228 STUDIES IN ADVANCED PHYSIOLOGY. 



of the capillaries there is no CO 2 , as the arterial blood just 

 coming from the lungs has practically no CO 2 in it (the air 

 itself having practically none) , so in further conformity with 

 Dalton's law the CO 2 will stream into the plasma of the 

 blood, the walls of the capillaries being entirely disre- 

 garded. But, as everyone knows, ordinary water (and 

 plasma is mainly water) cannot absorb very much CO 2 at 

 ordinary pressures. When we do want the water to absorb 

 larger quantities, as in the manufacture of soda-water, such 

 water must be subjected to very high gas pressures. The 

 difficult problem therefore presents itself of explaining how 

 the large amounts of CO 2 which are breathed out at each 

 breath are really carried by the blood. In the first place, 

 the corpuscles of the blood do not carry CO 2 . Blood with 

 the corpuscles left in it carries a little more CO 2 than when 

 the corpuscles are taken out, but this is due to the fact 

 that the corpuscles themselves are soaked with plasma, and 

 the plasma, like any liquid, will absorb certain amounts of 

 CO 2 . We must, therefore, look to the plasma itself as the 

 carrying agent of this gas. And yet by experiment it can 

 be conclusively shown that we breathe out much more CO 2 

 at each breath than could be absorbed by the plasma in that 

 time. In short, while some of the CO 2 is absorbed by the 

 plasma, some of it must be held in chemical combination. 

 Now just what this chemical combination is, physiologists 

 can not yet determine. It seems, however, highly probable 

 that as the blood is normally alkaline, much of the CO 2 in 

 the capillaries of the tissue unites with the alkaline sub- 

 stances of the blood and forms carbonates. To .even the 

 most elementary student in chemistry the following expla- 

 nation, though given unfortunately in technical, chemical 

 terms, will seem very clear: There are contained in ven- 

 ous blood certain quantities of (Na 2 CO 3 ) sodium carbon- 

 ate. When a liquid containing Na 2 CO 3 has added to it 

 CO 2 gas, it forms a new combination with this gas, and 

 there results Na H CO 3 , sodium bicarbonate. This Na H 

 CO 3 is possibly more familiar to us as ordinary baking- 



