Bird

Many - see section below.

Respiration

Birds respire by means of crosscurrent flow: the air flows at a 90 degree angle to the flow of blood in the lungs capillaries. 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. There are three parts involved in respiration:

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• the anterior air sacs (interclavicular, cervicals, and anterior thoracics),
• the lungs, and
• the posterior air sacs (posterior thoracics & abdominals).

It takes a bird two full breaths (inhaling and exhaling), to completely cycle the air from each inhalation through the lungs and out again. The air flows through air sacs and lungs as follows.

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• First inhalation: air flows through the trachea and bronchi into the posterior air sacs.
• First exhalation: air flows from the posterior air sacs to the lungs.
• Second inhalation: air flows from the lungs to the anterior air sacs.
• Second exhalation: air flows from the anterior sacs back through the trachea and out of the body.

In birds, at each inhalation and each exhalation fresh air flows through the lungs in only one direction. Birds are therefore able to diffuse more oxygen into their blood. Unlike humans and other mammals, there is no mixing of oxygen rich air and carbon dioxide rich air. Thus, the partial pressure of oxygen in a bird's lungs is the same as the environment. This is also why you would more likely see a bird on Mount Everest than, say, a mouse.

Related Topics:
Diffuse - Oxygen - Carbon dioxide - Partial pressure - Mount Everest - Mouse

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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.

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