238 A MANUAL OF PHYSIOLOGY 



In the serum the combined carbon dioxide exists chiefly 

 as carbonate and bicarbonate of sodium, the relative amount 

 of each depending on the quantity of carbon dioxide and of 

 other acids, such as phosphoric acid, which dispute with it 

 the possession of the bases. That its relations are peculiar, 

 however, is shown by the fact that from defibrinated blood 

 the whole of the carbon dioxide can in time be pumped out 

 without the addition of an acid to displace it from the bases 

 with which it is combined. It is hardly necessary to say 

 that this could not be done with a solution of sodium 

 carbonate. Yet when sodium carbonate is added to blood, 

 even in considerable amount, all the carbon dioxide in it 

 can be obtained by the pump. From serum a great deal, 

 but not the whole, of the carbon dioxide can be likewise 

 pumped out. The residue is set free on the addition of an 

 acid. 



The most satisfactory explanation seems to be that in the 

 serum there exist substances which can act as weak acids 

 in gradually driving out the carbon dioxide, when its escape 

 is rendered easier by the vacuum, but which, nevertheless, 

 do not affect litmus paper (since the reaction of serum is 

 alkaline). The quantity of these, however, is so small that 

 a portion of the carbon dioxide remains in the serum. The 

 proteids of the serum, such as serum-globulin, behave in 

 certain respects like weak acids, and may contribute to the 

 driving out of the carbon dioxide. 



When defibrinated blood is pumped out, the whole of the 

 carbon dioxide can be removed, apparently because sub- 

 stances of an acid nature pass from the corpuscles into 

 the serum and help to break up the carbonates. 



In the red corpuscles a portion of the carbon dioxide may 

 be in combination with alkalies. We know that the cor- 

 puscles contain alkalies, for the alkalinity of * laked ' blood 

 (pp. 34, 35), in which the red corpuscles have been broken 

 up, is found to be greater than that of unlaked blood, unless 

 a long time is allowed in the case of the latter for the 

 alkalies of the corpuscles to reach the acid used in titration 

 (Loewy). Some observers believe that a weak compound 

 of carbon dioxide can be formed with haemoglobin; for 



