How Birds Breathe

​Imagine that you’re struggling in the “death zone” on Everest, above 23,000 feet—taking one more step, gasping for breath. It’s called the death zone because once you’re up that high, you start dying; you can stay for only a limited length of time. Before this was well understood, many climbers died in the death zone.

​   But what’s that sound? Geese honking? Are you hallucinating because of low oxygen? No—a small group of geese is flying through a pass nearby.
       How does that even happen?
​
  Mammals and birds have very different breathing systems. In mammals, our lungs expand and contract to inhale and exhale. 

Bar-headed Geese, Dhanauri Wetlands, Uttar Pradesh, India. © 2019 Nitin Chandra.

Mammalian lungs never empty completely—a residual volume of gas always remains, and so the lungs never contain totally fresh air. This is called a “reciprocating” system. 
     By contrast, bird lungs are “flow-through.” They don’t expand and contract; instead, birds have air sacs that expand and contract, moving fresh air through their rigid lungs in one direction. ​​

With inhalation, the posterior (rear) air sacs fill with fresh air, and the anterior sacs pull in oxygen-depleted air from the lungs. During exhalation, the posterior sacs move fresh air into the lungs, and the anterior sacs move the “old” air out through the trachea. The result is a constant flow of fresh air through the lungs—much more efficient for gas exchange.

Avian Respiration. dynamicscience.com.au

​     The birds’ respiratory system makes it possible both to fly high and to fly long distances without stopping during migration. This efficient respiration also helps birds, which are warm blooded, to keep their metabolism high. They get plenty of oxygen.
    Birds evolved from the theropods, a dinosaur group that included such lovely beasts as Velociraptor and Tyrannosaurus. Mammals evolved from the cynodonts, a rather undistinguished, non-dinosaur reptile group. These groups had different breathing systems, which later resulted in the mammal–bird differences.
     What about bats, though? They are mammals, but they fly quite successfully, although not as high as some birds, nor for as long a period of time. As you’d expect, bats have several adaptations to support flight: very large hearts and lungs (compared to body size), a larger number of red blood cells than in other mammals, and more hemoglobin in those red blood cells. They, too, get plenty of oxygen.
     The Sherpa people of the Himalayas appear to have gene adaptations for better oxygen utilization, allowing them to perform much better at altitude than lowlanders do. As a group, they have lived at high altitudes (an average of 14,700 feet) for at least 6,000 years.

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For further reading:
Chase D. Mendenhall, “How Birds Breathe with Their Butts.” Carnegie Museum of Natural History. Click here

“How Air Sacs Power Lungs in Birds’ Respiratory System.” July 2020. Click here

J. N. Maina, “What it takes to fly: the structural and functional respiratory refinements in birds and bats.” J Exp Biol (2000) 203 (20): 3045–3064.
[Note: This is a highly technical article.] Click here

Emily Sohn, “The Science Behind The Super Abilities Of Sherpas.” May 28, 2017. NPR.org, Goats and Soda. Click here

Jennifer Jordan, The Last Man on the Mountain: The Death of an American Adventurer on K2. W.W. Norton & Co., August 1, 2011. [The story of Dudley Wolfe, a climber who died at high altitude before the dangers were fully understood. Available at Amazon, Barnes & Noble, Thriftbooks, and probably your local library.]