Air sac

Air sacs are spaces within an organism where there is the constant presence of air. Among modern animals,

Birds have a system of air sacs in their

Avian lungs have a bronchial system in which the air flows through dorsobronchi into the parabronchi before exiting via the ventrobronchi.[2] Gas exchange occurs at the parabronchi.^[2]

Avian pulmonary air sacs are lined with simple

In birds, gas exchange and volume change do not occur in the same place.[2] While gas exchange occurs in the parabronchi in the lungs, the lungs do not change volume much during respiration.^[9] Instead, voluminous expansion occurs in the air sacs.^[2][9] 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.^[10] Different air sacs alternate contraction and expansion, causing air motion, the fundamental mechanism of avian respiration.^[11] The compliance of the air sacs is related to the timing of all of the moving parts involved in respiration. ^[12]

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.^[9][13] These pneumatic bones are less vascularized than non-pneumatic bones and many pneumatic bones have pneumatic foramina (openings for air passage).^[9] Skeletal pneumaticity often originates developmentally as offshoots of the air sacs, especially in the synsacrum.^[9][14] Bone pneumaticity is generally found in the appendicular skeleton.^[9] Some birds, such as penguins or loons, have solid bones.^[14][15]

Other uses for air sacs

Water loss

In birds, some temperature control occurs in the respiratory system.^[16] Water vapor heats cool air during inhalation in the trachea, and increases its humidity.^[16] 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.^[16]

Diving

In diving birds, the air sacs can aid in helping birds with respiration.^[17] 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.^[17] This would then increase the uptake of oxygen stored in the respiratory system.^[17] In penguins, air sac volumes are constricted in deep dives to protect from the effects of water pressure.^[18] 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.^[18]

Song production

Air sacs play a role in

From about 1870 onwards scientists have generally agreed that the post-cranial skeletons of many dinosaurs contained many air-filled cavities (

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 birds' respiratory systems the most efficient of all animals'.^[27]

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.^{[28][29][30][31]}

Critics have claimed that, without avian air sacs, modest improvements in a few aspects of a modern reptile's

In advanced sauropods ("neosauropods") 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.^[36][37]

Evidence of air sacs has also been found in theropods. Studies indicate that fossils of coelurosaurs,^[38] ceratosaurs,^[35] 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.^[37] Aerosteon, a Late Cretaceous megaraptorid, had the most bird-like air sacs found so far.^[3]

Early

So far no evidence of air sacs has been found in ornithischian 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.^[41]

Development

Three explanations have been suggested for the development of air sacs in dinosaurs:^[3]

• Increase in respiratory capacity. This is probably the most common hypothesis, and fits well with the idea that many dinosaurs had fairly high
  metabolic rates
  .
• Improving balance and maneuvrability by lowering the
  rotational inertia
  . However this does not explain the expansion of air sacs in the quadrupedal sauropods.
• As a cooling mechanism. It seems that air sacs and feathers evolved at about the same time in
  coelurosaurs
  . If feathers retained heat, their owners would have required a means of dissipating excess heat. This idea is plausible but needs further empirical support.

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.^[41]

Calculations of the volumes of various parts of the sauropod Apatosaurus’ respiratory system support the evidence of bird-like air sacs in sauropods:

• Assuming that Apatosaurus, like dinosaurs' nearest surviving relatives crocodilians and birds, did not have a diaphragm, the dead-space volume of a 30-ton specimen would be about 184 liters. This is the total volume of the mouth, trachea and air tubes. If the animal exhales less than this, stale air is not expelled and is sucked back into the lungs on the following inhalation.
• Estimates of its tidal volume – the amount of air moved into or out of the lungs in a single breath – depend on the type of respiratory system the animal had: 904 liters if avian; 225 liters if mammalian; 19 liters if reptilian.

On this basis, Apatosaurus could not have had a reptilian respiratory system, as its tidal volume would have been less than its dead-space volume, so that stale air was not expelled but was sucked back into the lungs. Likewise, a mammalian system would only provide to the lungs about 225 − 184 = 41 liters of fresh, oxygenated air on each breath. Apatosaurus must therefore have had either a system unknown in the modern world or one like birds', with multiple air sacs and a flow-through lung. Furthermore, an avian system would only need a lung volume of about 600 liters while a mammalian one would have required about 2,950 liters, which would exceed the estimated 1,700 liters of space available in a 30-ton Apatosaurus′ chest.^[42]

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.^[43]

References