Cardiovascular, respiratory, and blood adjustments to hypoxia in the Japanese eel,Anguilla japonica

The cardioventilatory performance of the Japanese eel,Anguilla japonica, was evaluated during acute exposure to hypoxia. The eel became an oxygen conformer as ambient PO2 fell below the critical value of 110 mmHg. Although arterial and venous PO2 also fell progressively, the arterial O2 content remained constant down to an ambient PO2 of about 60 mmHg. Arterial blood O2 saturation was maintained at 85% even at 40 mmHg. The increase in the supply of O2 to the animal during hypoxia was due to a combination of adaptive adjustments: (1) an increase in ventilation: perfusion ratio brought about mainly be bradycardia; (2) an increase in respiratory exchange surface area which was manifested as an increase in branchial blood transit time and quantified as a rise in transfer factor, water-blood overlap coefficient and utilization (%); (3) an increase in blood O2 affinity and capacitance coefficient as a result of respiratory alkalosis and Bohr-Root shift and a decrease in haemoglobin allosteric modulator (GTP, ATP) concentrations in the RBC. These factors together helped to increase the efficiency of O2 transfer across the gills.