Lungs Avian lungs 

Avian lungs do not have alveoli as mammalian lungs do, they have Faveolar lungs. They contain millions of tiny passages known as para-bronchi, connected at both ends by the dorsobronchi. The airflow through the avian lung always travels in the same direction – posterior to anterior. This is in contrast to the mammalian body system, in which the direction of airflow in the lung is tidal, reversing between inhalation & 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 an equivalent weight mammal.

The lungs of birds are relatively small, but are connected to 8-9 air sacs that extend through much of the body, & are in turn connected to air spaces within the bones. The air sacs are smooth-walled, & do not themselves contribute much to respiration, but they do help to maintain the airflow through the lungs as air is forced through them by the movement of the ribs & flight muscles.

Because of the complexity of the body system, misunderstanding is common & it is incorrectly believed that it takes 2 breathing cycles for air to pass entirely through a bird's respiratory body system. A bird's lungs do not store air in either of the sacs between respiration cycles, air moves continuously from the posterior to anterior air sacs throughout respiration. This type of lung construction is called a circulatory lung, as distinct from the bellows lung possessed by other animals.