CH. xxiv.] DIFFUSION OF GASES. 377 



Diffusion of Gases within the Lungs. If two chambers 

 containing a mixture of gases in unequal amount are connected 

 together, a slow movement called diffusion takes place until the 

 percentage amount of each gas in each chamber is the same. Let 

 us suppose that one chamber contains a large quantity of oxygen 

 and a small quantity of carbonic acid ; and the other a small 

 quantity of oxygen and a large quantity of carbonic acid ; the 

 oxygen moves from the first to the second, and the carbonic acid 

 from the second to the first chamber. The pressure of a gas is 

 proportional to the percentage amount in which it is present in a 

 mixture. This is true for each gas in a mixture, the presence of 

 the others making no difference. 



In the atmosphere, for instance, the total barometric pressure 

 is 760 mm. of mercury ; the amount of oxygen in the air is 

 roughly one-fifth, and the pressure it exercises is also one-fifth 

 of 760; the nitrogen accounts for the other four-fifths. The 

 carbonic acid is present in such small quantities that the pressure 

 it exercises is only a fraction of a millimetre. 



In the alveolar air, however, the carbonic acid is present in 

 larger and the oxygen in smaller amount ; and in the intermediate 

 air passages there is an intermediate condition : hence as in the 

 two chambers we first considered, oxygen ditfuses down to the air 

 vesicles, and carbonic acid from them. These slow movements of 

 diffusion are assisted by the large draughts which are created in 

 the upper respiratory tract by the respiratory movements of the 

 chest. 



Gases of the Blood. From 100 volumes of blood, about 60 

 volumes of gas can be removed by the mercurial air-piimp. The 

 average composition of this gas in dog's blood is : 



Arterial blood. Venous blood. 



Oxygen . . . 20 . . . 8 to 12 



Nitrogen . . . I to a . . . i to 2 



Carbonic acid . 40 ... 46 



The nitrogen in the blood is simply dissolved from the air just 

 as water would dissolve it ; it has no physiological importance. 

 The other two gases are present in much greater amount than can 

 be explained by simple solution ; they are, in fact, chiefly present 

 in loose chemical combinations. Less than one volume of the 

 oxygen and about two of carbonic acid are present in simple 

 solution in the plasma. 



Oxygen in the Blood. The amount of gas dissolved in a 

 liquid varies with the prtfteure of the gas ; double the pressure 

 and the amount of gas dissolved is doubled. Now this does not 

 occur in the case of oxygen and blood ; very nearly the same 



