230 



How a Complex Ani?nal Uses Food unit iv 



Fig. 230 An '"iruii laiig."' This apparatus is used to produce breathing inoveinents for 

 paralyzed or weak patients. How does it work? (children's medical center, boston) 



the chest cavity the pressure of air inside 

 the chest cavity becomes less. The pres- 

 sure of air outside the lungs is greater 

 and air is pushed in. It moves through 

 the nose and mouth passages and through 

 the windpipe and bronchial tubes into 

 the air sacs. 



If you then make the chest cavity 

 smaller, the walls press on the air sacs, 

 thus raising the pressure of air inside. 

 Since the air pressure inside is greater 

 than the pressure outside, air moves out 

 of the sacs. It flows through the bron- 

 chial tubes and out of the nose and mouth 

 passages. This is exhaling. Under normal 

 conditions we inhale and exhale about 18 

 times a minute. You can make a model of 

 a chest cavity that can be made to 

 "breathe." See Exercise 5. 



In poliomyelitis (infantile paralysis) 

 the chest muscles and diaphragm are 

 sometimes paralyzed. An "iron lung" 

 (Fig. 230) is used to maintain the breath- 

 ing motions. Can you figure out how it 

 works.^ Be sure to check your reasoning. 



During the breathing movements as 

 the lungs fill up with air and rub against 

 the inside wall of the chest cavity there 

 might be friction betw^een these two 

 surfaces. But friction is reduced to a 

 minimum because the lungs are covered 

 and the cjiest walls are lined with thin, 

 moist, very smooth membranes, called 

 the pleura. 



Few air sacs are filled when you inhale. 

 Most of the air sacs, particularly those 

 that lie farthest away from the windpipe, 

 neither receive nor lose air during or- 



