An air sac is a small respiratory cavity. In birds, air sacs lead off the lungs and often enter the bones. They improve the efficiency of air ventilation in the lungs and contribute to the lightness of the bones. In many insects they occur as expansions in the tracheae (breathing tubes) and improve gas exchange by increasing the surface area available for respiration.

Many aquatic plants have developed air sacs, which enable them to float in water. Air sacs may be present in fully submerged marine plants, such as kelp, as well as floating, flowering freshwater plants. Some examples of plants with air sacs are the water primrose, giant bladder kelp, and the common bladderwort.