The air sacs are thin membranous structures connected to the primary or secondary bronchi via ostia and they comprise most of the volume of the respiratory system. Air sacs are poorly vascularized by the systemic circulation and do not directly participate in significant gas exchange but act as a bellows to ventilate the lungs. In most species, there are nine air sacs which can be considered in cranial and caudal functional groups (Maina, 1989). Air sac diverticulae may also penetrate the skeleton, but there are large species differences and the functional significance of such connections for respiration has not been established (Maina, 1989).
The cranial group consists of the paired cervical air sacs, the unpaired clavicular air sac, and the paired cranial thoracic air sacs. The cervical sacs directly connect to the first medioventral secondary bronchus. The clavicular air sac directly connects to the third medioventral secondary bronchus and may also have indirect connections via parabronchi to other cranial (medioventral) secondary bronchi in some species (e.g., chickens). The cranial thoracic air sacs generally connect to the third medioventral secondary bronchi and also to parabronchi originating from other cranial secondary bronchi in some species.
The caudal group consists of the paired caudal thoracic air sacs and paired abdominal air sacs. The caudal thoracic air sac is directly connected to the lateroventral secondary bronchus and may have indirect connections to other lateroventral or even cranial (medioventral) secondary bronchi in species with large amounts of neopulmonic parabronchi (e.g., chickens). The abdominal air sacs connect to the caudal end of the intrapulmonary primary bronchus and may have more indirect connections to parabronchi from laterodorsal secondary bronchi and the last mediodorsal secondary bronchi. Air sac connections with parabronchi are frequently grouped into a funnellike structure called the saccobronchus.