62 



RESPIRATORY MECHANISMS 



ing complexity of structure they combine a very large surface 

 area for the exchange of gases with very short diffusion dis- 

 tances within the air-space. 



This is brought out by the somewhat diagrammatic Figs. 

 34-37. In Fig. 34 the whole lung comprises only one 



"alveolus" with smooth walls 

 containing a dense network 

 of blood capillaries. In Figs. 

 35 and 36 there is an in- 

 creasing surface develop- 

 ment, but the whole is still 

 one cavity, allowing mixing 

 of the gases both by diffusion 

 and by convection, and the 

 entire surface is covered by 

 the respiratory epithelium. 

 In Fig. 37 we can distin- 

 guish both anatomically and 

 functionally between the air 

 passages, trachea and bron- 

 chi, in which practically no 

 exchange takes place with 

 the blood, and the alveolar 

 spaces. During inspiration air enters the alveoli from the 

 passages and from the outside, and when it flows in rapidly 

 it becomes mixed with the alveolar air. As a continuous 

 exchange takes place through the alveolar epithelium, diffu- 

 sion of oxygen and C0 2 molecules must also be a continuous 

 process within the air space. 



Fig. 38 is a diagram of a single unit in the human lung com- 

 prising a final bronchus, atrium, air-sacs, and hemispherical 

 alveoli. The size of such a unit is about 2 mm, and it is 

 evident that conditions for diffusion within it must be very 

 good. 



In the frog Rana esculenta, Krogh (1904) found that a lung 

 containing 5 ml air had a surface of nearly 100 cm 2 or 20 cm 2 

 per cubic centimeter. In man different estimates have been 



Fig. 34 



Fig. 35 



Fig. 34. Diagram of unialveolar 

 lung of Proteus, c.r. capillaries. (Ren- 

 ault.) 



Fig. 35. Diagram of Siren lung with 

 simple septa. (Renault.) 



