Among vertebrates, turtles are able to tolerate exceptionally low
oxygen tensions. We have investigated the compensatory mechanisms that regulate respiration and blood
oxygen transport in snapping turtles during short exposure to
hypoxia. Snapping turtles started to hyperventilate when
oxygen levels dropped below 10% O(2). Total ventilation increased 1.75-fold, essentially related to an increase in respiration frequency. During normoxia, respiration occurred in bouts of four to five breaths, whereas at 5% O(2), the ventilation pattern was more regular with breathing bouts consisting of a single breath. The increase in the heart rate between breaths during
hypoxia suggests that a high pulmonary blood flow may be maintained during non-ventilatory periods to improve arterial blood oxygenation. After 4 days of
hypoxia at 5% O(2), hematocrit,
hemoglobin concentration and multiplicity and intraerythrocytic organic
phosphate concentration remained unaltered. Accordingly,
oxygen binding curves at constant P(CO(2)) showed no changes in
oxygen affinity and cooperativity. However, blood pH increased significantly from 7.50+/-0.05 under normoxia to 7.72+/-0.03 under
hypoxia. The
respiratory alkalosis will produce a pronounced in vivo left-shift of the blood
oxygen dissociation curve due to the large Bohr effect and this is shown to be critical for arterial oxygen saturation.