How Air Sacs Power Lungs in Birds’ Respiratory System

Biological Strategy

Birds

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Resources are limited and the simple act of retaining them requires resources, especially energy. Living systems must constantly balance the value of resources obtained with the costs of resources expended; failure to do so can result in death or prevent reproduction. Living systems therefore optimize, rather than maximize, resource use. Optimizing shape ultimately optimizes materials and energy. An example of such optimization can be seen in the dolphin’s body shape. It’s streamlined to reduce drag in the water due to an optimal ratio of length to diameter, as well as features on its surface that lie flat, reducing turbulence.

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The most familiar forms of discharging gases are through respiration when many living systems release carbon dioxide, and when plants release oxygen as the end product of plant photosynthesis. Because gases cannot be effectively moved through pushing, a different kind of force is needed to expel them. Creating that force requires energy, even at the cellular level, so living systems must have efficient strategies worth the energy investment or use an external force. This typically entails strategies that build up pressure or use other forces to propel gases. For example, a human exhales about 15% of its spent air per breath. In contrast, when a whale surfaces, it exhales 90% of its spent air in just one spouting.

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Distribute Gases

Gases of particular importance to living systems are oxygen, carbon dioxide, and nitrogen. Oxygen and carbon dioxide are involved in respiration, so distributing these gases efficiently and effectively is important for a living system’s survival. However, gases are difficult to contain because they disperse easily. To accommodate this, living systems have strategies for confining gases and using gases’ properties to their advantage. For example, prairie dogs and mound-building termites build systems of tunnels and mounds that take advantage of wind to ventilate their underground homes.

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Store Gases

Living systems sometimes store gases, such as oxygen, to help maximize respiration or for a variety of other purposes. Gases are often difficult to store because they dissipate and can easily escape. Therefore, most gas storage in living systems can only be temporary. Some examples are fish swim bladders used to control buoyancy, and respiratory sacs in birds that help them maximize access to oxygenated air.

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Birds

Class Aves (“bird”): Eagles, hawks, sparrows, parrots

Birds are evolutionary engineering marvels. They are descended from dinosaurs, but are far from our idea of heavy, scaly reptiles. Of the specific adaptions that set them apart, most notable is flight—although some mammals can fly, birds take the prize for abundance in the skies. Many birds have hollow, lightweight skeletons and specially-designed wings to help them stay aloft. They also have feathers made of keratin that help them stay warm, attract mates, and improve navigation and aerodynamics in flight. In contrast to their dinosaur ancestors, they lack true teeth and have replaced them with specialized beaks and bills.

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The respiratory system of birds facilitates efficient exchange of carbon dioxide and oxygen by using air sacs to maintain a continuous unidirectional airflow through the lungs.

The Strategy

The avian respiratory system is notably different from the mammalian respiratory system, in both its structure and its ability to exchange gas as efficiently as possible.

It consists of paired lungs, which contain static structures with surfaces for gas exchange, and connected air sacs, which expand and contract causing air to move through the static lungs. A breath of oxygen-rich inhaled air remains in the respiratory system for two complete inhalation and exhalation cycles before it is fully spent and exhaled out the body.

When fresh air is first inhaled through a bird’s nares (nostrils), it travels through the trachea (a large tube extending from the throat), which splits into left and right primary bronchi (called “mesobronchi,” with each bronchus leading to a lung). The inhaled air travels down each primary bronchus and then divides: some air enters the lungs where gas exchange occurs, while the remaining air fills the posterior (rear) air sacs. Then, during the first exhalation, the fresh air in the posterior sacs enters the lungs and undergoes gas exchange. The spent air in the lungs is displaced by this incoming air and flows out the body through the trachea. During the second inhalation, fresh air again enters both the posterior sacs and the lungs. Spent air in the lungs is again displaced by incoming air, but it cannot exit through the trachea because fresh air is flowing inward. Instead, the spent air from the lungs enters anterior (forward) air sacs. Then, during the second exhalation, the spent air in the anterior sacs and in the lungs flows out through the trachea, and fresh air in the posterior sacs enters the lungs for gas exchange.

digram showing the flow of air through bird lungs during inhalation and exhalation

When a bird inhales, fresh air (blue) enters through the trachea and bronchus and flows into the lungs and posterior air sacs. The fresh air moving into the lungs displaces stale air (red) from the previous breath, moving it into the anterior air sacs. During exhalation, fresh air from the posterior air sac moves into the lungs, while stale air from the anterior air sacs is expelled through the bronchus and trachea.

This pattern of airflow through the respiratory system creates unidirectional (one-way) flow of fresh air over the gas exchange surfaces in the lungs. Furthermore, fresh air passes over the gas exchange surfaces during both inhalation and exhalation, resulting in a constant supply of fresh air enabling the bird to experience a near-continuous state of gas exchange within the lungs. This contrasts with mammalian lungs, which experience bidirectional (two-way) airflow over the gas exchange surfaces.

The efficiency of the avian respiratory system is owed in part to its unidirectional nature and in part to the structure of its parabronchial system (the smaller passages within the lungs). The air capillaries in the walls of the parabronchial system have a much larger overall surface area than that found in the mammalian respiratory system. The greater surface area allows a greater proportion of oxygen from each breath to be exchanged for carbon dioxide from the blood and tissues.

This summary features contributions from Alex Uhrich.

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Last Updated July 2, 2020

References

“Avian lungs are unique in that the air flows in only one direction, rather than in and out as in other vertebrates. How do birds control the air so that it flows through their lungs when they can only inhale and exhale through one trachea? The solution is a surprising combination of unique anatomical features and the manipulation of airflow. Supplementing the lungs is an elaborate system of interconnected air sacs, not present in mammals…Most birds inhale air through nostrils, or nares, at the base of the bill…Inhaled air moves next down the trachea, or windpipe, which divides into two bronchi and in turn into many subdividing stems and branches in each lung…Most of the lung tissue comprises roughly 1800 smaller interconnecting tertiary bronchi. These bronchi lead into tiny air capillaries that intertwine with blood capillaries, where gases are exchanged.

“Inhaled air proceeds through two respiratory cycles that, together, consist of four steps. Most of the air inhaled in step 1 passes through the primary bronchi to the posterior air sacs…In step 2, the exhalation phase of this first breath, the inhaled air moves from the posterior air sacs into the lungs. There, oxygen and carbon dioxide (CO2) exchange takes place as inhaled air flows through the air-capillary system. The next time that the bird inhales, step 3, the oxygen-depleted air moves from the lungs into the anterior air sacs. The second and final exhalation, step 4, expels CO2-rich air from the anterior air sacs, bronchi, and trachea back into the atmosphere.

“This series of four steps maximizes contact of fresh air with the respiratory surfaces of the lung. Most importantly, a bird replaces nearly all the air in its lungs with each breath. No residual air is left in the lungs during the ventilation cycle of birds, as it is in mammals. By transferring more air and air higher in oxygen content during each breath, birds achieve a more efficient rate of gas exchange than do mammals…The air-sac system is an inconspicuous, but integral, part of the avian respiratory system…Air sacs are thin-walled (only one or two cell layers thick) structures that extend into the body cavity and into the wing and leg bones…The air sacs make possible the continuous, unidirectional, efficient flow of air through the lungs.” (Gill 2007:143-147)

Book

Ornithology

Macmillan. | 01/03/2007 | Gill, Frank B.

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Reference

Journal article

Structure of the avian respiratory tract

Respiration Physiology | 12/02/2003 | Hans-Rainer Duncker

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