You know, I like to think I have a pretty good grasp on science, and biology, and how the world works.  And then some little fact trots along and knocks me right on my butt, thereby pointing out that the world is far more strange and complicated than we sometimes give it credit for.

This is one such fact, which I stumbled over last week: birds breathe in only one direction.  Most animals, such as ourselves, have "bidirectional respiration."  We inhale, fresh air comes into our lungs, it gets exchanged for all the carbon dioxide and other waste products, and then the old air is exhaled.  Fresh air in; stale air out.  This is also called "tidal respiration," because it's similar to the way the tides move in and out.

The main point of breathing, of course, is to bring oxygen into our bloodstream, and to get rid of the carbon dioxide.  If you think about it, bidirectional respiration is actually pretty inefficient.  It mixes the old air and the good air, so you're never inhaling completely clean air, and you're never exhaling completely stale air.

Birds found a clever way around this problem.  They basically have a circular respiratory system.  This is comprised of the trachea, the lungs, and a series of connected air sacks.  When a bird inhales, fresh air goes down its trachea and into its lungs.  It is then pushed into the air sacks.  It moves along through the sacks, giving oxygen into the blood and picking up carbon dioxide, then circles around and is exhaled through the trachea again.

Bird respiration is basically a four-step process. Inhale #1, exhale #1, inhale #2, exhale #2.  It takes all four steps for each "packet" of fresh air to move through the bird's body and be exhaled.

As I'm sure you know, a bird's bones are hollow (unlike our bones, which are full of marrow).  Some of these air sacs extend into some of the bones, which makes a bird's bones part of its respiratory system as well.  The air sacs are part of the humerus (their "arm bone"), the thigh bone, the vertebra, and the skull as well.

This of course opens birds up to a whole host of respiratory diseases and ailments that we have difficulty relating to.  This is also why it is so important to keep a bird's cage or coop clean - the ammonia fumes from their poop can easily damage their delicate respiratory system.  The same system which allows fresh air to sit and exchange with the blood gases is their downfall in this respect, because they can end up "breathing in" a lot more ammonia from the air.

One possible evolutionary reason for this unusual respiratory system is that at about the time birds were evolving, the world's oxygen levels were very low.  This was right after the time when the entire world was populated almost exclusively by one dinosaur, Lystrosaurus.  

Lystrosaurus reigned partly because it was able to cope with the poor oxygen levels left behind by the destruction of the biosphere by that big pesky comet.  Nearly stripped of plant life, for a while there the Earth's atmosphere had as little as half as much oxygen as it does today.

Birds no doubt benefited greatly by their more efficient, one-way respiratory system.  It's bizarre, but it certainly works for them!