BLOOD CHANGES IN THE LUNGS 



327 



ries from the heart (pulmonary arteries). The corpuscles 

 of this blood are laden with the carbon dioxide which they 

 have absorbed from the tissues. When these corpuscles 

 reach the lung capillaries their carbon dioxide passes, by 

 diffusion, through the 

 walls of capillary and 

 air sac into the cavi- 

 ties of the sacs. At the 

 same time, the hasmo- 

 globin in the corpus- 

 cles absorbs the oxygen 

 of the inspired air in 

 the sacs. Similarly, by 

 dialysis, the organic 

 compounds and water 

 are given off to the 

 sacs from the blood 

 plasma. In this way 

 the blood in the capil- 

 laries is enabled to 

 renew its supply of 

 oxygen, rid itself of its burden of carbon dioxide 

 and excess of water, and finally to return to the heart 

 (pulmonary veins). From there it is sent out to the 

 tissues again. This interchange, therefore, accounts for 

 the manner in which the oxygen gets into blood, and carbon 

 dioxide out of it. It is to be noted in this action that, 

 except for the tiny amounts of organic compounds given 

 off, no true wastes pass from the blood to the air in the air 

 sacs. On this account, breathing should not be spoken 

 of as the purifying of the blood. What actually happens 

 is the aerating of the blood, that is, an exchange of carbon 



Bronchiole 



Fig. 152 — Diagram ol two air cells, showing 

 the capillary network whicli covers them, 

 and at a the structures which intervene be- 

 tween the air and the blood are indicated ; 1, 

 mucous membrane of the air cell ; 2, submu- 

 cous meshwork ; 3, wall of capillary ; 4, 

 plasma in capillary ; 5, red blood corpuscle. 



