316 



Comparative Animal Physiology 



to be greater in the lish which have high oxygen tensions of loading and 

 unloading (Table 56, and Fig. 71). Krogh and Leitch^^i noted a greater CO2 

 sensitivity in the fresh-water pike and carp than in the marine cod and plaice. 

 The conclusion of more recent investigations^-^' '^■^- ^'^-^ is that differences in CO2 

 sensitivity are not related to marine fresh-water habitat per se, but represent 

 true hemoglobin differences of different species. The amount of oxygen which 

 can be taken up by a trout in the presence of COo is less than the amount 

 which can be taken up by a carp. The CO^ tension to which gills of all fish 

 are exposed is much lower than alveolar CO2, and the hemoglobin functions 

 at lower CO2 levels in fish than in terrestrial animals. A trout can be kept in 

 a bottle of water which contains ample oxygen, but as the COo in the water 

 increases, the fish is unable to use the oxygen. A catfish, on the other hand, 

 will endure tensions of CO2 at 400-500 mm. Hg for long periods. Black found 



TABLE 57. RELATION BETWEEN ALTITUDE AND RED CELLS, 



O. CAPACITY, AND t,/^ ,„, EOR ACCLIMATIZED ANIMALS 



Data combined from Dill,^'^' Hall, Dill, and Barron,'- Hall,™' '' 



and Keys, Hall, and Barron"'' 



