One-Way Lungs

Once upon a time at a job interview I was asked "What is the difference between a turbine engine and a piston engine?" This was before I understood that a question like that really means, "Demonstrate to the interview panel that you are a pilot who knows how things work on the airplanes she has flown, by describing the components of these two types of engines." I accepted it instead as the more interesting challenge of identifying the fundamental contrast between the two basic means of powering an airplane.

I was very pleased with my answer, as after only a moment of thought collection I stated that a piston engine represents a batch process, whereas a turbine engine is a continuous one. Concise and correct. Much of the work of chemical engineering is scaling chemical processes proven in a test tube (or perhaps--except in Texas--an Erlenmeyer flask), into much more efficient large scale continuous ones.

Fortunately, for the sake of the interview, I expanded on my thesis to explain that in a piston engine vapourized fuel is mixed with air and drawn into a cylinder where it is burned, produced power and is exhausted over and over again in discrete parcels, each power stroke contributing to the turning of the crankshaft as well as of the propeller, while in a turbine engine the fuel and air are continuously mixed and expelled. I think they were a little surprised by the initial angle of my answer, but satisfied that I knew my stuff. Later on, another interviewee said that his answer was that a piston engine is most efficient at low altitudes, while a turbine engine is most efficient at higher altitudes. I of course knew that, but such an operational difference hadn't occurred to me, even though it was equally true and relevant.

It was with that context that I learned how bird lungs work differently than humans. Somewhere between elementary school science class and basic first aid, you probably learned that when humans breathe air through our trachea, and so on into our lungs, it passes through tiny tubes called bronchioles which dead-end in clusters of tiny aveolar sacs. We inhale, filling the sacs with a mixture of mostly nitrogen and oxygen. Some of the oxygen diffuses through the walls of the aveoli into the capillaries surrounding them, in exchange for waste carbon dioxide. We exhale, expelling most but not all of the air, which now contains nitrogen unaffected by the process, somewhat depleted oxygen, and some carbon dioxide. Blood flows continuously through the capillaries, but during at least half the breathing cycle, the air in the aveoli is already partly depleted of oxygen. So it's not the most efficient process if you're interested in extracting all available oxygen, but it works pretty well for us, and our fellow mammals.

Birds, however, do it differently. They still breathe in though their mouths and noses into their trachea, but instead of bronchioles that end in aveolar cul-de-sacs that have to be filled and emptied, birds have parabronchi, tiny tubes which are surrounded by capillaries and have an intake at one end and an exhaust at the other. They're not called "intake" and "exhaust" because this is biology not engine design but you get the idea. Birds still breathe in and out in individual breaths, so to keep the air flow through the parabronchi continuous, birds have air sacs in addition to their lungs. While the bird is breathing in, outside air is going into the posterior air sacs and through the bird's lungs, the air going through the bird's lungs exits into the anterior air sacs. While the bird breathes out, air flows from the posterior air sacs through the lungs and out of the lungs and the anterior air sacs to the trachea. In other words, through use of temporary storage facilities, the bird is drawing fresh air through its lungs at the same time as it is exhaling stale air, and is expelling stale air from its lungs at the same time as it is drawing breath.

The bird method is more efficient at lower concentrations of oxygen, such as that found at the altitudes used by large migratory birds (see the figures at the end), or that of the Triassic era.

This is the coolest thing I learned all week. Look at the bird respiration link for more bizarre bird breathing stuff, like the fact that that air sacs are integrated into their bones, or that some birds have looping tracheas, way longer than they need to be.