134 RESPIRATION RESPIRATORY MOVEMENTS. 



nearly correct, it may be assumed that in the lungs, ^ to -g^ 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 c. c.) of fresh air are intro- 

 duced, sufficient only to fill the trachea and larger bronchial tubes, it is evi- 

 dent 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 respira- 

 tory apparatus and the air in the deeper parts of the lungs is constantly going 

 on by simple 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 con- 

 tains 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 by the fresh air. 



In obedience to the law established by Graham, that the diffusibility of 

 gases is inversely proportionate to the square root of their densities, the 

 penetration of atmospheric air, which is the lighter gas, to the deep portions 

 of the lungs would take place with greater rapidity than the ascent of the 

 air charged with carbon dioxide ; so that eighty-one parts of carbon dioxide 

 should be replaced by 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 vesicles, where the interchange of gases with the blood takes 

 place, maintains a nearly uniform composition. The process of aeration of 

 the blood, therefore, has little of that intermittent character which attends 

 the muscular movements of respiration, which would occur if the entire 

 gaseous contents of the lungs were changed with each respiratory act. 



