A STUDY OF RESPIRATION 



215 



FIG. 103. Two Air Sacs 

 with their Branches. 



expanded; but as expiration begins, 

 the elastic walls help to force back 

 through the branches of the windpipe 

 the air that has been taken into the 

 lungs (Fig. 103). 



Blood Supply. The pulmonary ar- 

 tery, as we have learned, arising from 

 the right ventricle, soon divides into 

 two branches, one for the right and 

 one for the left lung. Within the 

 lung tissue each blood vessel divides 

 into small arteries that follow the 

 course of the bronchial tubes to the = ending of a bronchial 



air sacs. Here the arteries communi- tube. 



.,. . . ... 6 = pouches from air sacs, 



cate with a maze of capillaries which 



run just beneath the thin lining of the air sacs. It is here 



that the exchange of 

 material takes place be- 

 tween blood and the in- 

 haled air, for the two 

 are separated only by 

 the extremely thin walls 

 of the air sacs and of 

 the capillaries. From 

 the pulmonary capillar- 

 ies of each lung the blood 

 is carried back to the left 

 auricle by two pulmon- 

 ary veins. 

 FIG. 104. Blood Capillaries (injected) in The lungs, like all 



Walls of Air Sacs. White Spaces are the other organs of the 



Cross Sections of Air Sacs. Dark Lines 



are the Capillaries, magnified about 30 body, have a certain 



times. Photographed through the mi- amO unt of work to do. 



Material must therefore 



be provided to supply the waste of the tissues. This is fur- 

 nished by a second set. of arteries (the bronchial arteries) that 



