252 RESPIRATORY FUNCTION OF THE BLOOD 



How the tissues unload the oxygen. 



These little barges 7-6/x in diameter squeeze along the capillary 

 vessels in the tissues. During their passage along a tube with a 

 diameter less than their own the corpuscles naturally undergo 

 distortion. This distortion has at least one effect on the loading 

 and unloading of the oxygen from the corpuscles. It puts on the 

 brake, slows down the corpuscles and gives the dock-labourers 

 and others opportunity to carry out their work. 



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. XXXI.). 



Increase in temperature increases the desaturation of haemo- 

 globin. 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 



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

 on the active masses of oxygen C and of haemoglobin C R , 

 i.e. v=k(C xC R ). 



Now k v the velocity constant of the saturation process, and & 2 , 

 the corresponding constant for desaturation, vary with the 

 temperature. We have seen (p. 247) that a, the absorption 

 coefficient of oxygen in blood, varies inversely with the tempera- 

 ture. 



where C u concent ration 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 7\ and T z 

 are related by the equation 



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

 of Hb unites with 1 gram molecule of oxygen). 



