THE TRANSPORT OF OXYGEN BY THE BLOOD OF FRESHWATER FISH

¹ From the Edward Martin Biological Laboratory, Swarthmore and the Department of Zoölogy, University of Pennsylvania

1. Oxygen capacities have been determined on the blood of four freshwater fish (catfish, carp, bowfin and common sucker). Individual variations are larger than average interspecific differences.

2. Significant differences between species were found for the affinity of the hemoglobin in whole blood for oxygen in the absence of CO₂ and at 15° C. in these freshwater fish as contrasted with the rather uniform oxygen affinity of most marine fish studied so far.

3. Significant differences between species were found for the effect of CO₂ on the affinity of the hemoglobin in whole blood for oxygen. For example, at 15° C. an increase in pCO₂ from 2 to 20 mm. causes an increase in oxygen pressure at half saturation of 8 mm. for the catfish and of 50 mm. for the common sucker.

4. Blood with high oxygen affinity has a low sensitivity to CO₂ (catfish). Blood with low affinity for oxygen has a high CO₂ sensitivity (common sucker).

5. It is suggested that since blood characterized by high oxygen affinity lacks a large CO₂ effect which could offset the disadvantage imposed at deoxygenation, the blood would function best in a high environmental temperature.

6. The greater sensitivity to CO₂ of fish blood as compared with mammalian blood was regarded by Krogh and Leitch as an adaptation to lower temperature. Evidence can now be presented to show that freshwater fish of higher CO₂ sensitivities are found in the colder habitats.

7. The very striking effects of CO₂ show that high pressures of oxygen might be developed within the fish by the presence of relatively low pressures of CO₂. The old suggestion that the Bohr effect may be responsible for the high percentage of oxygen in the swimbladder of many fish is thus given additional quantitative support.