Function edit Birds’ lungs obtain fresh air during both exhalation and inhalation, because the air sacs do all the “pumping” and the lungs simply absorb oxygen.
Scientists have generally agreed that, starting around 1870, many dinosaurs’ post-cranial skeletons contained numerous air-filled cavities (postcranial skeletal pneumaticity)[citation needed], particularly in the vertebrae. Both synapsids and archosaurs had pneumatization of the skull (such as paranasal sinuses), but only birds, non-avian saurischian dinosaurs, and pterosaurs had postcranial pneumatization.
These cavities were long thought to be merely weight-saving mechanisms, but Bakker hypothesized that they were linked to air sacs, similar to the ones that give birds’ respiratory systems the highest level of efficiency of any animal. [27].
John Ruben et al. disputed this, proposing that dinosaurs had a “tidal” respiratory system (in and out), driven by a hepatic piston mechanism resembling that of a crocodile: muscles primarily attached to the pubis pull the liver backwards, expanding the lungs during inhalation; when these muscles relax, the lungs return to their original size and shape, and the animal exhales. Additionally, they used this as evidence to refute the theory that birds descended from dinosaurs. [28][29][30][31].
Opponents have asserted that, in the absence of avian air sacs, slight enhancements in a few areas of a contemporary reptile’s circulatory and respiratory systems would allow it to attain 2050700% of the oxygen flow of a mammal of comparable size [32] and that the absence of avian air sacs would not stop the emergence of endothermy [33] Very few official refutations of Ruben et al. have been published in scientific journals. asserts that dinosaurs could not have had air sacs resembling those of birds; however, one counters that much of their evidence, derived from the severely flattened Sinosauropteryx fossil, makes it impossible to determine whether the liver was the proper shape to function as a component of a hepatic piston mechanism. [34] A few recent publications just state, without further explanation, that Ruben et al. argued against the presence of air sacs in dinosaurs. [35].
How Air Sacs Power Lungs in Birds Respiratory System
AskNature Team :
Because resources are scarce, even the act of retaining them takes resources, particularly energy. Living systems have an ongoing need to strike a balance between the costs and benefits of the resources they use; failing to do so can lead to death or stop reproduction. Living systems therefore optimize, rather than maximize, resource use. Optimizing shape ultimately optimizes materials and energy. The dolphin’s body form serves as an illustration of this kind of optimization. Its ideal length to diameter ratio and flat, turbulence-reducing surface features make it streamlined to minimize drag in the water.
The most common ways that gases are released are during respiration, which releases carbon dioxide from many living things, and during photosynthesis, which produces oxygen from plants. To effectively remove gases, a different kind of force is required as pushing is ineffective. Even at the cellular level, creating that force requires energy, so living systems either need to use an external force or have energy-efficient strategies worth the investment. This usually involves techniques that increase pressure or propel gases using other forces. For instance, a human breathes in roughly 2015% of the air it has to offer. Conversely, a whale exhales 90% of the air it has consumed in a single spouting when it surfaces.
Particularly crucial gases for biological systems are nitrogen, carbon dioxide, and oxygen. Since respiration requires both oxygen and carbon dioxide, a living system’s ability to distribute these gases effectively is essential to its survival. However, gases are difficult to contain because they disperse easily. Living systems have mechanisms for containing gases and taking advantage of their characteristics in order to adapt to this. For instance, mound-building termites and prairie dogs create networks of tunnels and mounds that use wind to ventilate their underground homes.
Sometimes, for a variety of reasons, living systems store gases, like oxygen, to help maximize respiration. Gases dissipate quickly and are easy to escape, making them challenging to store. As a result, most gas storage in biological systems is only possible momentarily. Fish swim bladders, which regulate buoyancy, and bird respiratory sacs, which enable birds to get the most oxygenated air possible, are two examples.
Class Aves (bird): Eagles, hawks, sparrows, parrots
Birds are evolutionary engineering marvels. Despite their ancestry in dinosaurs, they are not at all like the large, scaly reptiles that we image. The most remarkable of their unique adaptations is their ability to fly; while some mammals are able to do so, birds are the most numerous in the skies. To help them stay airborne, many birds have hollow, light skeletons and specially made wings. Additionally, they have keratin-filled feathers that aid in warmth retention, attract potential mates, and enhance aerodynamics and navigation while in flight. They have specialized beaks and bills in place of true teeth, unlike their dinosaur ancestors.
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In terms of both structure and capacity for optimal gas exchange, the avian respiratory system differs significantly from the mammalian respiratory system.
It is made up of paired lungs with stationary surfaces for gas exchange and linked air sacs that expand and contract to move air through the stationary lungs. Before it is completely used up and exhaled out of the body, a breath of oxygen-rich inhaled air stays in the respiratory system for two full cycles of inhalation and exhalation.
When a bird inhales air for the first time through its nares (nostrils), it passes through the trachea, a large tube that emerges from the throat, and divides into the left and right primary bronchi (also known as “mesobronchi”), each of which leads to a lung. After passing through each primary bronchus, the air that has been inhaled splits, with part of it going into the lungs for gas exchange and the rest filling the posterior, or rear, air sacs. The fresh air in the posterior sacs then enters the lungs during the first exhalation and undergoes gas exchange. This incoming air displaces the spent air in the lungs, which then exits the body through the trachea. Once more, fresh air enters the lungs and posterior sacs during the second inhalation. Incoming air once more displaces spent air in the lungs, but since fresh air is flowing inward, the spent air cannot leave through the trachea. Rather, the lungs’ expended air enters the anterior, or forward, air sacs. Next, during the second exhalation, fresh air from the posterior sacs enters the lungs for gas exchange while the spent air from the lungs and anterior sacs exits through the trachea. : Biomimicry Institute / Copyright © – All rights reserved.
A bird’s trachea, bronchus, lungs, and posterior air sacs all receive fresh air (blue) when it inhales. Stale air (red) from the previous breath is displaced by fresh air entering the lungs and is moved into the anterior air sacs. Stale air from the anterior air sacs is expelled through the bronchus and trachea during exhalation, while fresh air from the posterior air sac enters the lungs.
The respiratory system’s airflow pattern produces a unidirectional, or one-way, flow of fresh air over the lungs’ gas exchange surfaces. Moreover, fresh air is constantly supplied to the gas exchange surfaces during inhalation and exhalation, allowing the bird to maintain a nearly continuous state of gas exchange in its lungs. In contrast, the lungs of mammals experience two-way, or bidirectional, airflow across their gas exchange surfaces.
The avian respiratory system’s efficiency can be attributed to both its unidirectional function and the architecture of its parabronchial system, which consists of the smaller airways within the lungs. Compared to the respiratory system of mammals, the parabronchial system’s air capillaries have a far greater total surface area. A larger percentage of oxygen from each breath can be exchanged for carbon dioxide from the blood and tissues thanks to the increased surface area.
This summary features contributions from Alex Uhrich.
Evidence edit
Scholars have put forth arguments and evidence supporting the existence of air sacs in ceratosaurs, coelurosaurs, sauropods, and theropods Aerosteon and Coelophysis.
The lower back and hip vertebrae of advanced sauropods, or “neosauropods,” exhibit evidence of air sacs. Only the cervical (neck) vertebrae in early sauropods exhibit these characteristics. If the developmental sequence observed in bird embryos serves as a model, air sacs actually evolved before the skeletal channels that eventually house them. [36][37] Comparison between the air sacs of.
Evidence of air sacs has also been found in theropods. According to studies, there is evidence of air sacs in the fossils of coelurosaurs, ceratosaurs, and theropods Coelophysis and Aerosteon. One of the first dinosaurs whose fossils display evidence of air sac channels is Coelophysis, from the late Triassic. [37] The largest known bird-like air sacs were found in Aerosteon, a Late Cretaceous megaraptorid. [3].
It’s possible that early sauropodomorphs, such as the class of animals known as “prosauropods,” also possessed air sacs. Even though Plateosaurus and Thecodontosaurus have tiny, possibly pneumatic indentations, the indentations are still present. Based on the evidence for abdominal and cervical air sacs in sister taxa (sauropods and teropods), a 2007 study came to the conclusion that prosauropods most likely had them. The study came to the conclusion that although it was impossible to tell if prosauropods had a flow-through lung similar to that of birds, the air sacs were almost definitely present. [39] Reconstructing Plateosaurus’s air exchange volumethe amount of air exchanged with each breathshows that the animal had air sacs and uses them for lung ventilation. At 29 ml/kg, this ratio of air volume to body weight is comparable to that of geese and other birds and significantly higher than that of most mammals. [40].
As of right now, there is no proof that Ornithischian dinosaurs had air sacs. But since mammals also lack air sacs, this does not rule out the possibility that ornithischians had metabolic rates similar to those of mammals. [41].
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