Development, structure and function of a novel organ, the lung-air sac system of birds: To go where no 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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- "Infundibula from the floor of the atria open into the air capillaries , which are the gas exchanger . Atria , infundibula , and air capillaries are lined with the parabronchial epithelium that contains of granular cells , squamous atrial cells , and squamous respiratory cells ( reviewed in Maina , 2006b ) . The mor - phological similarity between the granular atrial cells in bird species with type II pneumocytes of the mam - malian lung has been reported ( Klika et al . "
ABSTRACT: The purpose of this study is to determine the possible changes in the localization of the four Epidermal Growth Factor Receptors and three ligands in quail lungs from the first day of hatching until the 125th after hatching using immunohistochemical methods. Immunohistochemical results demonstrated that four EGFRs and their ligands are chiefly located in the cytoplasm of cells. Additionally, ErbB4, AREG, and NRG1 are localized to the nucleus and nucleolus, but EGF is present in the nucleolus. ErbB2 was also found in the cell membrane. In the epithelium of secondary bronchi, the goblet cells only exhibited ErbB1 and ErbB2, whereas the basal and ciliated cells exhibited EGFRs and ligands immunoreactivity. The atrial granular cells displayed moderate levels of ErbB1-ErbB3 and EGF and strong levels of ErbB4, AREG, and NRG1 immunoreactivity. While the squamous atrial cells and squamous respiratory cells of air capillaries and endothelial cells of blood capillaries exhibited moderate to strong ErbB2, ErbB4, AREG, and NRG1 immunoreactivity, they had negative or weak ErbB1, ErbB3, and EGF immunoreactivity. The expression levels of ErbB2-ErbB4, EGF, AREG, and NRG1 were also detected in fibroblasts. Although ErbB2 was highly expressed in the bronchial and vascular smooth muscle cells, weak expression of ErbB1, ErbB3, AREG and EGF and moderate expression of ErbB4 and NRG1 were observed. Macrophages were only negative for ErbB1. In conclusion, these data indicate that the EGFR-system is functionally active at hatching, which supports the hypothesis that the members of EGFR-system play several cell-specific roles in quail lung growth after hatching. Microsc. Res. Tech., 2015. © 2015 Wiley Periodicals, Inc. © 2015 Wiley Periodicals, Inc.Microscopy Research and Technique 07/2015; DOI:10.1002/jemt.22544 · 1.17 Impact Factor
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- "Generally, the interatrial septa are inconspicuous and the atria are very shallow in small and metabolically highly active species of birds (Duncker 1974; Maina et al. 1982a) (Fig. 36) and in the ostrich (Maina and Nathaniel 2001). The atria project from the parabronchial lumen into the gas exchange tissue (Figs. 34–37) and give rise to 3 to 8 narrower passages, the infundibulae (McLelland 1989; Maina 2005). In the pigeon and mallard, the infundibulae are 25 to 40 lm wide and about 100 to 150 lm long (West et al. 1977). "
ABSTRACT: The avian respiratory apparatus is separated into a gas exchanger (the lung) and ventilators (the air sacs). Synchronized bellows-like movements of the cranial and caudal air sacs ventilate the lung continuously and unidirectionally in a caudocranial direction. With the lungs practically rigid, after their insertion into the ribs and the vertebrae and on attaching to the membranous horizontal septum, surface tension is not a constraining factor to the intensity that the gas exchange tissue can subdivide. Delicate, transparent, capacious and avascular, the air sacs are not directly involved in gas exchange. The airway system comprises of a three-tiered system of passageways, namely a primary bronchus, the secondary bronchi and the tertiary bronchi (parabronchi). The crosscurrent system is formed by the perpendicular arrangement between the mass (convective) air flow in the parabronchial lumen and the centripetal (inward) flow of the venous blood in the exchange tissue; the countercurrent system consists of the centrifugal (outward) flow of air from the parabronchial lumen into the air capillaries and the centripetal (inward) flow of blood in the blood capillaries, and; the multicapillary serial arterialization system is formed by the blood capillaries and the air capillaries where venous blood is oxygenated in succession at the infinite number of points where the respiratory units contact exchange tissue. Together with the aforementioned systems, features like large capillary blood volume, extensive respiratory surface area and thin bloodgas barrier accord high pulmonary diffusing capacity of O2 that supports the high metabolic capacities and energetic lifestyles of birds. Keywords Birds � Lung � Air sacs � Respiration �Development � Flight � OxygenJournal of Ornithology 07/2015; DOI:10.1007/s10336-015-1263-9 · 1.93 Impact Factor
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- "The air capillaries and blood capillaries form about 90% of the volume of the exchange tissue and intertwine closely. Three-dimensional computer reconstruction of serial sections (Woodward and Maina 2005) showed that the long-held notion that the air capillaries are straight "
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