat cells (epithelium) and a thin layer 

 of elastic tissue. This is the lung tissue. 

 Each sac can fold up into a small space 

 or stretch to a great size. In the walls of 

 each air sac there is a network of capil- 

 laries (see Fig. 227). 



If the lungs were a simple bag the 

 number of square inches of membrane 

 that could be brought in contact with 

 capillaries would be small. A large house 

 left as an empty shell would have rela- 

 tively httle wall surface inside. If, how- 

 ever, this same building were divided up 

 into apartments the interior wall surface 

 would be increased astonishingly. The 

 lung is built on the same principle. For 

 this reason, the lung membrane has an 

 enormous area. All of the area is in 

 contact with capillaries. Gases are con- 

 tinually diff^using into the capillaries and 

 out of them. Thus the composition of the 

 blood is rapidly changing as it flows 

 through the lungs. 



A B 



Fig. 229 Figure A shows the chest cavity while 

 you are exhaling. Figure B shows it while inhal- 

 ing. Co7//pare the position of the diaphragm in 

 A and B. What difference do you note? What 

 differejice do you observe in the ribs of the two 

 chest cavities? How do the ijwveinents of the 

 diaphragm and ribs cha?7ge the size of the cav- 

 ity? Explain why the changes in size cause 

 movement of air into and out of the lungs. 



The importance of elasticity in the 

 lungs. If the lungs were not elastic it 

 would be impossible for you to breathe, 

 that is, to inhale and exhale air. The 

 chest moves regularly every few seconds. 

 It grows larger and smaller, larger and 

 smaller. And so do the lungs. When the 

 chest cavity increases in size the lungs 

 follow suit; when it grows smaller, the 

 lungs also grow smaller. Because the 

 lungs are elastic they can change size 

 with the chest cavity. Some of you prob- 

 ably can enlarge your chest cavity more 

 than others. Try your chest expansion by 

 breathing out and measuring your chest 

 just below the armpits with a tape meas- 

 ure. Then take as deep a breath as you 

 can and again record the size of your 

 chest. 



How is the chest cavity changed in size? 

 The experiment you have just performed 



