Colleen Farmer was alone one night dissecting an alligator. Her focus was on blood flow in the heart, when suddenly, a hypothesis unfolded about animal lungs. In one sweep, she realized that what physiologists have assumed for decades about the evolution of airflow in alligators, other living reptiles, birds and maybe even dinosaurs might just be startlingly wrong.
Lungs sound simple: Air goes in, air goes out. But, like breathing itself, lungs are easy to take for granted and full of unexpected puzzles. In her windowless lab at the University of Utah, Farmer pondered two basic questions: Which direction does air really flow in lungs, and how did it evolve that way?
In people, air flows like the tides. Inhale, and air whooshes in. Exhale, and the air recedes along the same path, depleted of oxygen and laden with waste gas. Physiologists have believed that other vertebrates share this basic two-way tidal flow — except for birds.
Birds’ breathing was considered more efficient. There’s no downtime while used breath clears out of airways to make room for the next inhalation. Once fresh air reaches the lungs, it flows in one direction through an intricacy of loopy tubes in the crucial zones where blood vessels swap out gases. The evolution of more efficient lungs, biologists have argued, helped birds develop the metabolically expensive lifestyle of aerial athletes. Birds keep their bodies warm, regardless of the environment, and launch themselves into the skies.
There are no barrier gates or fleshy valves in bird lungs that create their remarkable one-way flow. Geometry does the trick. Quirky phenomena called “aerodynamic valves” steer air through open passageways, relying on the angles and shapes of the branching passageways.