APPLIED PHYSIOLOGY 



gists are coming round to the view that the part played 

 by the epithelium of the lungs is an active and not 

 merely a passive one, and cannot yet be explained by 

 the ordinary physical laws as we see them in operation 

 in non-living matter. As G. H. Lewes said nearly fifty 

 years ago :* ' Physical laws reveal only one part of the 

 mystery. Eespiration is not a simple physical act. It 

 is the function of a living organism, and as such receives 

 a specific character from that organism. No sooner do 

 we cease to regard the exclusively physical aspect of this 

 function no sooner do we fix our attention on the 

 organism and its influence, than the theory raised on 

 the simple laws of gaseous interchange suddenly totters 

 and falls.' 



One hundred volumes of arterial blood yield sixty 

 volumes of gas of the following composition : 



Oxygen ... 20 parts. 



Nitrogen 1-2 



Carbonic acid 40 ,> 



Of the oxygen less than one volume is in solution in 

 the plasma. The rest is combined with haemoglobin in 

 the red cells. Arterial blood is, however, not saturated 

 with oxygen. It is only about nine-tenths saturated, and 

 under ordinary conditions not more than one-third of 

 the combined oxygen is used. There is, therefore, a 

 considerable margin of oxygen to draw upon in 

 emergencies. Thanks to this, a very considerable 

 proportion of the haemoglobin in the blood can be 



* ' The Physiology of Common Life,' 1859, i., 878. 



