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.
"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 . "
"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). "
[Show abstract][Hide abstract] 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
Journal of Ornithology 07/2015; DOI:10.1007/s10336-015-1263-9 · 1.71 Impact Factor
"To gain insight into basal archosaur pulmonary anatomy, and to elucidate how and why the lungs of birds and those of the American alligator diverged, requires the careful study of a range of crocodilian and avian species. Whereas numerous studies are available for both anatomical and physiological aspects of avian lungs (Duncker, 1971; Brackenbury, 1972; Maina & Nathaniel, 2001; Maina, 2006; Farmer & Sanders, 2010), there are few studies of the crocodilian respiratory system, particularly studies that combine physiological and anatomical measurements. The clade Crocodylia is composed of at least two major lineages: Alligatoroidea, which includes the two extant alligator species and seven extant caiman species and Crocodyloidea, which includes the 13+ extant species of crocodiles. "
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The lungs of birds have long been known to move air in only one direction during both inspiration and expiration through most of the tubular gas-exchanging bronchi (parabronchi). Recently a similar pattern of airflow has been observed in American alligators, a sister taxon to birds. The pattern of flow appears to be due to the arrangement of the primary and secondary bronchi, which, via their branching angles, generate inspiratory and expiratory aerodynamic valves. Both the anatomical similarity of the avian and alligator lung and the similarity in the patterns of airflow raise the possibility that these features are plesiomorphic for Archosauria and therefore did not evolve in response to selection for flapping flight or an endothermic metabolism, as has been generally assumed. To further test the hypothesis that unidirectional airflow is ancestral for Archosauria, we measured airflow in the lungs of the Nile crocodile (Crocodylus niloticus). As in birds and alligators, air flows cranially to caudally in the cervical ventral bronchus, and caudally to cranially in the dorsobronchi in the lungs of Nile crocodiles. We also visualized the gross anatomy of the primary, secondary and tertiary pulmonary bronchi of C. niloticus using computed tomography (CT) and microCT. The cervical ventral bronchus, cranial dorsobronchi and cranial medial bronchi display similar characteristics to their proposed homologues in the alligator, while there is considerable variation in the tertiary and caudal group bronchi. Our data indicate that the aspects of the crocodilian bronchial tree that maintain the aerodynamic valves and thus generate unidirectional airflow, are ancestral for Archosauria.
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