164 



BIOLOGICAL CHEMISTRY 



corresponding animals. Thus the differences in the oxy- 

 hsemoglobin curves may be due to the saline constituents 

 associated with the haemoglobin in the red blood corpuscles. 



In Bar croft's experiments one factor was of extreme 

 importance, namely the pressure of carbon dioxide. Thus a 

 series of curves showing the dissociation of oxyhsemoglobin 

 with increasing concentration of carbon dioxide shows that 

 for the same pressure of oxygen the oxygen is given off more 

 easily the higher the pressure of carbon dioxide. 



TOO 



Q 10 20 30 40 &0 60 70 80 90 100 

 FIG. 38. Dissociation curve of Barcroft's blood. 



Exposed to o, 3, 20, 40 and 90 mm. CO 2 . 

 Ordinates = percentage saturation. 

 Abscissae = Oxygen pressure. 



(Frotn " Respiratory Function of the Blood" J. Barcroft, by permission of the Fditor of {he 

 " Journal of Physiology.") 



The physiological value of this relation is that it aids the 

 gaseous exchanges. In the capillaries of the tissue the pressure 

 of carbon dioxide is increased as the result of tissue activity. 

 There is also a low oxygen pressure owing to the use of oxygen 

 by the tissues. Both these factors favour the giving up of 

 oxygen from the oxyhaemoglobin. 



In the capillaries of the respiratory organs the oxygen 

 pressure is raised and the carbon dioxide escapes from the 

 blood. These two changes favour the absorption of oxygen 

 because the higher the oxygen pressure the more oxygen com- 

 bines with haemoglobin, and the lower the carbon dioxide 



