The avian respiratory system is physically distinct from the mammalian respiratory system, both in structure and in its ability to exchange gas as efficiently as possible.
The bird’s respiratory system 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 (used) 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 (a.k.a. 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.
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 airflow over the gas exchange surfaces.
The efficiency of the avian respiratory system is owed in part to its unidirectional nature and 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 the surface area, the more oxygen and carbon dioxide can be passed between blood and tissues, which makes for more efficient breathing.
This summary features contributions from Alex Uhrich.
“The respiratory system of birds is different in both structure and function from the respiratory system of mammals. Avian lungs are small, compact, spongy structures molded among the ribs on either side of the spine in the chest cavity. The dense tissues of avian lungs weigh as much as the lungs of mammals of equal body weight but occupy only about half the volume. Healthy bird lungs are well vascularized and light pink in color.
“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)