THE CHEMISTRY OF RESPIRATION 1183 



bloods of different animals, have been shown by Barcroft and Camis to 

 be dependent on the different saline content of the solution in the 

 various cases. Thus human haemoglobin solution, with a concentra- 

 tion of salts similar to that of dogs' blood, gives the same dissociation 

 curve as normal dogs' blood. 



More important is the effect of reaction, since, as we shall see, it is 

 the reaction of the blood, controlled especially by carbon dioxide 



100 



10 20 30 40 50 60 70 80 90 100 



FIG. 494. Dissociation curve of haemoglobin in various solvents. 

 I, in 0-9 per cent. KC1 ; II, in 0-7 per cent. NaCl ; III, in water. 

 (BARCROET.) 



tension, that determines the activity of the respiratory centres. In 

 Fig. 495 is represented the influence of varying tensions of carbon 

 dioxide, and in Fig. 496 the effect of slight additions of lactic acid 

 on the dissociation curve. It will be seen that the more acid the blood, 

 or the greater tension of carbon dioxide it contains, the more readily 

 does it undergo dissociation. This is especially marked at the very 

 high tension of 420 mm. carbon dioxide. It plays an important 

 part in the lower tensions, such as 40 and 80 mm. Hg. carbon 

 dioxide. It must be remembered that 40 mm. carbon dioxide repre- 

 sents approximately the normal carbon dioxide tension in the blood. 

 It is true that at 150 mm. oxygen pressure the bjood is practically 

 saturated with oxygen whatever (within physiological limits) the 



