Development, structure, and function of a novel respiratory organ, the lung-air sac system of birds: to go where no other vertebrate has gone.
ABSTRACT Among the air-breathing vertebrates, the avian respiratory apparatus, the lung-air sac system, is the most structurally complex and functionally efficient. After intricate morphogenesis, elaborate pulmonary vascular and airway (bronchial) architectures are formed. The crosscurrent, countercurrent, and multicapillary serial arterialization systems represent outstanding operational designs. The arrangement between the conduits of air and blood allows the respiratory media to be transported optimally in adequate measures and rates and to be exposed to each other over an extensive respiratory surface while separated by an extremely thin blood-gas barrier. As a consequence, the diffusing capacity (conductance) of the avian lung for oxygen is remarkably efficient. The foremost adaptive refinements are: (1) rigidity of the lung which allows intense subdivision of the exchange tissue (parenchyma) leading to formation of very small terminal respiratory units and consequently a vast respiratory surface; (2) a thin blood-gas barrier enabled by confinement of the pneumocytes (especially the type II cells) and the connective tissue elements to the atria and infundibulae, i.e. away from the respiratory surface of the air capillaries; (3) physical separation (uncoupling) of the lung (the gas exchanger) from the air sacs (the mechanical ventilators), permitting continuous and unidirectional ventilation of the lung. Among others, these features have created an incredibly efficient gas exchanger that supports the highly aerobic lifestyles and great metabolic capacities characteristic of birds. Interestingly, despite remarkable morphological heterogeneity in the gas exchangers of extant vertebrates at maturity, the processes involved in their formation and development are very similar. Transformation of one lung type to another is clearly conceivable, especially at lower levels of specialization. The crocodilian (reptilian) multicameral lung type represents a Bauplan from which the respiratory organs of nonavian theropod dinosaurs and the lung-air sac system of birds appear to have evolved. However, many fundamental aspects of the evolution, development, and even the structure and function of the avian respiratory system still remain uncertain.
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ABSTRACT: The Enantiornithes were the most taxonomically diverse bird group in the Mesozoic. Most of the known taxa are from Lower Cretaceous deposits of the Jehol Group in north-eastern China. A new specimen from the Jiufotang Formation in Jianchang, Liaoning Province, is described here; being a subadult individual at the time of death it had reached a relatively large size. The presence of uncinate processes, bicapitate and forked vertebral ribs, sternal ribs that were all of similar length, as well as the location of parapophyses and diapophyses on the thoracic vertebrae, may imply a rigid and volume-constant lung, and less efficient lung ventilation in enantiornithines. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113, 820–827.Biological Journal of the Linnean Society 11/2014; 113(3). DOI:10.1111/bij.12330 · 2.54 Impact Factor
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ABSTRACT: The avian respiratory system is separated into a lung (the gas exchanger) and the air sacs (the mechanical ventilators). The lung is intercalated between two functionally distinct sets of air sacs, a cranial and a caudal group. Avascular in nature, the air sacs are delicate, transparent, compliant and capacious. The lung is ventilated continuously and unidirectionally in a craniocaudal direction by synchronised bellows-like movements of the air sacs. Affixed to the ribs, the lung is virtually rigid. This has allowed for intense subdivision of the exchange tissue into very small respiratory units, the air capillaries, optimising the respiratory surface area. A thin blood–gas barrier is formed by restriction of cellular and connective tissue elements such as collagen and elastic tissue to the atria and the infundibulae. Complex vascular and bronchial systems are formed through elaborate morphogenetic processes. The airways comprise a primary bronchus, various secondary bronchi and numerous tertiary bronchi (parabronchi) that form a continuous loop. The directions of the air flow in the parabronchial lumen and that of the venous blood form a cross-current system, whereas the relationship between the air capillaries and blood capillaries is counter-current. A multicapillary serial arterialisation system is formed by the arrangement between the blood capillaries and air capillaries along the lengths of the parabronchi. A large volume of blood is exposed to air over an extensive surface area across a thin blood–gas barrier. Together with other physiological specialisations such as large tidal volume and cardiac output, these properties confer a high pulmonary diffusing capacity for oxygen. The exceptionally efficient gas exchange efficiency supports the high metabolic capacity and energetic lifestyle of birds. It should, however, be emphasised that since bats with a bronchioalveolar lung and insects with a tracheal system fly as well, if not better, than birds, the lung–air sac system is not a prerequisite for flight. It was one of the possible solutions to birds attaining life in the fast lane.Ostrich - Journal of African Ornithology 11/2009; 79(2):117-132. DOI:10.2989/OSTRICH.2008.79.2.1.575 · 0.43 Impact Factor
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ABSTRACT: Bar-headed geese cross the Himalayas on one of the most iconic high-altitude migrations in the world. Heart rates and metabolic costs of flight increase with elevation and can be near maximal during steep climbs. Their ability to sustain the high oxygen demands of flight in air that is exceedingly oxygen-thin depends on the unique cardiorespiratory physiology of birds in general along with several evolved specializations across the O2 transport cascade. ©2015 Int. Union Physiol. Sci./Am. Physiol. Soc.