Oxygen affinity of sea turtle blood

The oxygen binding properties of whole blood in adult sea turtles (green, Chelonia mydas; loggerhead, Caretta caretta) were measured as a function of pH, and carbon dioxide partial pressure using the 'Hemoscan Oxygen Dissociation Analyser' with modifications that improved the performance of the method. Under approximately physiological conditions (25.7 degrees C, PCO2 = 37 Torr; pH 7.45) there is a marked affinity difference between the two species (green, P50 = 29 Torr; loggerhead, 47 Torr). It is suggested that the large affinity difference between blood of green and loggerhead turtles may be related to differences in depths to which they typically dive. The Hill coefficient at 50% saturation is 2.4 in the green under these conditions, and 2.7 in loggerhead. Hill plots of both species curve upward and have significant regions with n approaching 1 at low saturation with the effect that the lower end of the binding curve is more hyperbolic than sigmoidal. The fixed acid and CO2 Bohr effects in both species are strongly saturation dependent, being very low at low saturation and increasing markedly at higher saturation. The fixed acid Bohr effects are similar in the two species but the green is more sensitive to CO2, at constant pH, and so has a larger CO2 Bohr effect. The hyperbolic shape of the lower end of the binding curves, and low pH sensitivity in this region, would tend to favor oxygen loading during a dive, when lung PO2 and blood pH are falling. These properties might therefore favor more complete utilization of the lung oxygen store. We suggest that diving vertebrates can usefully be divided into two groups, those that use the lung as the main oxygen stores and those that use tissue (blood and myoglobin) stores.