The respiratory system of birds efficiently transports oxygen via unidirectional air flow and air sac reservoirs.
“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 net 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.
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