RESPIRATION OF TETRAPODS 



59 



(b) 



mm. Hg. Pressure Intrapulmonic 



765 



.Pericardium 

 76( 



Lung 



m 



\ 



Diaphragm 

 (insp.) 



755 



750 



500 CCS. 



u 



-t 

 O 



Fig. 18. 



(a) Diagram to illustrate the basic structure of mammalian respir- 

 atory systems, (b) The changes in pressure in the pleural cavities and 

 within the lung are shown during a normal respiratory cycle, (b after 

 Ruch and Fulton, 1960.) 



of the alveolar air. This constancy contrasts with the marked 

 differences in composition of the inspired and expired air. 

 Inspired air contains 21 % of oxygen and 0-04% of carbon 

 dioxide, whereas expired air has only 16-4 % O2 but 4- 1 % carbon 

 dioxide. This percentage of carbon dioxide increases if a man 

 breathes slowly and deeply because an increasing proportion 

 comes from the alveolar air. With hyperventilation (forced 

 breathing) the CO2 content of the alveolar air falls and hence 

 that of the expired air. 



The gaseous exchange which takes place in the alveoli is 

 greatly facilitated by their enormous numbers. There may be 

 as many as 350 million alveoli in each lung of a man. Each 

 alveolus is less than | /x thick and is only IOOijl across.* The total 

 internal surface of the respiratory epithelium is therefore 



* Collapse of the alveoli on account of their small size and surface 

 tension forces is prevented by the presence of a thin lining of surface 

 active material. 



