RESPIRATION 



159 



the passage of oxygen into the blood may be a process of 

 diffusion. 



Land animals have an exchange between air and tissue 

 fluids. Insects possess tubes (tracheae) which pass from the 

 exterior to reach all parts of their bodies, but the usual 

 arrangement is an air sac or lung through which exchange 

 takes place between the contained gases and the circulating 

 fluid. 



Birds have lungs, and in addition they have air sacs. 

 The function of these air sacs is probably two-fold. First 

 of all they penetrate into the bones and in that way are useful 

 mechanically in that they permit a greater degree of strength 

 with less increase in weight, just as hollow bicycle tubing is 

 stronger for the same weight than solid 

 steel rods. The other function is respira- 

 tory. The thorax of flying birds must 

 be rigid for the attachment of muscles, 

 therefore the lungs cannot expand so 

 freely as in non-flying animals. The air 

 sacs pump air in and out through the 

 bronchi, thus mixing fresh air with the air 

 in the lung alveoli. 



Lungs are bellows which by expansion 

 and contraction draw in and expel air. 

 The structure of lungs is really that of a 

 number of minute bellows. The larger air 

 conducting tubes (trachea and bronchi) 

 divide and subdivide until a multitude of 

 extremely minute tubes are formed. Each 

 of these ends in a dilated extremity with a 

 number of pouches in its walls. 



The expansion of the lungs is really the 

 integrated expansion of the large number of these small air 

 sacs (alveoli). 



Gas exchange takes place between the air in the air sacs 

 and the blood in the capillaries covering the wall of the air 

 sac. The air in the conducting tubes is not of any use for 

 respiratory exchange. We distinguish between the air in 

 the tubes and in the alveoli by calling the former the air 

 in the dead space and the latter the alveolar air. If we 

 wish to measure the actual amount of air used in the alveoli 

 we must subtract the volume of the dead space from the 

 total volume breathed. 



From the paragraph above it is clear that the expired air 



FIG. 35. Lung 

 of mammal. 



The alveoli expand 

 allowing air to pass in 

 directly. Contraction 

 causes the air to be 

 forced out again. The 

 blood vessels form a 

 fine network over the 

 surface of the alveoli 

 and gas exchange 

 takes place between 

 the blood and air in 

 the air sacs. 



