PHYSICAL PKOCESSES IX CELLS. 



57 



erases applies also to the diffusion of several gases ; so that the general 

 rule may be formulated : If a number of gases exerting no chemical 

 influence on each other are allowed to enter a space, each gas will diffuse 

 itself uniformly through that space. 



If the amount of gas in any given space is small or large, or, in 

 other words, no matter what the gaseous pressure may be, another gas 

 will enter that space precisely as if it were a vacuum. 



The importance of these laws in explaining the mechanism of 

 gaseous interchange in respiration is very evident. The atmosphere is 

 composed of a mechanical mixture of about f nitrogen and | oxygen. 

 In the process of inspiration a variable amount of this 

 gaseous mixture is drawn into the lungs. It then meets 

 with a gaseous mixture which contains less ox}-gen than 

 the atmosphere (for a certain amount of the ox} T gen 

 taken in in previous inspirations has been removed by 

 the blood), and which contains a considerable volume of 

 carbon dioxide removed from the blood. 



Phenomena of diffusion, therefore, at once commence 

 between the air alread}' in the lungs and that which has 

 entered in inspiration. The air in the lungs becomes 

 gradually poorer in oxygen and richer in carbon dioxide, 

 as the air-cells are approached. Diffusion tends to equal- 

 ize this difference ; the oxygen of the inspired air diffuses 

 into the deeper portions of the lungs, the carbon dioxide 

 diffuses from the deeper to the upper portion, the process 

 being a constant one; for the difference in the relative 

 volumes of the two gases in the upper air-passages is 

 maintained by repeated expirations, by which C0 2 is 

 removed, anil inspirations, by which more oxygen is FlG ^ 



brought into the lungs. 



The COa formed in respiration by animal organisms, and thus 

 removed from them in respiration, distributes itself uniformly through 

 the atmosphere, so that there is everywhere a uniform percentage, 

 unless there is a local temporary increase ; but then this soon becomes 

 equalized by diffusion, permanent increase being prevented by the 

 absorption processes in the vegetable kingdom. 



The tension of in the atmosphere is far greater than that of C0 2 , 

 as the is present in far larger proportion, and conversely. 



Diffusion leads finally to the theoretical result, that all gases in any 

 given space, as in the atmosphere, exist under the same pressure ; when, 

 therefore, there is anywhere a temporary increase in the tension of a gas, 

 diffusion commences and tends to continue until there is a uniform 

 distribution and mixing of the different gases. 



