THE RESPIRATORY CHANGES IN THE LUNGS. 361 



follow the law of pressures. It exists in association with some substance or 

 substances in the blood, and its escape from the blood is a process of disso- 

 ciation. We cannot, however, speak of it as being associated, to the same 

 extent as is the oxygen, with the haemoglobin of the red corpuscles. So 

 fiir from the red corpuscles containing the great mass of the carbonic acid, 

 the quantity of this gas which is 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 cor- 

 puscles. 



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 quan- 

 tity (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 car- 

 bonic 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 corpus- 

 cles 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. 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 those of its dissociation) are determined by 

 the action of some substance or substances present in the corpuscles. 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 

 than the haemoglobin of the corpuscles plays a part in promoting the disso- 

 ciation of the sodium bicarbonate or even the carbonate, and thus keeping 

 up the carbonic acid of the entire blood. Other observers, however, main- 

 tain 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. 



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

 and venous blood seems to exist in a state of simple solution. 



THE RESPIRATORY CHANGES IN THE LUNGS. 



The Entrance of Oxygen. 



297. We have already seen that the blood in passing through the 

 lungs takes up a certain variable quantity (from 8 to 12 vols. per cent.) of 

 oxygen. We have further seen that the quantity so taken up, putting aside 

 the insignificant -fraction simply absorbed, enters into direct but loose com- 

 bination with the hemoglobin. In drawing a distinction between the oxy- 



