Continuous recordings have been made of pH, PO2, and PCO2 of arterial blood (pHa, PaO2, PaCO2) in an extracorporeal circulation during periods of hypoxia (inspired PO2 45–10 Torr) in sea bass, Morone labrax. During moderate hypoxia hyperventilation was accompanied by an increase in pHa. Continuation of moderate hypoxia for periods up to 24 h produces an increase in lactate and a consequent decrease in pHa. During deep hypoxia there is a very brief alkalosis as ventilation increases but a marked decrease in pHa as lactate levels rise. Recovery from hypoxia is associated with an increase in lactate concentration reaching values of more than 6 meq X l-1 following deep hypoxia, and pHa falls to 7.73. PaO2 recovers rapidly, but recovery of PaCO2 is not so rapid and together with the residual hyperventilation indicates that the fish is paying off an O2 debt.
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