DIGESTION AND RESPIRATION 533 



Bronchiole. 



Surf 



ace viev/ 



Ve-nul 



Alveolar duct 



Alveola.r sac 



Alveoli 



Arte^riole- 

 Capillaries 



Figure 26.9. A, Termination of the respiratory passages in the mammalian lung; 

 B, a further enlargement to show the dense capillary network covering a single 

 alveolus; C, an alveolus in section. Alveoli have a diameter of 0.2 to 0.3 mm. 



The bronchi branch profusely within the lungs and the walls of 

 the respiratory passages become progressively thinner (Fig. 26.7). Each 

 passage eventually terminates in an alveolar sac whose walls are so 

 puckered by pocket-shaped alveoli that it resembles a cluster of grapes 

 (Fig. 26.9). A dense network of capillaries is intimately associated with 

 the wall of the alveoli. Whether the capillaries themselves form the 

 wall of the alveoli, and hence are in direct contact with the alveolar 

 air, or whether they are separated from the alveolar lumen by a thin 

 layer of epithelium, has long been a controversial problem. In recent 

 years Low and others have studied the structure of the lung with the 

 electron microscope, and they find that the alveoli do have a very thin 

 epithelial wall of their own separating the lumen from the capillaries. 

 The plexus of capillaries covering the alveoli is so dense that little 

 space is left between the individual vessels. All this provides a huge 

 protected area for the exchange of gases. A large surface is, of course, 

 essential in a homoiothermic animal. A frog's lung is a hollow sac with 

 a few pockets in its wall, but the mammalian lung is greatly subdivided 

 internally and is like a fine-grained sponge. 



227. The Mechanics and Control of Breathing 



Mammalian lungs are ventilated by changing the dimensions of 

 the thorax and consequently the pressure within the lungs. During 

 normal, quiet inspiration, the size of the thorax is increased slightly, 



