Internal Anatomy of chicken:
The avian respiratory system delivers oxygen from the air to the tissues and also removes carbon dioxide. In addition, the respiratory system plays an important role in thermoregulation (maintaining normal body temperature). The avian respiratory system is different from that of other vertebrates, with birds having relatively small lungs plus nine air sacs that play an important role in respiration (but are not directly involved in the exchange of gases). The air sacs permit a unidirectional flow of air through the lungs. So, in bird lungs, more oxygen is available to diffuse into the blood.
Most birds have 9 air sacs:
- one interclavicular sac
- two cervical sacs
- two anterior thoracic sacs
- two posterior thoracic sacs
- two abdominal sacs
Functionally, these 9 air sacs can be divided into anterior sacs (interclavicular, cervicals, & anterior thoracics) & posterior sacs (posterior thoracics & abdominals). Air sacs have very thin walls with few blood vessels. So, they do not play a direct role in gas exchange. Rather, they act as a ‘bellows’ to ventilate the lungs (Powell 2000).
Birds can breathe through the mouth or the nostrils (nares). Air entering these openings (during inspiration) passes through the pharynx & then into the trachea (or windpipe).The trachea bifurcates (or splits) into two primary bronchi at the syrinx. The syrinx is unique to birds & is their ‘voicebox’. The primary bronchi enter the lungs & are then called Mesobronchi. Branching off from the mesobronchi are smaller tubes called dorsobronchi. The ventrobrochi, in turn, lead into the still smaller parabronchi. walls contain hundreds of tiny, branching, & anastomosing ‘air capillaries’ surrounded by a profuse network of blood capillaries (Welty and Baptista 1988). It is within these ‘air capillaries’ that the exchange of gases (oxygen and carbon dioxide) between the lungs and the blood occurs. After passing through the parabronchi, air moves into the ventrobronchi.
A schematic of the avian respiratory system, illustrating the major air sacs and their connections to the lung. (A) The lateral and dorsal direction of motion of the rib cage during exhalation is indicated by arrows. (B) The direction of airflow during inspiration. (C) The direction of flow during expiration (From: Plummer and Goller 2008).
During inhalation, all air sacs expand as inhaled air enters the posterior air sacs and lungs and, simultaneously, air moves out of the lungs and into the anterior air sacs. During exhalation, the air sacs diminish in volume as air moves (1) from the posterior air sacs through the lungs and (2) from the anterior air sacs and out of the body via the trachea.