A wonderful article by Walter Murch on how the birds came to have super-efficient lungs.
The answer seems to be that bar-headed geese, like all birds—hummingbirds, ostriches, pigeons—have super-efficient lungs. It makes our lungs—and the lungs of all mammals—look primitive. I’m sure when birds get together they gossip about how pathetic our lungs are!
All mammals, including us, breathe in through the same opening that we breathe out. Can you imagine if our digestive system worked the same way? What if the food we put in our mouths, after digestion, came out the same way? It doesn’t bear thinking about! Luckily, for digestion, we have a separate in and out. And that’s what the birds have with their lungs: an in point and an out point. They also have air sacs and hollow spaces in their bones. When they breathe in, half of the good air (with oxygen) goes into these hollow spaces, and the other half goes into their lungs through the rear entrance. When they breathe out, the good air that has been stored in the hollow places now also goes into their lungs through that rear entrance, and the bad air (carbon dioxide and water vapor) is pushed out the front exit. So it doesn’t matter whether birds are breathing in or out: Good air is always going in one direction through their lungs, pushing all the bad air out ahead of it.
How did birds get such great lungs? They inherited them from dinosaurs. Birds are dinosaurs! When I was growing up in the 1940s, there was a category in biology called Aves, which meant birds. But scientists have now folded Aves into a category called Dinosauria, and those dinosauria, like pigeons and seagulls and geese, are flying all around us today. If you want to know what a dinosaur probably tasted like, eat some chicken!
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