RESPIRATION BEYOND THE LUNGS 



397 



explain why hemoglobin should readily absorb 2 from the alveolar air, 

 but would fall far short of explaining how this 2 is readily released 

 when it is required in the tissues. Obviously there is some artificial con- 

 dition present in the above experiment which can not obtain in the nat- 

 ural environment of the blood. 



Since hemoglobin takes up 2 in proportion to its iron, it can not be 



10 ZO 30 W 30 60 70 80 <?0 100 



Fig. 138. Average dissociation curves. 



Ordinates Percentage saturation of hemoglobin with oxygen. 



Abscissae Tension of oxygen in mm. of mercury. 



Curve A Degree of saturation of pure hemoglobin solutions at varying pressures. 



Curve B Disregard this curve. 



Curve C Effect of 20 mm. CO2 pressure on above solution. 



Curve D The saturation curve in normal blood at 40 mm. carbon dioxide pressure. 



because of changes in the 2 combining part of the hemoglobin itself 

 that blood and pure hemoglobin solutions have dissimilar dissociation 

 curves, but rather because of differences in the environment in which the 

 hemoglobin acts. That this is so can be readily shown by plotting the 

 dissociation curve, not for a hemoglobin solution, but for blood itself 

 (D in Fig. 138). The results are very different. At a partial pressure 

 of 2 of about 60 mm. Hg that is, a lower pressure than exists in the 



