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Though some people find birds endlessly fascinating, these creatures are, in many ways, a very mundane part of life. Birds can be found on every continent in the world, and, with over 10,000 species in the avian roster, it makes sense that most people consider them commonplace. It’s easy to see a songbird fly by and think nothing of it, but their mastery of the skies is much more precise than one might think. Birds make flying seem effortless, but it’s an extremely exhausting task. Not only that, but many birds fly at altitudes where the air is thin and oxygen is hard to come by. Humans require months of acclimation to handle the rigors of high-altitude sports like mountaineering, but some birds can soar over the peak of Mount Everest as soon as their first migration. How is this possible?
Well, as it turns out, birds and mammals have very different ways of breathing.
To start with, let’s consider the mammalian respiratory system–the one most of us are familiar with. Breathing in mammals is a relatively simple affair. The diaphragm, a thin sheet-like muscle near the bottom of the ribcage, expands the space of the thoracic (chest) cavity when it flattens out. The lungs expand with the chest, creating a vacuum and drawing oxygen-laden air inward. Air enters through the nose and mouth, travels through tubes like the trachea to arrive in tiny little air sacs in the lungs. These air sacs, called alveoli, are surrounded by blood vessels called capillaries. Capillaries have incredibly thin walls, which allows the two vital gasses–oxygen and carbon dioxide–to move into and out of the bloodstream. Oxygen from the freshly-inhaled air moves inward, where it can be transported around the body to keep the cells alive. Carbon Dioxide, a waste product of cellular respiration, exits the bloodstream at the same time oxygen is flowing in. After a certain amount of time, muscles in the abdomen and chest work together to constrict the thoracic cavity and force the Carbon Dioxide out–along with whatever oxygen didn’t get absorbed in time. The end result is a bit...inefficient. Apart from the fact that not all oxygen in a breath can be absorbed, mammals experience an interrupted flow of oxygen. Whenever a mammal breathes out, there is a brief window where no new oxygen is available in the lungs. This is why we tend to breathe faster when we exert ourselves. Our bodies need a lot of oxygen to keep up the pace, and increasing the rate at which we breathe means we spend less time in the "pause" between breaths.
Birds do things a different way. For one thing, they don’t have diaphragms. The inflow (and outflow) of air into the body is instead managed entirely by certain muscles surrounding the ribcage. The biggest difference, however, is the fact that air takes a unique path through the avian respiratory system. In addition to lungs, birds have a series of air sacs in their bodies that help store oxygen and other gasses. The number and type of air sacs present can vary from species to species, but, in general, they at least have a pair of anterior (front) and posterior (rear) air sacs to support the function of their lungs. When a bird breathes in, air of course fills the lungs, but it also flows into storage in the posterior air sacs. As new air moves in, old, carbon dioxide-rich air doesn't "hang around" like it does in mammalian lungs. Instead, this old air is forced into the anterior air sacs. When a bird exhales, it doesn't empty out all of the gas in its body. The old air in the anterior air sacs gets pushed out, the increasingly carbon dioxide-rich air in the lungs gets pushed into the anterior air sacs, and the stored oxygen in the posterior air sacs moves up to fill the lungs. This innovation accomplishes two important things. First, it ensures that birds experience a constant flow of air to help fuel their flight. Unlike mammals, their lungs are full of oxygen when they inhale and when they exhale! Second, it helps keep their bodies nice and lightweight. Think about what happens when you breathe out all the air in your lungs while you're floating in the water. You start to sink, right? Flying is already an activity with a high energetic cost, so maintaining a constant buffer of buoyant gas reduces the amount of work it takes to keep a bird's body aloft.
(Image from AskNature)
This efficient form of respiration is called unidirectional airflow, because the air moves in one direction through a loop. Mammalian lungs, where gasses mingle together, experience bidirectional (two-way) airflow instead. Unidirectional airflow isn’t actually unique to birds (crocodiles, alligators, and some lizards also have respiratory systems where the oxygen flows in a one-way “loop”), but birds are certainly the ones who get the most "mileage" out of the trait. Without such an efficient system, it's unlikely they'd be able to supply their bodies with enough oxygen to maintain the rigors of flight.
Unidirectional airflow in birds isn’t a new discovery, but it continues to serve as an object of study for researchers of many disciplines. Just recently, in 2021, physicians finally pinned down the aerodynamics at play in birds’ lungs. Using a system of looping tubes, they were able to showcase why and how oxygen moves efficiently through them in a single direction. This heightened understanding holds promise for streamlining the movement of fluids in fields such as engineering and healthcare, among other things. It's pretty incredible that the common sparrow is itself powered by such a well-engineered system! Birds might be a common fixture of our lives, but there is so much more to them than meets the eye.
References & Further Reading
This brief article goes further into depth regarding the efficient systems in birds' lungs. While unidirectional airflow is certainly one part of the puzzle, there are other unique structures that enhance this system further! This article also lists more references to explore if you're interested in knowing more.
This bite-sized video by SciShow on YouTube explores the potential origins of unidirectional airflow in birds. If you found yourself intrigued by the idea that some reptiles also have unidirectional respiratory systems, this video is a great place to learn more!
If you're the sort of learner who likes to dive into the nitty-gritty details, this is the source for you. This extensive page covers a wide array of details and includes a number of helpful images, diagrams, and videos to cement the ideas. If you want to examine this topic at a collegiate level, this is a great first step!
This article details the recent physics experiment regarding the unidirectional airflow system found in birds' lungs. It includes videos of the enlightening project, plus an interview with the researchers who worked on it.
Yes, birds really can fly over the world's tallest mountain. In addition to highly specialized lungs, the bar-headed goose has a number of other unique modifications that makes it perfectly suited for tackling air so thin it has an oxygen concentration of 7%. Check out this article if you want to learn more!
A while ago, STEMx released an article about another surprising facet of birds: their intelligence. If you're of the mind that birds are pretty dumb, this article is a great introduction for why this might not be as true as you think.
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