Respiratory Physiology of House Sparrows in Relation to High-Altitude Flight

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Abstract
1. Some birds perform the strenuous activity of flapping flight at altitudes in excess of 6100 m. (20,000 ft.), an altitude at which resting, unacclimated man is in a state of incipient hypoxic collapse. This study investigates the respiratory problems associated with avian flight at 6100 m. 2. House sparrows and white mice were exposed to a simulated altitude of 6100 m. for an hour. Their metabolic rates were kept above the basal level by regulating environmental temperature at 5°C. At 6100 m. the sparrows consumed oxygen at 2.2 times the basal rate, used more oxygen and had higher heart and respiratory rates than at sea level. Body temperature decreased 2°C., but the birds were normally active. In contrast, the mice at 6100 m. were moribund and had body temperatures 10°C. below normal after 1 hr. Oxygen consumption and respiratory rates were less than at sea level. 3. At 6100 m. sparrows ventilated the respiratory parts of their lungs 77% more than at sea level. At sea level and 6100 m. respectively, the partial pressure of carbon dioxide in the lungs dropped from 39.5 to 27.2 mm. Hg, and the partial pressure of oxygen in the lungs dropped from 100 to 28 mm. The sparrows removed a greater proportion of the oxygen from the air ventilating the lungs at 6100 m. than they did at sea level. 4. Oxygen dissociation curves were determined for sparrow blood at various pH values. At 6100 m. the arterial blood of sparrows appears to be less than 24% saturated with oxygen. For the cardiovascular system to transport adequate oxygen to the tissues at this low level of saturation house sparrows must have a cardiac output that is at least triple the maximum known for mammals. 5. House sparrows in a hypobaric chamber could fly 2.5 m. and gain altitude at a simulated altitude of 6100 m. but not at 7620 m. (25,000 ft.). Budgerigars trained to fly in a wind tunnel in the hypobaric chamber could fly more than 20 min. at sea level and a maximum of 3 min. at an altitude of 3660 m. (12,000 ft.). A budgerigar trained to wear an oxygen mask while flying in the wind tunnel could fly at an altitude of 6100 m. 6. Calculations based on our present knowledge of avian physiology suggest how birds could be capable of flying at 6100 m. The possible advantages of flight at high altitudes are discussed.