Air sacs are spaces within an organism where there is the constant presence of air. Among modern animals, birds possess the most air sacs (9–11), with their extinct dinosaurian relatives showing a great increase^{[clarification needed]} in the pneumatization (presence of air) in their bones. Birds use air sacs for respiration as well as a number of other things.^{[clarification needed]} Theropods, like Aerosteon, have many air sacs in the body that are not just in bones, and they can be identified as the more primitive form of modern bird airways.^{[clarification needed]} Sauropods are well known for the large number of air pockets in their bones (especially vertebra), although one theropod, Deinocheirus, shows a rivalling number of air pockets.

Birds have a system of air sacs in their ventilation system. The air sacs work to produce a unidirectional flow where air enters and exits the lung at the same rate, contrasting the lungs of other tetrapods such as mammals where air enters and exits the lung in a tidal ventilation.

Avian lungs have a bronchial system in which the air flows through dorsobronchi into the parabronchi before exiting via the ventrobronchi. Gas exchange occurs at the parabronchi.

Avian pulmonary air sacs are lined with simple epithelial and secretory cells supported by elastin connective tissues. The air sacs themselves are either poorly vascularized or entirely avascular. No gas exchange occurs within them. There are five main air sacs in birds, three of which branch from the ventrobronchi, and two of which branch from the intrapulmonary bronchus connecting the dorsobronchi and ventrobronchi. The air sacs are usually paired, except for the clavicular air sac, creating a total of 9 air sacs. However, this morphology varies among bird species. Birds such as parrots have different air sac arrangements with partial fusion of the cervical air sacs, as well as connection between the claviclar and cranial thoracic air sacs. The morphologies of the individual air sacs also vary among bird taxa.

In birds, gas exchange and volume change do not occur in the same place. While gas exchange occurs in the parabronchi in the lungs, the lungs do not change volume much during respiration. Instead, voluminous expansion occurs in the air sacs. These volume changes cause pressure gradients between air sacs, with higher gradients causing more air to flow over the parabronchi during inhalation and lower gradients causing more air to flow over the parabronchi during exhalation. Different air sacs alternate contraction and expansion, causing air motion, the fundamental mechanism of avian respiration. The compliance of the air sacs is related to the timing of all of the moving parts involved in respiration.

Birds have hollow pneumatic bones. The hollow air spaces in bird bones outside of the head are connected to the air sacs in a way that a bird with a blocked windpipe and a bone broken in a manner where the inside of the bone was connected to the outside world could still breathe. These pneumatic bones are less vascularized than non-pneumatic bones and many pneumatic bones have pneumatic foramina (openings for air passage). Skeletal pneumaticity often originates developmentally as offshoots of the air sacs, especially in the synsacrum. Bone pneumaticity is generally found in the appendicular skeleton. Some birds, such as penguins or loons, have solid bones.

In birds, some temperature control occurs in the respiratory system. Water vapor heats cool air during inhalation in the trachea, and increases its humidity. The resulting evaporative water loss varies greatly and depends on several factors including air sac pressure and the subsequent rate of air flow through the trachea.

In diving birds, the air sacs can aid in helping birds with respiration. Movement of the muscles involved in diving can cause a pressure differential between the air sacs which would cause more air to move through the parabronchi. This would then increase the uptake of oxygen stored in the respiratory system. In penguins, air sac volumes are constricted in deep dives to protect from the effects of water pressure. Penguins have been found inflating their air sacs before dives and exhale much of the air during the deepest point of their dives to change buoyancy while descending and ascending during the dive.