Cardiopulmonary function in exercising bar-headed geese during normoxia and hypoxia

To investigate possible physiologic mechanisms that allow the bar-headed goose to perform strenuous physical activity when flying at high altitude (e.g., above 9,000 m), we measured cardiopulmonary variables during running exercise (treadmill; 0.6 m.sec-1; 2 degrees incline) while the bird breathed either normoxic (21% O2) or hypoxic (7% O2) gases via a face mask. 1. During normoxic exercise, O2 uptake rate doubled and both ventilation and cardiac output increased. Blood gases and pH in arterial, mixed venous and blood from the leg, however, remained virtually unaltered. 2. Hypoxia at rest stimulated ventilation to rise but not cardiac output. The birds reached a steady state with virtually unaltered O2 uptake. 3. Exercise during hypoxia further stimulated ventilation, resulting in elevated arterial PO2 and O2 content compared to hypoxia at rest. However, O2 uptake increased only slightly, and cardiac output did not rise over the resting hypoxic value. The hyperventilation resulted in respiratory alkalosis and increased CO2 output, with R values being as high as 2.0. 4. It is concluded that neither ventilation nor pulmonary gas transfer were the limiting step in supplying O2 to the working muscles during hypoxic exercise in our experiments. It is more likely that muscle blood flow or diffusion from muscle capillaries to mitochondria, or both, determined the aerobic capacity under these conditions.