98 



RESPIRATORY MECHANISMS 



tensions, so that exposure to C0 2 at a pressure of 10 mm or less 

 will increase the loading tension greatly. The curve (Fig. 55) 

 shows the effect on the paku. The C0 2 effect on the tension 

 of unloading is of course beneficial, while the effect on the 

 loading tension cannot become manifest in nature. Green 

 and Root (1933) showed further that above a certain concen- 

 tration of C0 2 , or any other acid, saturation with oxygen could 



20 30 40 50 60 70 80 90 100 

 Oxygen pressure in mm. Hg 



Fig. 55. Oxygen dissociation curves of paku blood at different C0 2 tensions. 



(Willmer.) 



not be reached at all in several marine fishes. In a toadfish 

 at Ph 6.5 the maximum saturation obtained at all 2 tensions 

 above 200 mm would correspond to 49% only of the oxygen 

 capacity. The biological significance of this remarkable 

 effect is not clear. It is suggested that it may have something 

 to do with the liberation of oxygen in the "gas gland" of the 

 swimming bladder. This suggestion is very plausible, but 

 even if proved would not go very far to explain the process of 

 gas secretion in this organ. 



