Airbag (bird)
The air sacs ( Sacci pneumatici ) of birds are thin-walled appendages of the lungs , which, like bellows, guide the air through the lungs. However, no gas exchange takes place in them. They are wafer-thin bags with a transparent wall. In addition to their function as the “engine of breathing”, they are also involved in voice training. High-frequency expirations (exhalations) are modulated in the vocal head ( syrinx ) to make birds sing. The third important function of the air sacs is to participate in thermoregulation through the release of heat through evaporation .
anatomy
tissue
Most of the air sacs are lined with a monolayer of squamous epithelium. In some cases, cubic and highly prismatic ciliated epithelial cells also appear. Elastic fibers and smooth muscles are embedded in its wall . It also contains individual nerve fibers , some of which have been described as originating from the vagus nerve in some birds , but whose function is not yet known.
Classification of the air sacs
The air sacs are created very early in the embryo during development . They arise in chickens on the 5th and 6th day of the embryo, simultaneously with the development of the lungs but before the formation of the secondary bronchi. On the 10th day the air sacs are already fully developed and afterwards only show a growth in size.
Six pairs of air sacs are created in the embryo, two pairs of which merge in almost all birds (with the exception of the great crested grebe ) as they develop into a uniform collarbone air sac . Only five pairs of air sacs are created in the turkey .
1 cervical air sac , 2 clavicle air sac ,
3 front and 4 rear chest air sac,
5 abdominal air sac ( 5 ' diverticulum in the pelvic girdle ), 6 lungs , 7 trachea
bones:
A clavicle , B raven bone ,
C shoulder blade , D notarium , E synsacrum ,
F pelvis , G femur ,
H sternum
- The left and right cervical air sacs ( Saccus cervicalis ) lie along the cervical spine and have bulges (diverticula) inside and outside the cervical vertebrae . In chicken birds, both cervical air sacs merge to form a main median chamber. The great crested grebe has no air sacs.
- The collarbone air sac ( Saccus clavicularis ) originally consists of two pairs of air sacs, which in birds (with the exception of the great crested grebe) merge median into an unpaired sac. Diverticula extend from the clavicle air sac inside and outside the chest . The latter also extend into the bones of the shoulder girdle and the humerus . These bones are thus air-filled (pneumatized) in birds. The humerus is not pneumatized in the ostrich . The functional significance of this pneumatization has not yet been finally clarified.
- The anterior thoracic air sacs ( Sacci thoracici craniales ) lie between two connective tissue membranes ( horizontal septum and obliquum ) within the chest and encompass the heart and the glandular stomach ( proventriculus ). In songbirds, they merge with the uniform collarbone air sac.
- The rear thoracic air sacs ( Sacci thoracici caudales ) lie directly against the body wall and are located behind the anterior chest air sacs. Turkeys do not have any (and are not created embryonally), chickens only have small posterior air sacs. In storks, they are divided so that they have 4 posterior breast air sacs.
- The abdominal air sacs ( Sacci abdominales ) lie as thin balloons between the intestinal loops and also pneumatize the pelvic girdle . In parrots , ostriches and passerines the abdominal air sacs are relatively small.
Number of air sacs
The birds have a maximum of 12 air sacs, but due to the merging of the four collarbone air sacs, as a rule only 9. The species-specific number varies between 7 and 11.
| Number of air sacs in some birds | ||
|---|---|---|
| 11 | Storks | rear chest air sacs subdivided again (i.e. 5 pairs + collarbone air sac) |
| 10 | Great crested grebe | 4 collarbone air sacs, but no cervical air sacs (i.e. 5 pairs) |
| 9 | As a rule | 4 pairs + uniform collarbone airbag |
| 8th | Domestic chicken | Unpaired neck and collarbone air sacs, 3 paired air sacs |
| 7th | Turkey | Unpaired cervical air sacs, posterior chest air sacs missing |
| Songbirds | unpaired collarbone air sac fused with front chest air sac | |
Airbag systems
There are two functional airbag systems:
- The cranial air sac system consists of the cervical, clavicle and the two front thoracic air sacs. They are ventilated via the so-called medioventral secondary bronchi (branches of the main bronchi directed towards the middle and downwards ) and are usually only active when breathing is increased.
- The caudal air sac system consists of the posterior thoracic and abdominal air sacs. They are connected via a lateroventral secondary bronchus or directly to the main bronchus.
Respiratory mechanics in birds
principle
The breathing of birds is fundamentally different from that of mammals . In mammals, the muscles responsible for inspiration (inhalation) and expiration (exhalation) change the volume of the chest and with it the volume of the lungs , causing air to flow in and out of the lungs. In reptiles, too, muscles change the volume of the chest for breathing, which means that the volume of the air sacs changes, while the lungs of birds cannot change the volume.
The most important inspiratory muscles are the rib appendages ( muscles appendicocostales ). Their contraction causes the chest to expand. The downward movement of the breastbone also contributes significantly to this process. This leads to a negative pressure in the body cavity and thus to an expansion of the air sacs, whereby air is sucked in through the lungs. The most important expiratory muscles are the abdominal muscles , which constrict the body cavity and thus displace the air from the air sacs. During both inhalation and exhalation, the air flows from back to front through the parabronchia of the lungs.
Resting breathing
In resting breathing (Fig. A) only the rear airbag system is usually active. From the two rear pairs of air bags, air is sucked through part of the lungs (so-called neopulmo ), and some fresh air also enters these air bags. This means that at the end of inhalation there is mixed air with a still usable proportion of oxygen in the rear air sacs. During exhalation, this mixed air is now once again passed through the lungs and thus used much more effectively than in mammals with their blind airways and the correspondingly high dead space .
This efficient system also compensates for the large volume of the trachea in birds, which is longer, thicker and approximately 4.5 times the dead space volume compared to mammals.
Forced breathing
With increased breathing (Fig. B), the anterior air sac system is added to normal breathing. It draws fresh air through the rest of the lungs (called palaeopulmo ). The stale air in this airbag system goes straight into the windpipe when you exhale and is not passed through the lungs again.
Species of animal species
In the case of bird species without neopulmo (e.g. penguins ) only fresh air gets into the rear air sacs; they use the front system when inhaling and the rear system when exhaling. In storks , the neopulmo is poorly developed.
Consequences from the breathing mechanics
When holding and manipulating birds, the complex breathing mechanics must be observed. When taking it in hand, care must be taken that the movement of the chest is not too restricted. If birds are placed on their backs, the burden on the other organs hinders the development of the abdominal air sacs and the gravity of the chest muscle hinders that of the breastbone, so that shortness of breath or even suffocation can quickly occur.
Evolutionary origin of the airbag system
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In numerous representatives of the theropods , the clade of dinosaurs from which modern birds emerged, vertebrae and ribs with pneumatic foramina comparable to those of recent birds have been identified, as has been the case with the primeval bird Archeopteryx . They indicate that even in the ancestors of the birds certain bones were filled with sacs (diverticula) of an air sac system (that is, pneumatized). Among the archosaurs , whose recent representatives are the crocodiles and birds , besides the theropods, which are considered the parent group of birds, other groups (the pterosaurs and the sauropods ) had pneumatized bones, so that some palaeontologists assume that there is no air sac system only a primeval characteristic of the birds, but that of a much larger and older group, the so-called ornithodirs .
The extent to which the function of the respiratory system was similar in recent birds and non-avian theropods is controversial: O'Connor and Claessens (2005) propose a form of perfusion breathing similar to that of birds for non-avian theropods. They demonstrated pneumatic openings in the cervical, thoracic and pelvic vertebrae in a well-preserved fossil specimen of the ancient theropod genus Majungatholus .
The fact that the neural arches of the thoracic vertebrae 12 and 13 show particularly small openings compared to the in front and behind them, they interpret as a separation between an anterior (cranial) and a posterior (caudal) air sac system, to which the diverticula in the anterior and posterior vertebrae and Neural arcs are to be assigned. O'Connor and Claessens consider the presence of a rear airbag system, which is mechanically easier to "ventilate" than a front one, to be a crucial prerequisite for a bird-like flow through the lungs (see resting breathing in recent birds, Fig. A).
Theropods, including early birds, had, in contrast to recent birds capable of flying, only a small sternum that barely occupies the front chest area and a three-rayed pelvis - the pubic bone was directed towards the abdomen and did not rotate parallel to the iliac bone and ischium , as in later birds . Obviously, up and down movements of the sternum and rotations of the pelvis ("pelvic breathing") during inhalation and exhalation did not play a role at the beginning of the bird's evolution. On the other hand, theropods, including primeval Mesozoic birds such as Archeopteryx , Confuciusornis and Sinornis, had interlocked abdominal ribs ( gastralia ). According to Claessens (2004), the contraction of the abdominal muscles (especially the rectus abdominis muscle ) pushed the abdominal ribs against each other and thus widened the back of the chest laterally. Thus, the contraction of the same muscles that contribute to the decrease in chest volume on exhalation in recent birds paradoxically made it possible to inhale in early birds and non-avian theropods. According to Carrier and Farmer (2000), this breathing mechanism, which is based on the displacement of a system of abdominal ribs, is also referred to as " cuirassal breathing " and is seen as a necessary prerequisite for primordial perfusion breathing . In later birds, with the loss of the ventral ribs, this form of chest contraction and expansion was replaced by other forms.
At which phylogenetic stage a breathing mechanism similar to that of today's birds first existed is still largely open. Before it was used for breathing, the air sac system in dinosaurs and pterosaurs could have had a thermoregulatory function and contributed to weight loss. Ruben et al. a. (1997, 1999) believe that fossils of the small theropod dinosaurs Sinosauropteryx and Scipionyx contain fossilized soft tissue of the chest and abdomen, which shows that the theropod's breathing is not similar to that of birds: Allegedly, the chest is through like crocodiles a diaphragm , the piston-like movement of which controlled inhalation and exhalation. However, this theory ignores the evidence of pneumatic foramina in large numbers of theropods. And that the fossils or skin shadows in the area of the abdomen of the specimens mentioned actually allow the above statements about the location and anatomy of the internal organs, is considered dubious among paleontologists.
Clinical significance
The air sac system is often affected by diseases of the respiratory tract. The construction of the air sacs means that, unlike mammals, birds cannot cough up foreign bodies. Air sac diseases are generally associated with severe shortness of breath.
- The most common bacterial pathogens of airbag ignition (aerosacculitis) are Escherichia coli , mycoplasmas ( Mycoplasma gallisepticum u. A.) And chlamydia ( ornithosis , Chlamydophila psittaci ), rare Pasteurella ( Pasteurella gallinarum ), Ornithobacterium rhinotracheale and Bordetella ( Bordetella avium ). To detect bacterial pathogens, irrigation analogous to bronchoalveolar lavage can be carried out.
- Aspergillus fumigatus is primarily involved in air sac mycoses caused by mold . The aspergillosis is the most common air bag infection.
- Aerosacculitis can also occur with viral diseases such as avian influenza ("bird flu"), Newcastle disease (atypical avian influenza), infectious bronchitis (IB) and infectious laryngotracheitis (ILT).
- Finally, parasites such as worms ( Serratospiculum spp.) Or mites ( air sac mite Cytodites nudus ) can also attack the air sac.
The air sacs are a limiting factor for the ultrasound examination of birds, as the tissue / air interface causes a total reflection of the sound waves . A direct application of isoflurane into the air sac is described as an anesthetic option .
literature
- BB Britt, PJ Makovicky, J. Gauthier, N. Bonde: Postcranial pneumatization in Archeopteryx. In: Nature . 395/1998, pp. 374-376, ISSN 0028-0836 .
- DR Carrier, CG Farmer: The evolution of pelic aspiration in archosaurs. In: Paleobiology. 26 (2) / 2000, pp. 271-293, ISSN 0094-8373 .
- LPAM Claessens: Dinosaur gastralia: origin, morphology, and function. In: Journal of Vertebrate Paleontology. 24 (1) / 2004, pp. 89-106.
- JR Codd, DF Boggs, SF Perry, DR Carrier: Activity of three muscles associated with the uncinate processes of the giant Canada Goose Branta canadensis maximus. In: Journal of Experimental Biology. 208/2005, pp. 849-857. ( Full text )
- MR. Duncker: The lung air sac system of birds. In: Results of anatomy and history of development . 45/1971, pp. 1-171, ISSN 0301-5556 .
- PM O'Connor, LPAM Claessens: Basic avian pulmonary design and flow-through ventilation in non-avian theropod dinosaurs. In: Nature. 436/2005, pp. 253-256, ISSN 0028-0836 .
- JA Ruben, TD Jones, NR Geist, WJ Hillenius: Lung Structure and Ventilation in Theropod Dinosaurs and Early Birds. In: Science . 278/1997, pp. 1267-1270.
- JA Ruben, CD Sasso, NR Geist, WJ Hillenius, TD Jones, M. Signore: Pulmonary Function ans Metabolic Physiology of Theropod Dinosaurs. In: Science. 283/1999, pp. 514-516.
- F.-V. Salomon (ed.): Textbook of poultry anatomy . Fischer-Verlag, Jena / Stuttgart 1993, ISBN 3-334-60403-9 .
- JH Smith, JL Meier, C. Lamke, PJ Neill, ED Box: Microscopic and submicroscopic anatomy of the parabronchi, air sacs, and respiratory space of the budgerigar (Melopsittacus undulatus). In: American Journal of Anatomy. 177/1986, pp. 221-242, ISSN 0002-9106 .
- MJ Wedel: Vertebral pneumaticity, air sacs, and the physiology of sauropod dinosaurs. In: Paleobiology. 29 (2) / 2003, pp. 243-255, ISSN 0094-8373 .
Web links
- birdrespiration (English)
