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Air sacs are spaces within an organism where there is the constant presence of air. Among modern animals, possess the most air sacs (9–11), with their extinct relatives showing a great increase in the pneumatization (presence of air) in their bones. Birds use air sacs for respiration as well as a number of other things. 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. 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.
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 Epithelium and 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 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 Lung 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 or , have solid bones.
For a long time these cavities were regarded simply as weight-saving devices, but Bakker proposed that they were connected to air sacs like those that make ' respiratory systems the most efficient of all animals'.
John Ruben et al. (1997, 1999, 2003, 2004) disputed this and suggested that dinosaurs had a "tidal" respiratory system (in and out) powered by a crocodile-like hepatic piston mechanism – muscles attached mainly to the pubis pull the liver backwards, which makes the lungs expand to inhale; when these muscles relax, the lungs return to their previous size and shape, and the animal exhales. They also presented this as a reason for doubting that birds descended from dinosaurs.
Critics have claimed that, without avian air sacs, modest improvements in a few aspects of a modern reptile's circulatory and respiratory systems would enable the reptile to achieve 50% to 70% of the oxygen flow of a mammal of similar size, and that lack of avian air sacs would not prevent the development of endothermy. Very few formal rebuttals have been published in scientific journals of Ruben et al.s claim that dinosaurs could not have had avian-style air sacs; but one points out that the Sinosauropteryx fossil on which they based much of their argument was severely flattened and therefore it was impossible to tell whether the liver was the right shape to act as part of a hepatic piston mechanism. Some recent papers simply note without further comment that Ruben et al. argued against the presence of air sacs in dinosaurs.
In advanced sauropods ("") the vertebrae of the lower back and hip regions show signs of air sacs. In early sauropods only the cervical (neck) vertebrae show these features. If the developmental sequence found in bird embryos is a guide, air sacs actually evolved before the channels in the skeleton that accommodate them in later forms. Full text currently online at and Detailed anatomical analyses can be found at Evidence of air sacs has also been found in theropods. Studies indicate that fossils of Coeluridae, This is also one of several topics featured in a post on Naish's blog, - note Mirischia was a coelurosaur, which Naish believes was closely related to Compsognathus. Ceratosauria, and the theropods Coelophysis and Aerosteon exhibit evidence of air sacs. Coelophysis, from the late Triassic, is one of the earliest dinosaurs whose fossils show evidence of channels for air sacs. Aerosteon, a Late Cretaceous Megaraptora, had the most bird-like air sacs found so far.
Early , including the group traditionally called "prosauropods", may also have had air sacs. Although possible pneumatic indentations have been found in Plateosaurus and Thecodontosaurus, the indentations are very small. One study in 2007 concluded that prosauropods likely had abdominal and cervical air sacs, based on the evidence for them in sister taxa (theropods and sauropods). The study concluded that it was impossible to determine whether prosauropods had a bird-like flow-through lung, but that the air sacs were almost certainly present. A further indication for the presence of air sacs and their use in lung ventilation comes from a reconstruction of the air exchange volume (the volume of air exchanged with each breath) of Plateosaurus, which when expressed as a ratio of air volume per body weight at 29 ml/kg is similar to values of geese and other birds, and much higher than typical mammalian values.
So far no evidence of air sacs has been found in dinosaurs. But this does not imply that ornithischians could not have had metabolic rates comparable to those of mammals, since mammals also do not have air sacs.
Dinosaur respiratory systems with bird-like air sacs may have been capable of sustaining higher activity levels than mammals of similar size and build can sustain. In addition to providing a very efficient supply of oxygen, the rapid airflow would have been an effective cooling mechanism, which is essential for animals that are active but too large to get rid of all the excess heat through their skins.
Calculations of the volumes of various parts of the sauropod Apatosaurus respiratory system support the evidence of bird-like air sacs in sauropods:
The palaeontologist Peter Ward has argued that the evolution of the air sac system, which first appears in the very earliest dinosaurs, may have been in response to the very low (11%) atmospheric oxygen of the Carnian and Norian ages of the Triassic Period. (2025). 9780309141239, National Academies Press. ISBN 9780309141239
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