Birds
Cutaneous respiration is insignificant in birds. The almost
exclusive respiratory organ is the lungs. Like mammals, birds
have two lungs connected to a trachea and ventilated by an
aspiration pump. Beyond this, however, the structural similarities
are few. For example, there are no blind-ended alveoli in
and out of which air moves. Instead, the conducting passages
branch repeatedly and eventually form numerous tiny, oneway
passageways, the parabronchi, that permit air to flow
through the lungs. Small air capillaries open off the walls of
each parabronchus, and gas exchange with the blood actually