MECHANISM AND CHEMISTRY OF RESPIRATION 203 



no free oxygen is present in the tissues, since, as we shall see 

 later, the muscles are using up oxygen as fast as they can get 

 it. Here then, among the tissues where the oxygen is nearly 

 absent, or under very slight pressure, the corpuscles give up 

 the oxygen they are carrying and turn to a bluish red color 

 as they flow out from the capillaries into the veins and back 

 to the heart. 



Nitrogen in Respiration. Nitrogen composes about four- 

 fifths of the air we breathe, but so far as the respiratory pro- 

 cesses are concerned, it simply dilutes the oxygen of the air. 

 Some of it is doubtless absorbed by the blood plasma but none 

 of it is used in the body and practically none of it is given off 

 from the body in the form of gas. Hence the blood comes 

 back to the lungs from the tissues with just as much nitrogen 

 as it had originally. 



Breathing Pure Oxygen. The statement that nitrogen 

 dilutes the air is, however, open to a misapprehension. If 

 oxygen is as active a gas as we believe it to be, what will be 

 the effect if, instead of breathing an atmosphere which con- 

 tains only one-fifth oxygen, we breathe pure oxygen? We 

 might at first imagine that we should absorb much more 

 oxygen. Conversely, we might suppose that, if there were less 

 than the usual amount of oxygen in the air, we could not 

 obtain enough. We often think that the reason air in a 

 room becomes unpleasant and depressing is that it contains 

 so little oxygen. All of these impressions are mistakes. Since 

 the haemoglobin can absorb a certain quantity of oxygen 

 and no more, it can obtain this amount perfectly well from 

 ordinary air, or indeed from air containing less oxygen than 

 usual. If, therefore, one should breathe pure oxygen, the 

 blood would take on practically no more than it does from 

 ordinary air (a slightly greater amount might be taken in by 

 the blood plasma, for that combination is a mere mixture). 

 So far as the oxygen goes, there is probably enough in any air 

 we breathe to furnish the blood properly for effective working. 



