CHAP, ii.] RESPIRATION. 461 



being of such a kind that while the haemoglobin readily com- 

 bines with the oxygen of the air to which it is exposed, dis- 

 sociation readily occurs at low pressures, or in the presence of 

 indifferent gases, or by the action of substances having a greater 

 affinity for oxygen than has haemoglobin itself. The difference 

 between venous and arterial blood, as far as oxygen is con- 

 cerned, is that while in arterial blood the haemoglobin holds 

 nearly its full complement of oxygen and may be spoken of as 

 nearly wholly oxyhaemoglobin, in venous blood the haemoglobin 

 is to a large but variable extent, reduced ; and the character- 

 istic colours of venous and arterial blood are in the main due to 

 the fact that the colour of reduced haemoglobin is purple, while 

 that of oxyhaemoglobin is scarlet. 



The relations of the Carbonic Acid in the Blood. 



283. The presence of carbonic acid in the blood appears 

 to be determined by conditions more complex in their nature 

 and at present not so well understood as those which determine 

 the presence of oxygen. The carbonic acid is not simply dis- 

 solved in the blood ; its absorption by blood does not 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 dissociation. We cannot however speak of it as 

 being associated, in the same definite and clear way as is the 

 oxygen, with the haemoglobin of the red corpuscles. 



Several facts seem to support the view that the carbonic 

 acid exists associated with some substance or substances in the 

 plasma, but at the same time indicate that the conditions of its 

 association (and therefore 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 associa- 

 tion 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 at least of this gas is in some definite 

 way associated with the red corpuscles. Further investigations 

 are necessary before the matter can be said to have been placed 

 on a satisfactory footing. 



The relations of Nitrogen in the Blood. 



284. The small quantity of this gas which is present in 

 both arterial and venous blood seems to exist in a state of sim- 

 Dle solution. 



