Lung

The lung is the essential organ of respiration in air-breathing vertebrates.

Avian lungs

Birds have a complex but highly efficient crosscurrent exchange system in their lungs, accompanied by air sacs to control the flow of gas through it. See bird respiration for a detailed account of this system.

Related Topics:
Bird - Bird respiration

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The lungs of birds differ significantly from those of mammal. In addition to the lungs themselves, birds have posterior and anterior air sacs (typically nine) which control air flow through the lungs, but do not play a direct role in gas exchange. They have a flow-through respiration system.

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When a bird inhales, air flows in through the trachea to the posterior air sacs, while air currently within the lungs flows into the anterior air sacs. When the bird exhales, the fresh air now contained within the posterior air sacs is driven into the lungs, and the stale air now contained within the anterior air sacs is expelled through the trachea and into the atmosphere. Two complete cycles of inhalation and exhalation are, therefore, required for one breath of air to make its way through the avian respiratory system.

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Avian lungs do not have alveoli, as mammalian lungs do, but instead contain millions of tiny passages known as parabronchi, connected at either ends by the dorsobronchi and ventrobronchi. Air flows through the honeycombed walls of the parabronchi and into air capillaries, where oxygen and carbon dioxide are traded with cross-flowing blood capillaries by diffusion, a process of crosscurrent exchange.

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The purpose of this complex system of air sacs is to ensure that the airflow through the avian lung is always traveling in the same direction - posterior to anterior. This 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.

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