DIFFUSION OF AIR IN THE LUNGS 115 



cubic centimeters) of air, and ascertained that the volume had dimin- 

 ished by sixty-one cubic inches (1000 cubic centimeters), or a little 

 more than one-fiftieth. Adopting the approximations of Davy and 

 Cuvier, applied to the human subject, as nearly correct, it may be 

 assumed that in the lungs one-seventieth to one-fiftieth of the inspired 

 air is lost. 



Diffusion of Air in the Lungs. When it is remembered that with 

 each inspiration, but about twenty cubic inches (327.7 cubic centimeters) 

 of fresh air are introduced, sufficient only to fill the trachea and larger 

 bronchial tubes, it is evident that some forces must act by which this 

 fresh air finds its way into the air-cells, and the vitiated air is brought 

 into the larger tubes, to be expelled with the succeeding expiration. 



The interchange between the fresh air in the upper portions of the 

 respiratory apparatus and the air in the deeper parts of the lungs is 

 constantly going on by diffusion aided by the active currents or impulses 

 produced by the alternate movements of the chest. In the respiratory 

 apparatus, at the end of an inspiration, the atmospheric air, composed of 

 a mixture of oxygen and nitrogen, is introduced into the tubes with a 

 considerable impetus and is brought into contact with the gas in the 

 lungs, which is heavier, as it contains a certain quantity of carbon 

 dioxide. Diffusion then takes place, aided by the elastic lungs, which 

 are gradually forcing the gaseous contents out of the cells, until a 

 certain portion of the air loaded with carbon dioxide finds its way to 

 the larger tubes, to be thrown off in expiration, its place being supplied 

 with fresh air. 



In accordance with the law that the diffusibility of gases is in 

 inverse ratio to the square root of their densities, the penetration of 

 atmospheric air, which is the lighter, to the deep portions of the lungs 

 would take place with greater rapidity than the ascent of air charged 

 with carbon dioxide ; so that eighty-one parts of carbon dioxide should 

 be replaced with ninety-five parts of oxygen. It is found, indeed, that 

 the volume of carbon dioxide exhaled is always less than the volume of 

 oxygen absorbed. This diffusion is constantly going on, so that the air 

 in the pulmonary alveoli, where the interchange of gases with the 

 blood takes place, maintains a nearly uniform composition. The pro- 

 cess of aeration of the blood, therefore, has little of that intermittent 

 character which attends the muscular movements of respiration. 



