Ch and dissociation 329 



tension the percentage saturation of haemoglobin with oxygen 

 at these low oxygen tensions becomes very small. 



In the lung. CO2 passes from the blood into the alveolar air, and 

 the hydrogen ion concentration of the plasma tends to decrease. 

 At the ordinary alveolar tension of oxygen this has little effect on 

 the combination of oxygen and haemoglo})in, })ut where the 

 alveolar tension of oxygen is reduced, as at high altitudes, the 

 exchange of gases occurs under conditions in which the COg tension 

 is increasingly important, and the adequate removal of the COg, 

 and consequent decrease of H^ concentration in the plasma, is 

 an important factor in permitting the picking up of an adequate 

 load of oxygen by the haemoglobin. 



From curve 82 it may be seen that, if the exchange of gases 

 were to take place in the lungs at an oxygen tension of 50 mm., 

 and if the curve indicated by the broken line represented the 

 haemoglobin dissociation curve at the COg tension of venous 

 blood, and the thick line represented the same curve at the COg 

 tension of the alveolar air, then the mere passage of the COo from 

 the blood into the alveolar air would enable the haemoglobin to 

 take up more oxygen. Thus the action of the CO^ on the reaction 

 between haemoglobin and oxygen is such that the taking up of oxygen 

 in the lungs and the unloading of it in the tissues are both facilitated. 

 So it appears that the COg tension of the blood is an important 

 factor in defining the oxygen dissociation curve, and that the con- 

 stant K in Hill's equation must be a function of the carbon-dioxide 

 tension. 



It has been found that Hill's equation may be written 



H,C03 + 7-7 _ [Hb] [0,r 

 0-014 [HbOg] ' 



where K = [^^^^3] + 7-7 ^ 



0014 



How the tissues unload the oxygen. 



(1) Distortion. The corpuscle is considered as a fluid drop with 

 an outer coat of orientated lipoid matter. The colloidal haemo- 

 globin is dispersed through the liquid contents. These little 

 barges, 8-8/x in diameter (p. 313), 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. 



