Development, structure, and function of a novel respiratory organ, the lung-air sac system of birds: to go where no other vertebrate has gone

School of Anatomical Sciences, Faculty of Health Sciences, University of the Witwatersrand, 7 York Road, Parktown 2193, Johannesburg, South Africa.
Biological Reviews (Impact Factor: 9.79). 12/2006; 81(4):545-79. DOI: 10.1017/S1464793106007111
Source: PubMed

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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    • "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). "
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    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 � Oxygen
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    • "A recent review by Duncker (2004) has not helped resolve the situation since it maintains the short form of the names (dorsobronchi, ventrobronchi, etc.) and has not clarified the number of the various categories of secondary bronchi, or even their threedimensional arrangement. A comprehensive review of development, structure, and function furnished by Maina (2006) has not addressed the confusion in nomenclature of the secondary bronchi. At the parabronchial level, the air conduits form a honeycomb-like structure and are separated by the interparabronchial septa, which carry blood vessels and nerves (King and McLelland, 1984; Maina, 1982, 1988). "
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    Microscopy Research and Technique 09/2008; 71(9):689-702. DOI:10.1002/jemt.20608 · 1.17 Impact Factor
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    ABSTRACT: The possibility of dual-band signal using relativistic backward wave oscillator with a slow-wave system excited by a single beam was described. We demonstrate that a regime of generation at several close frequencies with the same transverse field structure can be rather simply realized in a sectioned system with a stepwise variation of mismatch between the beam and synchronous wave. In this case of high-power relativistic BWO with the microwave system representing sections of a corrugated waveguide, a change of synchronism conditions can provided by variation of the corrugation period.
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