Birds have an extensive respiratory system.  Like mammals, birds have 2 lungs connected to a trachea and ventilated by an aspiration pump.  However, beyond that, birds have a unique respiratory system.  Air is inhaled through the external nares and enters through the internal nares.  Air travels through the pharynx and into the larynx which is protected from food and liquids by the epiglottis that covers the glottis opening.  Continuous with the larynx is the trachea.  The trachea is supported by lateral cartilaginous rings that prevent collapsing while allowing expansion of the trachea.  Air then splits at the Syrinx (sound mechanism) and travels down the bronchi (mesobronchi).  The bronchi branches repeatedly and form parabronchi (one-way passageways) that lead to the lungs.  Small capillaries open off the parabronchi; this is the site of gas exchange with the blood capillaries.  Avian lungs are located in the dorsal wall of the thoracic cavity.  Nine air sacs are connected to the lungs.  These air sacs extend into the bones, replacing marrow.  There is still much debate over the function of avian air sacs; -theories include lightening the bird, cooling the testes, providing lift, etc.  However none of these are feasible.  The current proposal is that air sacs function as bellows.  Avian lungs themselves do not change shape with rib cage motion.  Compression and expansion of the rib cage acts on the air sacs, drawing the air through them and into the lungs (Kardong, 2002).
    A bird's respiratatory system is adapted to its oxygen demands.  Birds have ten times more respiratory surface area than any other mammal of the same size.  Also, oxygenated air passes across the lung with each inspiration.

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