The Science Journal of the American Association for Respiratory Care

1998 OPEN FORUM Abstracts

How Can a Bird Fly Over Mt. Everest? or How to Get Oxygen Out of the Air When There Is Hardly Any There.

Craig Black, PhD. RRT, University of Toledo Community and Technical College, Toledo, OH 43606.

Bar-headed Geese (Anser indicus) have flown over the Himalayas for thousands of years during their annual migration between their wintering grounds in India and their breeding grounds in China and Mongolia. Although many birds migrate over the Himalayas, most fly through passes at altitudes below 20,000 feet. Bar-headed Geese preferentially fly over the highest peaks and have been observed flying at over 30,000 feet. Barometric pressure at this altitude is approximately 250mmHg and the PO_{2} of dry air is only 52mmHg. If this air is inspired and humidified at the birds body temperature of 41°C, then PIO_{2} becomes 41.6mmHg, only 28% of that found at sea level. For the Bar-headed Goose, the O_{2} requirement for flight is approximately equivalent to that for a person running at maximum speed or about 12-15 times greater than basal metabolism. Normally mammals do not live above about 18,000 feet, although humans have climbed to nearly 30,000 feet without supplemental O_{2}. At this altitude, the extreme hypoxia limits their activity to no more than slow walking. Therefore, Bar-headed geese are much more efficient at extracting available O_{2} from air than any mammals.

Research has shown that Bar-headed geese have several adaptations which combine to increase the their ability to support the metabolic demands of flight at this altitude.

1. The lungs of all birds have a totally different design from that of the mammalian lung, one which allows pure inspired air to come to within only a few microns of the air-blood exchange surface. This allows the A-a gradient to be reduced to only 1-2mmHg. Mammals can reduce the A-a gradient to no less than about 30mmHg.

2. The hemoglobin of Bar-headed geese has higher than normal hemoglobin-O_{2} affinity, which shifts the Hb-O_{2} curve to the left, thereby increasing O_{2} saturation levels under hypoxic conditions.

3. The bird lung structure allows for more complete removal of CO_{2} from the blood. Flying birds may therefore have a relative alkalosis which also shifts the Hb-O_{2} dissociation curve to the left, further increasing O_{2} saturation levels under hypoxic conditions.

4. There is some evidence suggesting that Bar-headed geese are capable of increasing capillary density in muscle tissue. This decreases the diffusion distance for O_{2} from capillaries to cells, allowing the same amount of O_{2} to diffuse at a smaller partial pressure gradient.

5. Bar-headed geese can increase cerebral blood flow during hypoxia, allowing for increased O_{2} delivery to the brain.

The Himalayan Mountain range is a relatively "new" geological feature, created by the collision of two massive continental plates. Biologists suggest that Bar-headed geese actually pre-date the creation of the Himalayas and have been following the same migration route for considerably longer than the Himalayas have been present. Each year as the Himalayas were pushed higher and higher, there was selection for those individual birds which could fly at higher and higher altitudes. Thus, the Barheaded goose has become one of the most extraordinary of all high-altitude animal species.

The 44th International Respiratory Congress Abstracts-On-Disk®, November 7 - 10, 1998, Atlanta, Georgia.

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