How birds breathe, in 184 words
February 3, 2022
Windpipe and lungs in pink, air sacs in teal. Steps 1 and 3 happen at the same time — one breath of air is moving through the lungs and into the air sacs in back (1) at the same time as an earlier breath of air is moving out of the lungs and into the air sacs up front (3). Steps 2 and 4 happen at the same time as well — the air sacs in back are blowing air through the lungs (2) while the air sacs in front are blowing air out the windpipe (4). Each breath of air is inside the bird for two inhalations and exhalations.
Our lungs are made up of millions of tiny bags. Breath in, fill the bags with fresh air, breathe out, empty the bags of spent air. But bird lungs are very different. They’re made up of millions of tiny tubes, like bundles of drinking straws, and those tubes are connected to big, empty air sacs, like balloons that spread throughout the body. When birds breathe in, some of the air goes through the lungs, and some skips the lungs and goes into the air sacs. Then when the bird breathes out, the air in the air sacs gets pushed through the tubes in the lungs. So birds get oxygen-rich air blown through their lungs both when they inhale and when they exhale. The lungs and air sacs of birds also send mini air sacs into the skeleton, and these create air-filled spaces inside the bones, akin to our sinuses. These air spaces in the skeleton are the footprint of the respiratory system. A lot of extinct dinosaurs have the same pattern of air spaces in their skeletons, so we think they breathed like birds.
— Jessie Atterholt and Matt Wedel
Sauropod Vertebra Picture of the Week 
February 3, 2022 at 6:52 am
So what is this? For reasons that will become apparent in a week, it seemed like it would be useful to be able to point people to a description of how birds breathe that was (1) really short, and (2) written in ordinary English. I enlisted Jessie Atterholt’s help, partly because she is a fearsomely talented writer, and partly because she’d already made a sweet diagram of bird breathing that I really wanted to run with the text. And here we are.
This should be obvious, but just in case: we’ve both made our little contributions, but Jessie and I are summarizing decades of works by other folks here. And there are other reasons besides air-filled bones to think that non-avian dinosaurs breathed like birds, we’re just focusing on that line of evidence because that’s the one we work on and know best.
Exciting news next Thursday — stay tuned!
February 3, 2022 at 8:42 am
hopefully this relates to a fossil containing well preserved soft tissue from the saurpod respiratory system …
I mean, after reading Mike’s paper on necks, that seems impossible, but you never know …
February 3, 2022 at 9:10 am
Hmm … So air moves through the lungs twice, in different directions, in steps one and two? And here was I, thinking that the description “avian flow-through lung” meant air always moved in one direction.
February 3, 2022 at 9:37 am
This is the most fascinating little piece of science I’ve ever read. I didn’t know I needed to know this. Thank you for this lovely little piece.
February 3, 2022 at 10:32 am
aside: I do find it interesting that bird-type lungs are the closest (as far as I know) that tetrapods come to the pass-through system some non-tetrapod fish have, where water enters through the mouth, passes over the gills, and then exits through the gill slits (or operculum, if present)
Makes me wonder if a hypothetical animal who had fully developed pass-through lungs would have water loss problems in dry environments.
February 3, 2022 at 4:24 pm
Mike wrote:
Hmm … So air moves through the lungs twice, in different directions, in steps one and two? And here was I, thinking that the description “avian flow-through lung” meant air always moved in one direction.
Air does move unidirectionally in the gas-exchanging parabronchi, but bidirectionally in the larger bronchi that feed the parabronchi — see the second and third images in this post. That subtlety is something we lost in going for brevity; it would take a lot of additional words to even define parabronchi in informal language.
llewelly wrote:
Makes me wonder if a hypothetical animal who had fully developed pass-through lungs would have water loss problems in dry environments.
By “fully developed pass-through lungs” do mean air entering and exiting through different holes in the body, the way that water enters through the mouths of fish and leaves through their gill slits?
In any case, I don’t think it would be much of a problem. Extant desert animals are pretty good at recapturing moisture from the respiratory system. Birds in particular are amazing at this: the fancy aerodynamic valving of their lungs means that they can exclude the lungs from the airflow during thermoregulatory panting, and the long trachea and nasal cavities are good at catching moisture on the way out. Probably my favorite paper title of all time is Withers et al. (1981), “Desert ostrich exhales unsaturated air” — a novel and surprising finding related clearly in just five words that make a complete sentence. The upshot is that desert birds lose about half as much water in a given amount of time as similarly-sized mammals — they’re not just a little better at living in deserts, they’re phenomenal at it.
Where I’m going with this is that a terrestrial animal with a separate out-pipe from the lungs could presumably put the same moisture-recapturing stuff in its ‘posterior trachea’ and recapture just as much moisture as an ostrich.
Withers, P.C., Siegfried, W.R., & Louw, G.N. 1981. Desert ostrich exhales unsaturated air. South African Journal of Science, 77(12): 569-570.
February 3, 2022 at 4:35 pm
Thank you, Matt, that is exactly what I was asking – and yes, it does seem moisture-capturing posterior trachea would solve the problem nicely.
February 3, 2022 at 5:48 pm
I initially thought that this might have been an abstract that had been run through tl;dr papers (which unfortunately appears to be under maintenance at the time of posting), but the fact that it was written from scratch is pretty impressive!
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February 28, 2022 at 1:57 am
My big unanswered question about avian respirology is why the … hose leading from the head to the lung complex is often (even absurdly) hyper-extended, sometimes even with extra loops. It seems like its full length is necessarily full of dead air, that just came out of the lungs on the last breath and will have to go back in before the fresh stuff can get in.
(I understand that “why” questions are a temptation to teleology, but sometimes they have plausibly insightful answers anyway.)
February 28, 2022 at 2:14 am
My understanding is that the volume of the posterior thoracic and (especially) abdominal air sacs is so great that it simply overwhelms the tracheal dead space. Also, there is always some mixing of dead air with fresh air in those posterior air sacs, so the air blown through the lungs on exhalation isn’t as fresh as the stuff coming in during inspiration — but also, the avian lung is so ridiculously good at gas exchange that air that would be unacceptable for most mammals is still tractable for birds, many of which can survive at altitudes that would kill us.
Darren wrote about the long tracheas of birds way back in 2009, on TetZoo v2 (link).
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