How Do Birds Breathe?
Courtesy of Florian Glawogger, Unsplash
Despite the fact that birds are extremely energetic and active creatures, have you ever seen one gasping for air? We examine the amazing ways in which birds manage to maintain their busy lives without giving up in a pile of exhaustion.
If you have ever spent more than a few minutes observing birds, you will undoubtedly quickly discover that they are constantly moving, especially when you are attempting to locate them through binoculars or take a picture of them. They need a lot of energy because they are always moving, whether it is to avoid danger, to pursue during the mating season, or simply to find food.
In order to maintain their high metabolic rates, all birds prefer diets high in fat and protein, but in order to meet their enormous oxygen demands, their respiratory systems also need to be extremely effective.
Mammals use the diaphragm, a slab of muscle, to help them breathe by allowing air to enter and exit their lungs. Humans have nearly all of their rib cages in the chest cavity, which expands to make room for our lungs when we breathe in and contracts when we exhale.
However, since birds lack a diaphragm, their breathing is assisted by unique muscles that are attached to the sternum and rib cage. These muscles move air downward and forward during inhalation and upward and backward during exhalation.
This produces a motion of expansion and contraction that helps the even more intricate air-flow system inside the chest cavity, which we’ll discuss shortly.
Thus, similar to humans, birds breathe through their mouths and noses through a tube called a trachea, but that is where all similarities end. The two big sacs that make up the human lungs are filled with smaller air sacs called alveoli, which are grouped around tubes called bronchi and resemble clusters of grapes on a vine.
When air enters our bodies, it is drawn down the trachea and then through the bronchi into the alveoli, or air sacs, where osmosis is responsible for the gas exchange of carbon dioxide and oxygen.
Our blood and tissues absorb the rich, life-giving oxygen from the air, while the potentially lethal byproduct carbon dioxide is forced into the now-old, oxygen-starved air, which we then exhale, forcing the harmful carbon dioxide back out into the atmosphere.
Bidirectional flow is the term for this in-and-out, two-way flow that occurs during breathing in all mammals. It’s crucial to understand that after the oxygen leaves the alveoli, the air inside becomes stale and leftover. It is now filled with carbon dioxide, which we breathe out again through the same mechanism, briefly depriving ourselves of oxygen as we exhale. Bidirectional airflow is therefore actually quite inefficient.
The energy required by birds to maintain flight is immense, so it’s imperative that oxygen enters the body and carbon dioxide exits the body simultaneously.
As a result, the respiratory systems of birds have developed into incredibly effective mechanisms. Birds have relatively small lungs that are encircled by nine air sacs that are positioned around the outside of the lungs, rather than having two large lungs that house the air sacs.
Unlike ours, the air sacs’ thin walls have nothing to do with gas exchange. Rather, these air sacs direct the flow of air like bellows. Rather than in the air sacs, the exchange of waste carbon dioxide and essential oxygen actually occurs inside capillaries that cover the exterior of tiny air passages called parabronchi, which are located inside the small lungs.
Courtesy of Cruithne9, Wikimedia Commons
Small air sacs are located near the base of the neck, in front of the lungs, and larger ones are located at the back of the bird, past the lungs, and are referred to as posterior sacs.
This bird’s remarkably effective respiratory system is based on the variation in the size and location of its air sacs throughout its body.
âImagine a bird breathing in for the very first time. When fresh, oxygenated air enters the trachea, the posterior air sacs are pulled outward by the muscles of the rib cage and sternum, which forces the fresh air down the trachea and into the larger sacs at the back of the bird, inflating them. Then, when the bird exhales, the muscles tighten, causing the larger sacs to dilate and forcing air out of the body and back into the lungs.
The air then rushes through the tiny parabronchi due to high pressure, allowing oxygen to be absorbed by the body and drawing carbon dioxide into the now “stale” air. When the bird inhales again, the stale air in its lungs is drawn into the smaller front sacs, which subsequently expand with the bad air, while more fresh air rushes into the posterior sacs as before.
Muscles cause the front sacs to collapse when the bird exhales a second time, pushing the stale air up the trachea and out into the environment. Recall that oxygenated air inhaled during the second breath is now drawn into the lungs from the posterior sacs once more, and so forth.
This indicates that, unlike mammals, which use an in-and-out system, birds use a single direction for airflow through their lungs: in from the back and out the front. It also means that birds actually require two breaths to complete a “cycle” of respiration. Because of this one-way flow, birds never run out of oxygen.
Though birds cannot pant like dogs can, they can still cool themselves by opening their mouths, as you may have noticed when a bird appeared to be out of breath. But don’t worrytheir incredible lungs are operating at maximum capacity, keeping the aerobic cycle steadyjust like breathing.
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About the author: Our writer and researcher for the Bird Buddy blog is Sim Wood. She is currently remodeling her Slovenian property with her spouse and making do without a plan. She is also proficient in 72 bird species’ calls and songs. Favorite bird: shoebill.
Chase Mendenhall works at the Carnegie Museum of Natural History as an assistant curator for birds, ecology, and conservation. It is encouraged for museum staff members to blog about the special experiences and insights they have had from working there.
Because a bird’s breathing takes two cycles to complete a single breath, the air that passes through its lungs is unidirectional, constantly fresh, and oxygen-rich. The gas exchange region of a bird’s anatomy is arranged into a network of parallel tubes that carry deoxygenated blood into the lung in the opposite direction of the air flow. Bird lungs are small and rigid. Birds’ efficient “counter-current” gas exchange, which is exclusive to their lungs, helps some species, like the Bar-headed Goose (Anser indicus), soar over Mount Everest without issue. However, because mammalian lungs never completely expel stale air during exhalation, human explorers struggle to breathe fresh air at 29,029 feet above sea level. Instead, they yearn to be able to use their butts to continuously breathe in fresh air, just like birds do.
A bird’s breath enters through its nares, or nostrils, and passes through the trachea before entering a number of posterior air sacs in its thorax and rumpits butts. The same breath that a bird exhales enters the lung, where carbon dioxide is expelled and oxygen is absorbed, rather than leaving the body as it does in mammals. The same breath of air leaves the lungs and enters the anterior air sacs when a bird inhales twice. The stale air exits the bird’s body through the nares during its second and final exhale.
Because of their highly efficient respiratory system, birds are able to sustain their flight even at high altitudes where oxygen is in short supply. The fact that birds have static lungs and breathe unidirectionally via air sacs throughout their bodies rather than the diaphragms found in other land animals is a crucial characteristic of avian respiration.
FAQ
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