Respiration - 359 



SOFT PALATE 

 PHARYNX 



ESOPHAGUS 

 TRACHEA 



NASAL PASSAGES 

 HARD PALATE 

 NOSTRIL 

 MOUTH 



JAW BONE 



TONGUE 



SALIVARY GLANDS 



LARYNX 



LEFT LUNG, SECTION 



ALVEOLI 

 PLEURA 



GALL BLADDER 



STOMACH 



\ I SPLEEN 



K\ \ / PANCREAS 



Fig. 19-3. The human respiratory system, semidiagrammatic. 



atmosphere. Accordingly lung-breathing ani- 

 mals have developed methods of ventilating 

 the recesses of the lungs. Unless air in the 

 lung sac is changed from moment to mo- 

 ment, oxygen is soon depleted from the pul- 

 monary air, and carbon dioxide accumulates 

 to toxic levels. 



In higher vertebrates the lungs arise as a 

 simple tubular outgrowth from the floor of 

 the pharynx. This lung bud pushes posteri- 

 orly — parallel and ventral to the esophagus 

 — until it reaches the thorax. Here the tube 

 forks, sending a branch to each side of the 

 thorax. Each branch gives rise to a lung, 

 which eventually occupies almost all the cor- 

 responding side of the thoracic cavity. 



The unbranched portion of the air passage 

 (Fig. 19-3) becomes the larynx and trachea, 

 and the two main forks become the bronchi. 

 On entering the lung each bronchus gives 

 rise to many branches, which form the 

 smaller bronchioles in all parts of the 

 lung. Eventually each bronchiole terminates 



blindly, by leading into a cluster of tiny 

 air chambers, the aeveoli (Fig. 19-4) Each 

 alveolus lies in intimate contact with a net- 

 work of capillaries; in the human lung, there 

 is an alveolar surface of more than 1000 

 square feet across which an aeration of the 

 blood occurs. All the many branches of the 

 "bronchial tree" are held in place by elastic 

 connective tissue that fills in the interalveo- 

 lar spaces; and each lung is covered as a 

 whole by its own external epithelium, called 

 the pleura. 



BRONCHIOLE 

 INFUNDIBULUM 



ALVEOLUS 



Fig. 19-4. Structure of a small portion of the human 

 lung. 



