Mammalia: Visceral Systems 



617 



.'-Alveolar Sac 



Fig. 472. Diagram of a lung lobule, showing the subdivision of a bronchiole 

 into alveolar ducts, sacs, and alveoli. Respiratory epithelium may extend into 

 the bronchioles. (After Bremer. Courtesy, Neal and Rand: "Chordate Anatomy," 

 Philadelphia, The Blakiston Company.) 



When the lungs are filled, the air-spaces are distended and the elastic 

 fibers are stretched. Emptying the air-spaces during expiration depends 

 largely on contraction of the elastic tissue, assisted more or less by 

 muscular contraction. 



The terminal respiratory structures are of minute dimensions. In 

 man the nonrespiratory bronchioles have diameters of 1.0 mm. to 0.5 

 mm. The respiratory bronchioles are 0.5 mm. or less in diameter. A 

 human alveolus is about 0.1 mm. in diameter at the end of expiration 

 and may swell to a diameter of 0.3 mm. during inspiration. Attempts 

 have been made to estimate the number of alveoli. The number varies 



