ELI ON EARTH: Bird Breath

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Monday, August 24, 2015, 12:58 am
Aron Sousa

Image: A hawk (probably sharp-shinned) in MSU’s Beal Gardens earlier this year.

Simply put, East Lansing birds, like the one shown above, breathe differently from East Lansing reptiles and mammals, including this author.

Despite many, many years focused on biology—admittedly the biology of one species—this author was shocked to learn that birds breathe very differently from mammals and reptiles. Last week I wrote about sandhill cranes and their double-length trachea, and in my research for that piece, I learned that air flows only one way in bird lungs. Air flows in and out of the lungs like a bellows in mammals and reptiles, but, in birds, air flows to and through the lungs in one direction.

Birds have a series of air sacs that hold air before it goes into and after it comes out of their lungs. These air sacs allow the air they breathe to only pass in one direction through their lungs. Here’s how a “breath” of air moves through the bird over two cycles of inhalation and exhalation:

  1. First inhalation – air flows through holes in the beak (nares) into the windpipe (trachea) and into the air sacs near the back of the bird (posterior or caudal air sacs).
  2. First exhalation – the air flows from the caudal air sacs and into the lungs, moving from the back toward the front of the bird.
  3. Second inhalation- the air flows out of the lungs and up the front (cranial or anterior) air sacs.
  4. Second exhalation – the air moves out of the cranial air sacs through the trachea and out the nares.


In people, the mammal I know most about, air comes in through our nostrils (nares), through the windpipe (trachea) and into the lungs during inhalation and out the same path on exhalation. At rest adult humans move about half a liter of air per breath, but if you are exercising hard, or need more oxygen in high altitude, each breath might move about four liters of air in and out. At full capacity, the human respiratory system holds 5-6 liters of air, which means that 1-2 of those liters don’t get exhaled during a breath in heavy work. If you did exhale all of the air in your lungs, they would collapse into something like a wet, sticky ball of tissue paper.

The air here in East Lansing is about 21% oxygen, and people can pull one quarter to a third of the oxygen out of the air and into their bloodstream. For those of you keeping score at home, the 1-2 liters of old air in the lungs have already had one quarter to a third of the oxygen removed, and so as the old air and new air mix in your lungs, the amount of oxygen in your lungs is slightly less than in the air you breath in.

By contrast, the air moving into bird lungs is always fresh and fully oxygenated, because the old air does not have to stay behind to keep the lungs inflated. The unidirectional flow of bird air prevents mixing of new and old air, and so birds have more oxygen in their lungs than people do.

In addition, bird lungs are made up of small tubes that air flows through rather than being like the mammalian system of tiny air pockets (alveoli) that air must flow in to and then back out of again. The tube system keeps the air moving efficiently. (For those of you interested in birds as dinosaurs, there is good fossil evidence that bird-related dinosaurs had the same bone-air sac connections as we see in modern birds.)

This system (check out the animations here) allows birds to have lungs that weigh about half as much as a similarly-sized mammal, and keeping one’s mass low is all important if you are going to fly. Another advantage of this breathing system comes at higher altitudes, where there is much less oxygen in the air. At high altitudes, birds have a few a few extra percentage points of oxygen to draw on, which can make a big difference when flying.


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