MECHANISM AND CHEMISTRY OF RESPIRATION 205 



the blood has gone through the capillaries it has become loaded 

 with this gas. About 40% of what the blood carries is in loose 

 combination with the proteid materials in the corpuscles, but 

 this does not at all effect the freedom with which oxygen 

 combines with the iron of the haemoglobin. About 60% of 

 the carbon dioxid is carried in the plasma; a small part of this 

 (about 2%) is simply dissolved in the plasma, while the rest of 

 it is temporarily combined with other elements in the blood 

 stream. 



When the blood comes to the air sacs in the lungs it finds con- 

 ditions opposite to those among the tissues; the amount of 

 carbon dioxid in the inhaled air is very small, and the pressure 

 is so low that the blood immediately lets go its hold on its 

 C0 2 , which thus passes at once into the air sacs. Thus carbon 

 dioxid gas is breathed out at every expiration, while new air is 

 breathed in. If this inhaled air should contain too much CO 2 , 

 the blood could not rid itself of its own quota of the gas, and 

 the person would soon die. 



It has been found that air containing 8-9% of carbon 

 dioxid produces severe discomfort. 



This gas, which is heavier than ordinary air, is sometimes 

 found in abundance in deep wells or mines where it accumu- 

 lates, making the air distinctly poisonous. Men who must 

 descend into such wells frequently first lower a lighted candle. 

 If it will burn, the air is safe to breathe; if not, no person can 

 safely enter. 



BREATHING AND EXERCISE 



Since without a supply of oxygen one cannot live, and since 

 one would soon be poisoned by carbon dioxid if he could not 

 dispose of it, the necessity of breathing is evident. We can 

 easily see, too, why breathing will increase in rapidity if one 

 exercises the muscles vigorously, as, for instance, in running. 

 Breathing must be accelerated SQ a.s to keep up a larger supply 



