62O 



PRINCIPLES OF GENERAL PHYSIOLOGY 



tension in tissues varies from zero to some 10 to 20 mm. of mercury, dependent, of 

 course, on the rate at which it is consumed in relation to that at which it is 

 supplied. Take the case of 10 mm. From curve IV of the figure we see that 

 at this tension haemoglobin is only 56 per cent, saturated, so that the difference 

 between 56 per cent, and 93 per cent., namely 37 per cent., represents that 

 available for the tissue. On the contrary, take curve II, at 25 ; at 100 mm., 

 we have about 98 per cent, saturation ; at 10 mm. 88 per cent., a difference of 

 1 per cent. only. The advantage of the warm-blooded animal is plain. 



The different position of the equilibrium at different temperatures must 



70 



SO 



90 



1 on 



10 20 30 40 50 60 70 80 90 100 



FIG. 190. DISSOCIATION CURVES OF OXYH^EMOGLOBIN AT DIFFERENT TEMPERATURES. 



Ordinates percentage of reduced haemoglobin. 



Abscissae tension of oxygen in mm. of mercury. 



Curves I, II, III, IV, and V correspond to 16, 25, 32% 38, and 49 C. respectively. 



Note that the higher the temperature, the less oxygen is held by haemoglobin at a given tension 

 of the jjas. 



(Barcroft and Hill, Jl. Physid., 39, 422.) 



obviously be due to the greater acceleration by temperature of the dissociation of 

 oxyhsemoglobin than that of the taking up of oxygen. Experiments on this 

 question will be found in Barcroft's book (1914, Chapter XL), together with 

 curves. 



It may be noted that the effect of temperature is the same as that on cases of 

 typical adsorption, where it is due to the negative temperature coefficient of 

 surface energy. 



Now, since raising the temperature causes dissociation of oxyhsemoglobin, 

 van't HofPs principle of mobile equilibrium tells us that the " combination " must 

 be associated with evolution of heat. Further, van't Hoff has worked out a 

 formula relating the position* of equilibrium to the heat evolved on combination. 



