Avian Lungs
Avian lungs do not have alveoli as mammalian lungs do; birds have Faveolar lungs, which contain millions of tiny passages called parabronchi. There are air vesicles, called atria, which project radially from the walls of the parabronchi. The gas exchange tissues are set into the walls of the atria and gases travel via diffusion between the gas exchange tissues and the lumen of each parabronchus. There are two categories of parabronchi. The paleopulmonic parabronchi are found in all birds and air flows through them in the same direction – posterior to anterior during inhalation and exhalation. Some bird species also have neopulmonic parabronch where the air flow is bidirectional. The paleopulmonic unidirectional airflow is in contrast to the mammalian system, in which the direction of airflow in the lung is tidal, reversing between inhalation and exhalation.
By utilizing a unidirectional flow of air, avian lungs are able to extract a greater concentration of oxygen from inhaled air. Birds are thus equipped to fly at altitudes at which mammals would succumb to hypoxia. This also allows them to sustain a higher metabolic rate than most equivalent weight mammals. Note than some species of small bats have a higher mean total morphometnc pulmonary diffusing capacity for oxygen than equivalent weight birds but this is the exception and is not the rule.
The lungs of birds are relatively small, but are connected to 8-9 air sacs that extend through much of the body, and are in turn connected to air spaces within the bones. The air sacs, although thin walled, are poorly vascularized, and do not themselves contribute much to gas exchange, but they do act like bellows to ventilate the lungs. The air sacs expand and contract due to changes in the volume of the combined thorax and abdominal cavity. This volume change is caused by the movement of the sternum and ribs and this movement is often synchronized with movement of the flight muscles.
Because of the complexity of the system, misunderstanding is common and it is incorrectly believed that it takes two breathing cycles for air to pass entirely through a bird's respiratory system. Air is not stored in either the posterior or anterior sacs between respiration cycles, air moves continuously from the posterior to the anterior of the lungs throughout respiration. This type of lung construction is called a circulatory lung, as distinct from the bellows lung possessed by other animals.
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