The ability of crocodilian haemoglobins to bind HCO3 has been appreciated for more than half a century, but the functional implication of this exceptional mechanism has not previously been assessed in vivo. Therefore, the goal of the present study was to address the hypothesis that CO2 primarily binds to haemoglobin, rather than being accumulated in plasma as in other vertebrates, during diving in caimans. Here, we demonstrate that CO2 primarily accumulates within the erythrocyte during diving and that most of the accumulated CO2 is bound to haemoglobin. Furthermore, we show that this HCO3 binding is tightly associated with the progressive blood deoxygenation during diving; therefore, crocodilians differ from the classic vertebrate pattern, where HCO3 accumulates in the plasma upon excretion from the erythrocytes by the Cl/HCO3 exchanger.

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