234 



THE INTERNAL ENVIRONMENT OF THE RODY 



Part III 



Esophoqus 



Swim 

 bladder 



^ 



u 



\JI 



To other 

 lung 



FISH 



SALAMANDER 



FROG 



To other 

 lung 



To other 

 lung 



TOAD 



REPTILE 



DETAIL OF 

 REPTILES LUNG 



Fig. 13.9. The evolution of lungs shows a great increase in the area of lining 

 exposed to air, and the close association of air and blood, the latter circulating in 

 capillaries between the lining and the covering of the lungs. The great develop- 

 ment of the lining is emphasized in this figure. The lining is the membrane 

 through which oxygen and carbon dioxide pass to and from the blood, i.e., where 

 external respiration occurs. 



by branches of the vagus nerves. These control the size of the passageways 

 through the bronchioles, many of which are closed in ordinary shallow breath- 

 ing. In the disease of asthma large numbers of them are closed spasmodically. 

 The capillary-covered alveoli are the real functional structures of the lung, 

 the part of it in which the major exchange of gases takes place (Fig. 13.10). It 

 is estimated that there are 400 million of these in human lungs and that four 

 to five quarts of blood pass through the lungs per minute during rest, and at 

 least 20 times that during violent exercise. 



Passage of Air to and from the Lungs. Air is normally inhaled through the 

 nostrils into the nasal chambers. There it is broken into eddying currents as it 

 comes in touch with the warm, ciliated, mucous epithelium that covers the 

 turbinate bones that hang down like curtains into the nasal chambers (Fig. 

 13.12). This combination of structures constitutes an air conditioner, heater, 



