Why do birds tend to show greater tolerance to high-altitude hypoxia than do mammals?

Question

Using the bar-headed goose as an example, list three evolutionary adaptations that enable this species to successfully migrate at high altitudes, such as over the Himalayas at over 9000 m.

Answer 1

Answer: Hypoxia is a condition where the body is deprived of adequate oxygen. The lung anatomy of birds differs from mammals and is advantageous at high altitudes. In birds, unidirectional ventilation, a thin gas exchange surface, and cross-current flow result in very efficient gas exchange and oxygen extraction. Birds have higher capillary density and smaller muscle fibers compared to mammals, which results in better gas exchange at the muscles. Furthermore, birds tolerate hyperventilation better than mammals, so they obtain more oxygen at high altitudes.
When we compare lowland birds with bar-headed geese we observe three striking differences. Bar-headed geese have larger lungs for their body size, so the greater surface area provides higher oxygen diffusion capacity. Bar-headed geese also have hearts with a higher density of capillaries, which improves oxygen delivery to their hearts. Mitochondria in muscle cells of bar-headed geese is found much closer to capillaries, thus reducing the distance between mitochondria and blood, and resulting in a higher diffusion capacity for oxygen.