THE RESPIRATORY SYSTEM 



339 



bronchioli, which in general are lined with a simple cuboidal non-respira- 

 tory epithelium. There is an elaborate network of capillaries in the 

 walls of the alveoU, so that only two extremely thin membranes separate 

 the blood in the capillaries from the air in the alveoU. J. S. Haldane has 



ALVEOLAR SAC 



Fig. 283. — Diagram of a lung lobule showing the subdivision of a bronchiolus into 

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

 (Redrawn after Bremer.) 



shown that during this exchange a constant quantity of carbon dioxide 

 is maintained in the alveoli and that the rhythm of breathing is dependent 

 upon this factor. 



Lungs are very elastic, and their elasticity is increased by the smooth 

 muscle fibers which extend into the connective tissue of the lungs as far 

 as the alveolar sacs but not into the walls of the alveoli. 



