CHAP. IL] RESPIRATION. 571 



present in a volume of serum is according to some observers 

 actually greater than that which is present in an equal volume of 

 blood, i.e. an equal volume of mixed corpuscles and serum ; that is 

 to say, the carbonic acid is much more largely associated with the 

 serum (or, in the living blood, with the plasma) than with the red 

 corpuscles. 



When serum is subjected to the action of the mercurial pump, 

 by far the greater part of the carbonic acid is given off; but a 

 small additional quantity (2 to 5 vols. per cent.) may be extracted 

 by the subsequent addition of an acid. This latter portion may 

 be spoken of as 'fixed' carbonic acid in distinction to the larger 

 ' loose ' portion which is given off to the vacuum. When however 

 the whole blood is subjected to the vacuum until the carbonic 

 acid ceases to be given off, the subsequent addition of acid is 

 said not to set free any further quantity ; so that when serum is 

 mixed with corpuscles all the carbonic acid may be spoken of 

 as ' loose ' ; and it is stated that the excess of carbonic acid in 

 a quantity of serum over that present in the same bulk of entire 

 blood, corresponds to the fixed portion in serum which has to 

 be driven off by an acid. Moreover, even those who maintain 

 that the quantity of carbonic acid in entire blood is less than that 

 in an equal volume of serum, admit that the carbonic acid exists 

 in some way or other at a higher pressure in, and is more readily 

 given off from entire blood than from serum. 



If these statements be accepted it seems probable that the 

 carbonic acid exists associated with some substance or substances 

 in the serum, or rather plasma, but that the conditions of its 

 association (and therefore of its dissociation) are determined by 

 the action of some substance or substances present in the cor- 

 puscles. It has been suggested that the association of the 

 carbonic acid in the plasma is with one or other of the proteids 

 of the plasma ; but it has also been suggested that the association 

 is one with sodium as sodium bicarbonate, and further that the 

 haemoglobin of the corpuscles plays a part in promoting the 

 dissociation of the sodium bicarbonate or even the carbonate, 

 and thus keeping up the carbonic acid of the entire blood. Other 

 observers however maintain that the plasma does not hold this 

 exclusive possession of the carbonic acid, but that a considerable 

 quantity of this gas is in some way associated with the red 

 corpuscles. Indeed further investigations are necessary before the 

 matter can be said to have been placed on a satisfactory footing. 



The relations of the Nitrogen in the Blood. 



353. The small quantity of this gas which is present in both 

 arterial and venous blood seems to exist in a state of simple solu- 

 tion. 



