abdominal air sac

The abdominal air sac is an air-filled respiratory structure found in birds and some reptiles. It forms part of the highly efficient unidirectional airflow system that allows these animals to extract oxygen from air more effectively than the tidal breathing system used by mammals.

The abdominal air sac is positioned in the caudal (tail-end) portion of the body cavity, posterior to the digestive organs. In birds, it is typically the largest of the nine air sacs in the respiratory system.

The abdominal air sac functions as a bellows that moves fresh air through the lungs in one direction during both inhalation and exhalation. It does not itself participate in gas exchange but is critical for maintaining the continuous flow of oxygenated air across the parabronchial lung tissue.

In birds, air sacs can be infected by aspergillosis (a fungal infection) or other respiratory pathogens, leading to air sacculitis. Rupture of air sacs due to trauma can cause subcutaneous emphysema. These conditions are significant concerns in avian veterinary medicine.

Unlike lungs, which are the site of actual gas exchange, the abdominal air sac is a thin-walled membranous structure that stores and moves air but does not itself absorb oxygen or release carbon dioxide. It works in tandem with the lungs to enable the unidirectional flow system.