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Air sac

Air Sacs in Avian Respiration
The dinosaurs, birds included, have a system of air sacs in their ventilation system. The air sacs in dinosaurs 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 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 intrapulomonary bronchus connecting the dorsobronchi and ventrobronchi. However, there are birds, such as parrots, which have different air sac arrangements.

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 casing 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.

Water Loss
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.

Diving
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 to inflate 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.

Song Production
Air sacs play a role in song production in songbirds and related birds, with some studies hypothesizing that the air sac may be involved as a resonating chamber. The pressure of air in the air sac is also heavily involved in song production, as different males singing the same song have similar modulations in air sac pressure. Changes in air pressure patterns are indicative of respiratory muscle activity and the airflow around the syrinx, the primary vocalization organ of songbirds. The portion of the neural pathways which control respiration during vocalization changes air sac pressure to control vocal intensity. The pressure in the interclavicular air sac is highly correlated with the fundamental frequency of birdsong in doves. Birdsong primarily occurs in expiration and therefore syllables and fundamental frequency are highly correlated with increased interclavicular air sac pressure. Changes in air sac pressure also affect the length of the trachea which can also affect the fundamental frequency. In a species of tyrannid (the sister group to true songbirds), birds have two different sources of sound around the trachea. At high air sac pressures, the two sound sources have different frequencies, while at low pressure they have the same frequency. The generation of bird trills involves modulation of the pressure in air sacs. Since so many aspects of birdsong depend on air sac pressure, there is a trade off between trill rate and the duration of each call, though this has not been studied in depth.