330 RESPIRATORY FUNCTION OF THE BLOOD 



(2) Another physical factor comes into play, viz., alterations 

 of temperature, and that has a profound effect on both the amount 

 of gas liberated and the speed at which it is handled. The 

 temperature in the lung where oxygen is taken on board is usually 

 less than 37° C, while the temperature of active tissue may be 

 greater (see Chap. XXXII.). Increase in temperature increases 

 the desaturation of haemoglobin. The amount of desaturation 

 brought about by an increase in temperature may be calculated 

 from the laws of van't Hoff and Arrhenius. The process of 

 saturation and desaturation may be represented by the reaction 

 formula 



Hb + Oo :;^ HbO^. 



The velocity of this reaction depends, other things being equal, 

 on the active masses of oxygen, C,), and of haemoglobin, C^., 



i.e. V = k (Co X Cjj). 



Now A'l, the velocity constant of the saturation process, and k2, 

 the corresponding constant for desaturation, vary with the 

 temperature. We have seen (p. 323) that a, the absorption 

 coefficient of oxygen in blood, varies inversely with the temperature. 



where Cjj = concentration of oxyhaemoglobin and p = oxygen 

 pressure. 



This value, K, is constant for each temperature, and by the law 

 of van't Hoff the values of K for any two temperatures T^ and To 

 are related by the equation 



-q T., - T. 



(e = base of Napierian logs, q = heat evolved when 1 gram 

 molecule of Hb unites with 1 gram molecule of oxygen). 



For example, let us try to determine what desaturation would 

 arise from raising the temperature from 36° C. to 39° C. 



Here T, = 273 + 36 = 309 absolute, 



Ta = 273 + 39 = 312 

 and q = 28,000 cals., 



- 28000 3 



If ^36 be 30 per cent., then K^^ is equal to 30e-°'^3^'^ = 19-4, 

 we find that haemoglobin which was 30 per cent, saturated at 

 36° becomes only 19-4 per cent, saturated at 39°. 



