Oxygen delivery to the heart and brain during hypoxia: Pekin duck vs. bar-headed goose

To investigate mechanisms that may allow birds to tolerate extreme high altitude, we acutely exposed unanesthetized bar-headed geese (Anser indicus) and Pekin ducks (Anas platyrhynchos) to 0.21, 0.10, and 0.05 inspired fractional concentrations of O2 (FIO2). In both birds, arterial O2 partial pressure (PaO2) was about 95 Torr at 0.21 FIO2, 45 Torr at 0.10 FIO2, and 28 Torr at 0.05 FIO2. Hyperventilation occurred at both levels of hypoxia, with PaCO2 decreasing to about 7 Torr at 28 Torr PaO2. At 28 Torr PaO2, arterial O2 content (CaO2) in geese (10.4 vol%) was significantly higher than in ducks (4.1 vol%). As PaO2 declined from about 95 to 28 Torr, both cerebral and coronary blood flow (determined by using the radioactive microsphere method) increased more than fivefold in ducks but less than threefold in geese. At both levels of hypoxia, O2 delivery (flow X CaO2) to the heart and brain of geese was the same as or higher than that of ducks. The unique control of cerebral and coronary O2 delivery exhibited by both species of birds may be related to their remarkable tolerance to severe hypocapnic hypoxia.