FUNCTIONAL ADAPTATION 



MAMMALIAN RESPIRATION 



327 



to its respective loading and unloading tensions than is conferred by the 

 variation of structure of the Hiifner unit; on the other hand it makes it far 

 more sensitive to change in its macroenvironment, such as pH or partial 

 pressure of carbon dioxide (Chapter VI). 



9. FUNCTIONAL ADAPTATION — MAMMALIAN 

 RESPIRATION 



9.1. Significance of Sigmoid Dissociation Curve 



In view of the detail in which this has been discussed -elsewhere 

 (119,11^1), we will deal with it very briefly. As Barcroft has pointed 

 out, the sigmoid dissociation curve enables a well-adapted pigment 



100 



p(02), mm. 



120 



Fig. 2. Loading and unloading tensions of pigments with hyperbolic and 

 sigmoid dissociation curves. 



to deliver the maximum (|uantity of oxygen between limits of the 

 loading and unloading tensions set by the environment of the pigment. 

 This can be clearly seen from Figure 2, where a sigmoid dissociation 

 curve is compared with two calculated on Hufner's theory. 



The loading and unloading tensions of the environment in which 

 the pigment must function are 120 mm. and 30 mm., respectively. 



