Researchers Explain Secret to How Birds Breathe Air in One Direction

Breathing in and out. It’s so simple we often forget we’re doing it, but birds have mastered an even more efficient trick that’s been long-shrouded in mystery — breathing so that the air in their lungs flows in one direction. A team of mathematicians and physicists now say they’ve come up with the explanation for how it’s possible.

In the journal Physical Review Letters, researchers at New Jersey Institute of Technology and New York University have shown how birds can continuously pass oxygenated air through their lungs even when they exhale, using a combination of lab experiments and computer simulations to capture the complex flow physics at work throughout networks of loop-shaped airways and air sacs that make up avian lungs. 

The study has revealed a flow dynamic responsible — small tornado-like airflows, or vortices, arise inside these loopy lung structures that lead to unidirectional air flow even during exhalation. The team says the finding could be a key puzzle piece for better understanding the energy-efficient, aerobic advantage many birds possess for soaring great distances and high altitudes.

“For nearly 100 years, biologists have known bird lungs are much different than our own, made up by tons of these loop structures that provide a constant one-way stream of air, as opposed to our lungs’ branch-like structures that slosh air back and forth as we breathe. However, no one has been able to pin down exactly how these unusual one-way flows arise, and that’s what our study allows us to see in a very simple way,” said Anand Oza, assistant professor of mathematics at NJIT and study co-author. 

“From a physics and engineering perspective, it is really fascinating because this type of directed flow is almost always accomplished through valves that open and close just at the right times to push fluid in the right direction. … In birds, it seems this could be happening naturally due to a property of the air itself that is generated inside these loops, directing the flow almost like valves would do.”

To investigate, researchers at NYU’s Applied Math Lab at the Courant Institute built their own simplified version of bird lungs in the lab — designing a scaled down tube-replicate of the avian respiratory network and filling it with water instead of air. As they recreated the process of inhalation and exhalation in birds using an oscillating piston in the lower loop, the experiment showed a distinct one-way flow pattern in the upper loop, while the flow simply oscillated back and forth in the lower loop. In live bird lungs, this would allow for fresh air to continuously pass over oxygen-absorbing tissues.

Video: Experiments on a network with two loops, the lower subject to oscillations that mimic breathing and the upper developing one-way flow. (Credit: NYU’s Applied Mathematics Laboratory)

“This is in essence what happens inside lungs, but now we could actually see and measure — and thus understand — what was going on,” explained Leif Ristroph, associate professor at NYU’s Courant Institute of Mathematical Sciences and the senior author of the paper, in a statement. “The way this plays out is that the network has loops and thus junctions, which are a bit like ‘forks in the road’ where the flows have a choice about which route to take.” 

To dive deeper, Oza and NJIT mathematics professor Christina Frederick designed computer simulations to independently confirm the phenomenon from Ristroph’s lab and explore the complex fluid dynamics at play, particularly at these forks in the road.

“The initial experiments left a question … we still had no idea why the flow moves the way it does,” said Frederick. “When we began to run our simulations, something immediately popped out … what we showed is that vortices are responsible for the pumping action, as they plug up part of the network at the junction points.”

Video: Analogous simulations showing vortices generated near the network junctions. (Credit: New Jersey Institute of Technology)

“Inertia tends to cause the flows to keep going straight rather than turn down a side street, which gets obstructed by a vortex,” said Oza. “This ends up leading to one-way flows and circulation around loops because of how the junctions are hooked up in the network.” 

For now, along with offering new insights into how birds breathe so efficiently, Oza says the study could potentially open the doors for engineering applications inspired by the mechanism uncovered in the paper. 

“Engineers are always looking for more energy-efficient and robust designs for pumping fluids like fuel or coolant through mechanical systems,” said Oza. “What our study offers is a proof of concept … In this case, Mother Nature’s design might inspire engineers to take advantage of the flow mechanism in bird lungs to produce a similar pumping action, without the need for moving parts or valves that are prone to breaking.”