PURE AIR 111 



see that besides a watery substance called plasma, 

 it contains thousands of minute disks commonly 

 named corpuscles. Now, these tiny boats in the 

 blood carry oxygen to the tissues of the body. On 

 their circuit through the capillaries the blood also 

 collects carbonic acid gas and other poisonous matter, 

 which it bears to the lungs. 



Now let us consider the marvelous change that takes 

 place here. In the cells of the lungs is the air with its 

 oxygen. In the capillaries, separated from the air 

 only by the most delicate of membranes, is the blood 

 with its freight of carbonic acid gas. The separating 

 membrane is so thin that the law of diffusion of gases 

 still governs. The blood gives up much of its poison- 

 ous carbon dioxid through the thin membrane, 

 receiving in return the purifying oxygen, which at 

 once changes its color to a bright red. The oxygen 

 is then rapidly borne by the blood through the pul- 

 monary vein to the left side of the heart. The beat- 

 ing (contracting) of the heart sends it thence through 

 the aorta and its branching arteries to the remotest 

 capillaries of the body. 



While passing through these capillaries, the blood 

 deposits its precious freight of oxygen wherever tissues 

 are ready to be burned. It also collects carbonic 

 acid gas where the oxidized tissues have produced it, 

 and comes back to the lungs again to give up its dark 

 cargo of poisonous gases and receive a fresh supply of 

 oxygen in return. The many thousands of air-cells 



