Respiration in birds

Flying is an activity that demands an extraordinary amount of metabolic energy. During flight, the active muscles of birds require and produce very large amounts of the cellular “energy currency” known as adenosine triphosphate (ATP) to power their contractions. To accomplish this, the flight muscles must be well supplied with oxygen extracted from the atmosphere. The flight muscles furthermore generate large amounts of carbon dioxide as a waste product of their metabolism, and this must be eliminated from the blood.

These facts make it tempting to propose that birds need a specially designed respiratory system because their need for oxygen must be greater than that of comparably sized mammals. Many textbooks say exactly this. Studies have shown that resting birds and comparably sized resting mammals both consume nearly identical amounts of oxygen. The argument says that flying birds must consume more oxygen than mammals exercising at high levels of performance. Further studies have shown that flying birds consume ten to fifteen times more oxygen than when at rest, and in general, birds can extract 25 percent more oxygen from the air than mammals. This is no greater than the increase in oxygen consumption that any well-conditioned human athlete can attain.

The primary reasons that birds require a special respiratory system become clear when the structural limitations of size and weight imposed by flight and the extreme environments encountered during high-altitude flight (such as low oxygen availability) are taken into consideration. The respiratory system of a bird needs to be relatively light, compact, and efficient in order to maximize the animal’s chances of survival. The avian respiratory system has evolved successfully, which is evident if one observes birds in steady flight at altitudes so high that the scarcity of oxygen would cause most mammals to become comatose even when totally inactive.

The Avian Respiratory System

This sort of respiratory performance has been achieved, within the imposed size and weight restrictions, through the evolution of the avian respiratory system into a unique and very efficient structure. Typical mammalian lungs expand and collapse inside the body with each breath (the lungs have a variable volume), and there is a bidirectional flow of gas into and then back out of them. In contrast, the bird’s two lungs have a nearly constant volume through which most of the inhaled gas flows in one direction. This is possible because of the presence of several large, thin-walled air sacs that join the lungs and assist in their ventilation, much in the manner of a system of bellows to push air through the lungs.

The inhaled air initially follows an anatomic pathway in birds that resembles that of mammals—through a trachea (windpipe) that then divides into two tubes called the primary bronchi, each of which enters a lung. At this point, the anatomy of the bird becomes different as the primary bronchi actually passes completely through the avian lung to terminate in paired abdominal air sacs. These air sacs also have connections to several hollow bones in the bird’s legs. Near the posterior end of the primary bronchi, furthermore, there is a pair of posterior thoracic air sacs that are connected to the primary bronchi via laterobronchi. Joined to the primary bronchi via one of several secondary tubes called the ventrobronchi are paired anterior thoracic air sacs and, depending on the bird species, usually several other anteriorly located air sacs.

Very soon after entering the lung, four ventrobronchi branches from a primary bronchus. Farther along the primary bronchus, there are as many as ten additional secondary bronchi, called the dorsobronchi, which also leave the primary bronchus. The dorsobronchi and ventrobronchi both divide many times as they penetrate the lung tissue. Eventually, the dorsobronchi become connected with the ventrobronchi by thousands of tiny parabronchi, each of which is only about one millimeter in diameter.

The parabronchial walls are like a lattice, perforated extensively with pockets called atria, which themselves have indentations known as infundibula. Interconnecting the infundibula are numerous tiny air capillaries approximately five micrometers in diameter. The lattice itself is made up of the capillary blood vessels into which the oxygen and the carbon dioxide diffuses during the exchange of these gases between the air and the blood. This region of parabronchi and the many tiny air passages extending from them, all of which are enmeshed by the very dense network of blood capillaries, make up the gas exchange area of the avian lung.

Inhalation and Exhalation

The bronchi, primary and secondary, and the air sacs do exchange gas with the blood during avian respiration. Their only function is to help move the air through the actual gas exchange organs, the lungs. To achieve the unidirectional flow of air through the lungs, the inhaled, oxygen-rich air passes through the primary bronchi directly into the posterior air sacs, which are being expanded as a result of the movements of inhalation. While this is happening, the air that is already in the gas exchange areas of the lungs is being pulled out of them and into the anterior air sacs, which are also experiencing expansion during inhalation. Replacing this air in the lungs is a portion of the oxygen-rich air from the primary bronchi that does not enter the posterior air sacs but flows into the posterior lung regions and is directed forward to the anterior areas.

During exhalation, both the posterior and anterior air sacs are compressed, and their contents emptied. The air from the posterior air sacs enters the gas exchange areas of the lungs, not yet having lost any of its oxygen, and flows in a posterior-to-anterior direction. The air from the anterior air sacs, loaded with the carbon dioxide which had diffused out of the blood capillaries, enters the primary bronchi and is exhaled by the animal. The cycle then repeats itself with the next inhalation. Birds do not have diaphragms like humans but use their ribs and specialized air sacs to exhale air.

During the cycles of inhalation and exhalation, the air is always passing through the gas exchange areas of the avian lungs in a single posterior-to-anterior direction. The air passing through the gas exchange areas of the lung also always contains a relatively constant and high percentage of oxygen that can diffuse into the blood inside the lung’s blood capillaries. The blood entering these capillaries is rich in carbon dioxide, which diffuses out of the blood and into the air as it passes through the lung on its way to the anterior air sacs and eventually out of the animal.

A scientist at the University of Florida was studying a red-tailed hawk's anatomy using computed tomography (CT) scans and accidentally discovered it had a specialized air sac. Further examination of the hawk and sixty-eight other bird species confirmed this air sac is present in all soaring birds, like eagles and osprey, between their upstroke and downstroke muscles. They named it the subpectoral diverticulum (SPD). The researchers concluded that the SPD modifies the biomechanics of the bird's flight muscles to allow them to soar more easily. The absence of the extra sac is believed to be a product of evolution.

Blood and Oxygen

The primary oxygen transporter is the avian hemoglobin molecule, and it does not perform significantly better than the hemoglobin found in mammals. In most species, there is little or no difference in the amount of hemoglobin contained in avian blood except in diving birds. In studies of diving birds, scientists found higher levels of myoglobin and hemoglobin, which carry oxygen in the blood, allowing them to inhale fewer times while maintaining necessary blood oxygen levels. This ensures they can dive and catch prey. If their oxygen levels lower, they can continue diving by actively redirecting oxygenated blood to their brain, vital organs, and the muscles used in diving. Diving birds also have larger erythrocytes—red blood cells that store oxygen, increasing the time diving birds can hold their breath. Penguins also inflate their air sacs to adjust their buoyancy when chasing prey underwater.

The flow of the blood with respect to the gas flow through the gas exchange regions is a crucial factor. The flow of blood through the pulmonary (lung) blood capillaries is in a direction that permits the efficient extraction of the air’s oxygen. While it was previously believed that there was a countercurrent flow of air and blood, with the air moving through the lung in a posterior-to-anterior direction and the blood flowing through the lung in an anterior-to-posterior direction, it is now known to be a crosscurrent system. This crosscurrent system has the air still passing in a posterior-to-anterior direction, but the blood flows in a combination of the opposite direction and at a right angle to the airflow. This critical design feature, combined with those mentioned in the next paragraph, permits the avian lung to be the most efficient of any known gas exchange organ for an air-breathing animal.

To comprehend the compact and efficient design of the avian lung, it is necessary only to measure the ratio of the surface area it uses for gas exchange to the lung volume. This value is approximately ten times greater than the ratio found in mammals, allowing more blood per unit of volume of the lung to be present at any time. This is really of importance only when it is noted that avian lungs have more blood in their capillaries per unit of volume of the lung than mammalian lungs. Since only the blood in the capillaries can pick up oxygen and get rid of carbon dioxide, the value of such features is obvious: small and very efficient lungs.

Avian Anatomy and Physiology

The primary techniques utilized in the study of the avian respiratory system come from the fields of anatomy and physiology. Modern work involves the use of electron microscopes, miniature gas flow meters, and even tiny radio transmitters that can send signals from sensors implanted in a freely moving animal.

The connection of the airways with the large air sacs was first reported in the seventeenth century. In the eighteenth century, pioneering studies were performed, which showed the interesting fact that some hollow bones of the bird were connected to the respiratory system. It is, in fact, possible for a bird to breathe through one of these bones if its normal airway is blocked and the hollow center of the bone is opened to the atmosphere.

The rate at which birds consume oxygen can be measured in much the same way as it is in other animals. This can involve the use of a respirometer, a device to record the volumes of air inhaled and exhaled during breathing. It can also indicate how much oxygen is retained by the animal for its survival. Other methods are also used if the experimental demands preclude the use of such a device.

Scientists have even sampled the air at different places in the respiratory system of birds. To do this, it is necessary to insert small tubes into the regions of interest (for example, into the different air sacs) and to withdraw small samples of the gas found within these structures at different times during the breathing cycle. The samples are then tested for their oxygen and carbon dioxide content. The results can reveal whether the gas has already experienced exchanges of oxygen and carbon dioxide with the blood and whether the gas is being replaced at regular intervals or is only being stored there. The latter question involves using a marker gas mixture that the animal first inhales for one or more breaths and that is then replaced by normal air for inhalation. The rate of disappearance of the marker gas indicates how quickly the gas content of the region being studied is replaced.

Similarly, miniature gas flow meters have been developed to investigate the direction of the gas flow within the various regions of the respiratory system. Inserting these into the airways of larger birds shows that gas flows through the bird's lung in a unidirectional way. The results of such investigations have allowed scientists to discard incorrect hypotheses about the manner in which the inhaled air is distributed throughout the bird’s respiratory system and about the functions of various respiratory structures.

Biochemical studies of avian blood measure the same factors as are measured in other animal blood tests. The quantities of interest are the hemoglobin content of the blood (important because it carries essentially all the oxygen transported by the blood), the ability of avian hemoglobin to carry oxygen, the effects of various factors on the hemoglobin-oxygen interactions (such as the normally occurring acids of the blood, carbon dioxide, and temperature), and the ability of the hemoglobin to transport carbon dioxide. It is important to learn about these factors because of the intimate relationships between the respiratory organs and the blood (as transporter of the oxygen and carbon dioxide between the lungs and the other tissues of the body). These studies have revealed no dramatic differences between avian and mammalian blood with respect to the blood’s respiratory functions.

These and many other methods are used to investigate avian respiratory systems. As a result of these studies, scientists know that a bird’s ability to maintain sustained flight at high altitudes results not from any special properties of its blood but is based on the very special design of its respiratory organs.

Altitude and Oxygen Demand

Regardless of the animal, there is one common need for survival—oxygen. Oxygen must be delivered at a rate high enough to permit the organism’s cellular functions to take place, most of which require a continuous and large supply of adenosine triphosphate (ATP). The energy-rich ATP molecules are made in large quantities by the processes of aerobic cellular respiration (oxygen-dependent cellular respiration). Without sufficient oxygen, rates of aerobic cellular respiration sufficient for maintaining life are impossible. Correctly interpreting the value of the avian respiratory system requires an understanding of the characteristics and demands of avian life. Because of the high activity levels of flying birds, but especially because of the elevations at which this activity occurs and the consequent reduction of oxygen availability, birds require very efficient respiratory systems. To provide them with the required oxygen within the context and demands of their lifestyle, they have evolved a distinctive respiratory system. The anatomical, physiological, behavioral, and chemical components of the bird are all exquisitely adapted to these demands, not as independent entities but as an integrated system.

The unique structure of the avian respiratory system is the most efficient air-breathing organ among vertebrates. It can extract oxygen from the air so efficiently that prolonged flight is possible at altitudes where the low availability of oxygen is disabling to most vertebrates, even at rest. The mechanics of this efficient respiration system may have applications for engineering and developing high-efficiency oxygen extraction devices. Large-scale applications of this device could enrich the atmosphere with oxygen for the survival of animals in hostile environments. Regardless of these possible applications, avian respiration provides a remarkable example of a creative solution to environmental demands resulting from natural selection.

Principal Terms

Adenosine Triphosphate (ATP): The energy currency of cell metabolism in all organisms

Aerobic: Requiring free oxygen; any biological process that can occur in the presence of free oxygen

Diffusion: The net movement of molecules from an area of high concentration to one of lower concentration as a result of random molecular movements

Hemoglobin: A protein in vertebrate red blood cells that carries oxygen and carbon dioxide

Metabolism: The sum of all chemical processes occurring within a cell or living organism

Respiration: The utilization of oxygen in air-breathing vertebrates, the inhalation of oxygen, and the exhalation of carbon dioxide

Bibliography

Baumel, Julian J., ed. Handbook of Avian Anatomy: Nomina Anatomica Avium. 2nd ed. Oxford, Nuttall Ornithological Club, 1993.

Carey, Sarah. "UF Researchers: Soaring Birds Use Their Lungs to Modify Mechanics of Flight." University of Florida Health, 12 June 2024, ufhealth.org/news/2024/uf-researchers-soaring-birds-use-their-lungs-to-modify-mechanics-of-flight. Accessed 15 Sept. 2024.

Randall, David, Warren Burggren, and Kathleen French. Eckert Animal Physiology Mechanisms and Adaptations. 5th ed. New York City, W.H. Freeman and Company, 2002.

Farner, Donald S., James R. King, and Kenneth C. Parkes, eds. Avian Biology. Philadephia, Elsevier, 1997.

Hildebrand, Milton. Analysis of Vertebrate Structure. 6th ed. Hoboken, John Wiley & Sons, 2015.

"How Do Birds Breathe? (Everything Explained)." Bird Fact, 22 July 2022, birdfact.com/articles/how-do-birds-breathe. Accessed 5 July 2023.

Lovette, Irby J., et al. The Cornell Lab of Ornithology Handbook of Bird Biology. 3rd ed., Hoboken, John Wiley & Sons, 2020.

Proctor, Noble S. Manual of Ornithology: Avian Structure and Function. New Haven, Yale University Press, 1993.

Ritchison, Gary. In a Class of Their Own: A Detailed Examination of Avian Forms and Functions. New York City, Springer, 2023.

Scanes, C. G., and Sami Dridi. Sturkie’s Avian Physiology. 7th ed., Cambridge, Academic Press, 2022.

Schmidt-Nielsen, Knut. How Animals Work. Cambridge University Press, 1972.

Schmidt-Nielsen, Knut. “How Birds Breathe.” Scientific American, vol. 225, Dec. 1971, pp. 72-79.

Welty, Joel Carl. The Life of Birds. 4th ed. Philadelphia, W. B. Saunders, 1988.

Woakes, A. J., M. K. Grieshaber, and C. R. Bridges, eds. Physiological Strategies for Gas Exchange and Metabolism. Cambridge, Cambridge University Press, 1991.