Inspiratory aerodynamic valving in the avian lung: functional morphology of the extrapulmonary primary bronchus

A critical assessment of the cellular defences of the avian respiratory system: are birds in general and poultry in particular relatively more susceptible to pulmonary infections/afflictions?

In commercial poultry farming, respiratory diseases cause high morbidities and mortalities, begetting colossal economic losses. Without empirical evidence, early observations led to the supposition that birds in general, and poultry in particular, have weak innate and adaptive pulmonary defences and are therefore highly susceptible to injury by pathogens. Recent findings have, however, shown that birds possess notably efficient pulmonary defences that include: (i) a structurally complex three-tiered airway arrangement with aerodynamically intricate air-flow dynamics that provide efficient filtration of inhaled air; (ii) a specialised airway mucosal lining that comprises air-filtering (ciliated) cells and various resident phagocytic cells such as surface and tissue macrophages, dendritic cells and lymphocytes; (iii) an exceptionally efficient mucociliary escalator system that efficiently removes trapped foreign agents; (iv) phagocytotic atrial and infundibular epithelial cells; (v) phagocytically competent surface macrophages that destroy pathogens and injurious particulates; (vi) pulmonary intravascular macrophages that protect the lung from the vascular side; and (vii) proficiently phagocytic pulmonary extravasated erythrocytes. Additionally, the avian respiratory system rapidly translocates phagocytic cells onto the respiratory surface, ostensibly from the subepithelial space and the circulatory system: the mobilised cells complement the surface macrophages in destroying foreign agents. Further studies are needed to determine whether the posited weak defence of the avian respiratory system is a global avian feature or is exclusive to poultry. This review argues that any inadequacies of pulmonary defences in poultry may have derived from exacting genetic manipulation(s) for traits such as rapid weight gain from efficient conversion of food into meat and eggs and the harsh environmental conditions and severe husbandry operations in modern poultry farming. To reduce pulmonary diseases and their severity, greater effort must be directed at establishment of optimal poultry housing conditions and use of more humane husbandry practices.

Perspectives on the Structure and Function of the Avian Respiratory System: Functional Efficiency Built on Structural Complexity

Among the air-breathing vertebrates, regarding respiratory efficiency, the avian respiratory system rests at the evolutionary zenith. Structurally, it is separated into a lung that serves as a gas exchanger and air sacs that mechanically ventilate the lung continuously and unidirectionally in a caudocranial direction. Largely avascular, the air sacs are delicate, transparent, compliant and capacious air-filled spaces that are not meaningfully involved in gas exchange. The avian lungs are deeply and firmly attached to the vertebrae and the ribs on the dorsolateral aspects, rendering them practically rigid and inflexible. The attachment of the lung to the body wall allowed extreme subdivision of the exchange tissue into minuscule and stable terminal respiratory units, the air capillaries. The process generated a large respiratory surface area in small lungs with low volume density of gas exchange tissue. For the respiratory structures, invariably, thin blood-gas barrier, large respiratory surface area and large pulmonary capillary blood volume are the foremost adaptive structural features that confer large total pulmonary morphometric diffusing capacities of O2. At parabronchial level, the construction and the arrangement of the airway- and the vascular components of the avian lung determine the delivery, the presentation and the exposure of inspired air to capillary blood across the blood-gas barrier. In the avian lung, crosscurrent-, countercurrent- and multicapillary serial arterialization systems that stem from the organization of the structural parts of the lung promote gas exchange. The exceptional respiratory efficiency of the avian respiratory system stems from synergy of morphological properties and physiological processes, means by which O2 uptake is optimized and high metabolic states and capacities supported. Given that among the extant animal taxa insects, birds and bats (which accomplished volancy chronologically in that order) possess structurally much different respiratory systems, the avian respiratory system was by no means a prerequisite for evolution of powered flight but was but one of the adaptive solutions to realization of an exceptionally efficient mode of locomotion.

Flow Rectification in Loopy Network Models of Bird Lungs

We demonstrate flow rectification, valveless pumping, or alternating to direct current (AC-to-DC) conversion in macroscale fluidic networks with loops. Inspired by the unique anatomy of bird lungs and the phenomenon of directed airflow throughout the respiration cycle, we hypothesize, test, and validate that multiloop networks exhibit persistent circulation or DC flows when subject to oscillatory or AC forcing at high Reynolds numbers. Experiments reveal that disproportionately stronger circulation is generated for higher frequencies and amplitudes of the imposed oscillations, and this nonlinear response is corroborated by numerical simulations. Visualizations show that flow separation and vortex shedding at network junctions serve the valving function of directing current with appropriate timing in the oscillation cycle. These findings suggest strategies for controlling inertial flows through network topology and junction connectivity.

Anatomy, ontogeny, and evolution of the archosaurian respiratory system: A case study on Alligator mississippiensis and Struthio camelus

The avian lung is highly specialized and is both functionally and morphologically distinct from that of their closest extant relatives, the crocodilians. It is highly partitioned, with a unidirectionally ventilated and immobilized gas-exchanging lung, and functionally decoupled, compliant, poorly vascularized ventilatory air-sacs. To understand the evolutionary history of the archosaurian respiratory system, it is essential to determine which anatomical characteristics are shared between birds and crocodilians and the role these shared traits play in their respective respiratory biology. To begin to address this larger question, we examined the anatomy of the lung and bronchial tree of 10 American alligators (Alligator mississippiensis) and 11 ostriches (Struthio camelus) across an ontogenetic series using traditional and micro-computed tomography (µCT), three-dimensional (3D) digital models, and morphometry. Intraspecific variation and left to right asymmetry were present in certain aspects of the bronchial tree of both taxa but was particularly evident in the cardiac (medial) region of the lungs of alligators and the caudal aspect of the bronchial tree in both species. The cross-sectional area of the primary bronchus at the level of the major secondary airways and cross-sectional area of ostia scaled either isometrically or negatively allometrically in alligators and isometrically or positively allometrically in ostriches with respect to body mass. Of 15 lung metrics, five were significantly different between the alligator and ostrich, suggesting that these aspects of the lung are more interspecifically plastic in archosaurs. One metric, the distances between the carina and each of the major secondary airways, had minimal intraspecific or ontogenetic variation in both alligators and ostriches, and thus may be a conserved trait in both taxa. In contrast to previous descriptions, the 3D digital models and CT scan data demonstrate that the pulmonary diverticula pneumatize the axial skeleton of the ostrich directly from the gas-exchanging pulmonary tissues instead of the air sacs. Global and specific comparisons between the bronchial topography of the alligator and ostrich reveal multiple possible homologies, suggesting that certain structural aspects of the bronchial tree are likely conserved across Archosauria, and may have been present in the ancestral archosaurian lung.

Robust Unidirectional Airflow through Avian Lungs: New Insights from a Piecewise Linear Mathematical Model

Avian lungs are remarkably different from mammalian lungs in that air flows unidirectionally through rigid tubes in which gas exchange occurs. Experimental observations have been able to determine the pattern of gas flow in the respiratory system, but understanding how the flow pattern is generated and determining the factors contributing to the observed dynamics remains elusive. It has been hypothesized that the unidirectional flow is due to aerodynamic valving during inspiration and expiration, resulting from the anatomical structure and the fluid dynamics involved, however, theoretical studies to back up this hypothesis are lacking. We have constructed a novel mathematical model of the airflow in the avian respiratory system that can produce unidirectional flow which is robust to changes in model parameters, breathing frequency and breathing amplitude. The model consists of two piecewise linear ordinary differential equations with lumped parameters and discontinuous, flow-dependent resistances that mimic the experimental observations. Using dynamical systems techniques and numerical analysis, we show that unidirectional flow can be produced by either effective inspiratory or effective expiratory valving, but that both inspiratory and expiratory valving are required to produce the high efficiencies of flows observed in avian lungs. We further show that the efficacy of the inspiratory and expiratory valving depends on airsac compliances and airflow resistances that may not be located in the immediate area of the valving. Our model provides additional novel insights; for example, we show that physiologically realistic resistance values lead to efficiencies that are close to maximum, and that when the relative lumped compliances of the caudal and cranial airsacs vary, it affects the timing of the airflow across the gas exchange area. These and other insights obtained by our study significantly enhance our understanding of the operation of the avian respiratory system.

Birds and mammals have similar metabolic demands and cardiovascular systems, but they have evolved drastically different respiratory systems. A key difference in birds is that gas exchange occurs in rigid tubes, through which air flows unidirectionally during both inspiration and expiration. How this unidirectional flow is generated, and the factors affecting it, are not well understood. It has been hypothesized that the unidirectional flow is due to aerodynamic valving resulting from the complex anatomical structure. To test this hypothesis we have constructed a novel mathematical model that, unlike previous models, produces unidirectional flow through the lungs consistently even when the amplitude and frequency of breathing change. We have investigated the model both analytically and computationally and shown the importance of aerodynamic valving for generating strong airflow through the lungs. Our model also predicts that the timing of airflow through the lungs depends on the relative compliances of the different airsacs that exist in birds. The lumped parameters approach we use means that this model is generally applicable across all birds.

Osteological and Soft-Tissue Evidence for Pneumatization in the Cervical Column of the Ostrich (Struthio camelus) and Observations on the Vertebral Columns of Non-Volant, Semi-Volant and Semi-Aquatic Birds

Postcranial skeletal pneumaticity (PSP) is a condition most notably found in birds, but that is also present in other saurischian dinosaurs and pterosaurs. In birds, skeletal pneumatization occurs where bones are penetrated by pneumatic diverticula, membranous extensions that originate from air sacs that serve in the ventilation of the lung. Key questions that remain to be addressed include further characterizing (1) the skeletal features that can be used to infer the presence/absence and extent of PSP in birds and non-avian dinosaurs, and (2) the association between vertebral laminae and specific components of the avian respiratory system. Previous work has used vertebral features such as pneumatic foramina, fossae, and laminae to identify/infer the presence of air sacs and diverticula, and to discuss the range of possible functions of such features. Here, we tabulate pneumatic features in the vertebral column of 11 avian taxa, including the flightless ratites and selected members of semi-volant and semi-aquatic Neornithes. We investigate the associations of these osteological features with each other and, in the case of Struthio camelus, with the specific presence of pneumatic diverticula. We find that the mere presence of vertebral laminae does not indicate the presence of skeletal pneumaticity, since laminae are not always associated with pneumatic foramina or fossae. Nevertheless, laminae are more strongly developed when adjacent to foramina or fossae. In addition, membranous air sac extensions and adjacent musculature share the same attachment points on the vertebrae, rendering the use of such features for reconstructing respiratory soft tissue features ambiguous. Finally, pneumatic diverticula attach to the margins of laminae, foramina, and/or fossae prior to their intraosseous course. Similarities in PSP distribution among the examined taxa are concordant with their phylogenetic interrelationships. The possible functions of PSP are discussed in brief, based upon variation in the extent of PSP between taxa with differing ecologies.

Similarity of Crocodilian and Avian Lungs Indicates Unidirectional Flow Is Ancestral for Archosaurs

Patterns of airflow and pulmonary anatomy were studied in the American alligator (Alligator mississippiensis), the black caiman (Melanosuchus niger), the spectacled caiman (Caiman crocodilus), the dwarf crocodile (Osteolaemus tetraspis), the saltwater crocodile (Crocodylus porosus), the Nile crocodile (Crocodylus niloticus), and Morelet's crocodile (Crocodylus moreletii). In addition, anatomy was studied in the Orinoco crocodile (Crocodylus intermedius). Airflow was measured using heated thermistor flow meters and visualized by endoscopy during insufflation of aerosolized propolene glycol and glycerol. Computed tomography and gross dissection were used to visualize the anatomy. In all species studied a bird-like pattern of unidirectional flow was present, in which air flowed caudad in the cervical ventral bronchus and its branches during both lung inflation and deflation and craniad in dorsobronchi and their branches. Tubular pathways connected the secondary bronchi to each other and allowed air to flow from the dorsobronchi into the ventrobronchi. No evidence for anatomical valves was found, suggesting that aerodynamic valves cause the unidirectional flow. In vivo data from the American alligator showed that unidirectional flow is present during periods of breath-holding (apnea) and is powered by the beating heart, suggesting that this pattern of flow harnesses the heart as a pump for air. Unidirectional flow may also facilitate washout of stale gases from the lung, reducing the cost of breathing, respiratory evaporative water loss, heat loss through the heat of vaporization, and facilitating crypsis. The similarity in structure and function of the bird lung with pulmonary anatomy of this broad range of crocodilian species indicates that a similar morphology and pattern of unidirectional flow were present in the lungs of the common ancestor of crocodilians and birds. These data suggest a paradigm shift is needed in our understanding of the evolution of this character. Although conventional wisdom is that unidirectional flow is important for the high activity and basal metabolic rates for which birds are renowned, the widespread occurrence of this pattern of flow in crocodilians indicates otherwise. Furthermore, these results show that air sacs are not requisite for unidirectional flow, and therefore raise questions about the function of avian air sacs.

The Evolution of Unidirectional Pulmonary Airflow

Conventional wisdom holds that the avian respiratory system is unique because air flows in the same direction through most of the gas-exchange tubules during both phases of ventilation. However, recent studies showing that unidirectional airflow also exists in crocodilians and lizards raise questions about the true phylogenetic distribution of unidirectional airflow, the selective drivers of the trait, the date of origin, and the functional consequences of this phenomenon. These discoveries suggest unidirectional flow was present in the common diapsid ancestor and are inconsistent with the traditional paradigm that unidirectional flow is an adaptation for supporting high rates of gas exchange. Instead, these discoveries suggest it may serve functions such as decreasing the work of breathing, decreasing evaporative respiratory water loss, reducing rates of heat loss, and facilitating crypsis. The divergence in the design of the respiratory system between unidirectionally ventilated lungs and tidally ventilated lungs, such as those found in mammals, is very old, with a minimum date for the divergence in the Permian Period. From this foundation, the avian and mammalian lineages evolved very different respiratory systems. I suggest the difference in design is due to the same selective pressure, expanded aerobic capacity, acting under different environmental conditions. High levels of atmospheric oxygen of the Permian Period relaxed selection for a thin blood-gas barrier and may have resulted in the homogeneous, broncho-alveolar design, whereas the reduced oxygen of the Mesozoic selected for a heterogeneous lung with an extremely thin blood-gas barrier. These differences in lung design may explain the puzzling pattern of ecomorphological diversification of Mesozoic mammals: all were small animals that did not occupy niches requiring a great aerobic capacity. The broncho-alveolar lung and the hypoxia of the Mesozoic may have restricted these mammals from exploiting niches of large body size, where cursorial locomotion can be advantageous, as well as other niches requiring great aerobic capacities, such as those using flapping flight. Furthermore, hypoxia may have exerted positive selection for a parasagittal posture, the diaphragm, and reduced erythrocyte size, innovations that enabled increased rates of ventilation and more rapid rates of diffusion in the lung.

Ventilation patterns of the songbird lung/air sac system during different behaviors

Unidirectional, continuous airflow through the avian lung is achieved through an elaborate air sac system with a sequential, posterior to anterior ventilation pattern. This classical model was established through various approaches spanning passively ventilated systems to mass spectrometry analysis of tracer gas flow into various air sacs during spontaneous breathing in restrained ducks. Information on flow patterns in other bird taxa is missing, and these techniques do not permit direct tests of whether the basic flow pattern can change during different behaviors. Here we use thermistors implanted into various locations of the respiratory system to detect small pulses of tracer gas (helium) to reconstruct airflow patterns in quietly breathing and behaving (calling, wing flapping) songbirds (zebra finch and yellow-headed blackbird). The results illustrate that the basic pattern of airflow in these two species is largely consistent with the model. However, two notable differences emerged. First, some tracer gas arrived in the anterior set of air sacs during the inspiration during which it was inhaled, suggesting a more rapid throughput through the lung than previously assumed. Second, differences in ventilation between the two anterior air sacs emerged during calling and wing flapping, indicating that adjustments in the flow pattern occur during dynamic behaviors. It is unclear whether this modulation in ventilation pattern is passive or active. This technique for studying ventilation patterns during dynamic behaviors proves useful for establishing detailed timing of airflow and modulation of ventilation in the avian respiratory system.

The skeletal kinematics of lung ventilation in three basal bird taxa (emu, tinamou, and guinea fowl)

In vivo visceral and skeletal kinematics of lung ventilation was examined using cineradiography in two palaeognaths, the emu (Dromaius novaehollandiae) and the Chilean tinamou (Nothoprocta perdicaria), and a basal neognath, the helmeted guinea fowl (Numida meleagris). Upon inspiration, the thorax expands in all dimensions. The vertebral ribs swing forward and upward, thereby increasing the transverse diameter of the trunk. The consistent location of the parapophysis throughout the dorsal vertebral series, ventral and cranial to the diapophysis, ensures a relatively uniform lateral expansion. An increase in the angle between the vertebral and the sternal ribs causes the sternal ribs to push the sternum ventrally. Owing to the greater length of the caudal sternal ribs, the caudal sternal margin is displaced further ventrally than the cranial sternal margin. When observed in lateral view, sternal movement is not linear, but elliptical. The avian thorax is highly constrained in its movement when compared with crocodylians, the other extant archosaur clade. Birds lack a lumbar region and intermediate ribs. Sternal ribs are completely ossified, and have a bicondylar articulation with the sternum. Considering the importance of pressure differences between cranial and caudal air sac complexes for the generation of unidirectional air flow in the avian lung, it is hypothesized that a decrease in the degrees of freedom of movement of the avian trunk skeleton, greater expansion of the ventrocaudal trunk region, and elliptical sternal movement may represent specific adaptations for fine-tuned control over air flow within the complex avian pulmonary system.

Singing with reduced air sac volume causes uniform decrease in airflow and sound amplitude in the zebra finch

Song of the zebra finch (Taeniopygia guttata) is a complex temporal sequence generated by a drastic change to the regular oscillations of the normal respiratory pattern. It is not known how respiratory functions, such as supply of air volume and gas exchange, are controlled during song. To understand the integration between respiration and song, we manipulated respiration during song by injecting inert dental medium into the air sacs. Increased respiratory rate after injections indicates that the reduction of air affected quiet respiration and that birds compensated for the reduced air volume. During song, air sac pressure, tracheal airflow and sound amplitude decreased substantially with each injection. This decrease was consistently present during each expiratory pulse of the song motif irrespective of the air volume used. Few changes to the temporal pattern of song were noted, such as the increased duration of a minibreath in one bird and the decrease in duration of a long syllable in another bird. Despite the drastic reduction in air sac pressure, airflow and sound amplitude, no increase in abdominal muscle activity was seen. This suggests that during song, birds do not compensate for the reduced physiological or acoustic parameters. Neither somatosensory nor auditory feedback mechanisms appear to effect a correction in expiratory effort to compensate for reduced air sac pressure and sound amplitude.

The avian lung-associated immune system: a review

The lung is a major target organ for numerous viral and bacterial diseases of poultry. To control this constant threat birds have developed a highly organized lung-associated immune system. In this review the basic features of this system are described and their functional properties discussed. Most prominent in the avian lung is the bronchus-associated lymphoid tissue (BALT) which is located at the junctions between the primary bronchus and the caudal secondary bronchi. BALT nodules are absent in newly hatched birds, but gradually developed into the mature structures found from 6-8 weeks onwards. They are organized into distinct B and T cell areas, frequently comprise germinal centres and are covered by a characteristic follicle-associated epithelium. The interstitial tissue of the parabronchial walls harbours large numbers of tissue macrophages and lymphocytes which are scattered throughout tissue. A striking feature of the avian lung is the low number of macrophages on the respiratory surface under non-inflammatory conditions. Stimulation of the lung by live bacteria but not by a variety of bacterial products elicits a significant efflux of activated macrophages and, depending on the pathogen, of heterophils. In addition to the cellular components humoral defence mechanisms are found on the lung surface including secretory IgA. The compartmentalisation of the immune system in the avian lung into BALT and non BALT-regions should be taken into account in studies on the host-pathogen interaction since these structures may have distinct functional properties during an immune response.

A systematic study of the development of the airway (bronchial) system of the avian lung from days 3 to 26 of embryogenesis: a transmission electron microscopic study on the domestic fowl, Gallus gallus variant domesticus

In the embryo of the domestic fowl, Gallus gallus variant domesticus, the lung buds become evident on day 3 of development. After fusing on the ventral midline, the single entity divides into left and right primordial lungs that elongate caudally while diverging and shifting towards the dorsolateral aspects of the coelomic cavity. On reaching their definitive topographical locations, the lungs rotate along a longitudinal axis, attach, and begin to slide into the ribs. First appearing as a solid cord of epithelial cells that runs in the proximal-distal axis of the developing lung, progressively, the intrapulmonary primary bronchus begins to canalize. In quick succession, secondary bronchi sprout from it in a craniocaudal sequence and radiate outwards. On reaching the periphery of the lung, parabronchi (tertiary bronchi) bud from the secondary bronchi and project into the surrounding mesenchymal cell mass. The parabronchi canalize, lengthen, increase in diameter, anastomose, and ultimately connect the secondary bronchi. The luminal aspect of the formative parabronchi is initially lined by a composite epithelium of which the peripheral cells attach onto the basement membrane while the apical ones project prominently into the lumen. The epithelium transforms to a simple columnar type in which the cells connect through arm-like extensions and prominently large intercellular spaces form. The atria are conspicuous on day 15, the infundibulae on day 16, and air capillaries on day 18. At hatching (day 21), the air and blood capillaries have anastomosed profusely and the blood-gas barrier become remarkably thin. The lung is well developed and potentially functionally competent at the end of the embryonic life. Thereafter, at least upto day 26, no further consequential structures form. The mechanisms by which the airways in the avian lung develop fundamentally differ from those that occur in the mammalian one. Compared with the blind-ended bronchial system that inaugurates in the mammalian lung, an elaborate, continuous system of air conduits develops in the avian one. Further studies are necessary to underpin the specific molecular factors and genetic processes that direct the morphogenesis of an exceptionally complex and efficient respiratory organ.

Respiratory and reproductive paleophysiology of dinosaurs and early birds

In terms of their diversity and longevity, dinosaurs and birds were/are surely among the most successful of terrestrial vertebrates. Unfortunately, interpreting many aspects of the biology of dinosaurs and the earliest of the birds presents formidable challenges because they are known only from fossils. Nevertheless, a variety of attributes of these taxa can be inferred by identification of shared anatomical structures whose presence is causally linked to specialized functions in living reptiles, birds, and mammals. Studies such as these demonstrate that although dinosaurs and early birds were likely to have been homeothermic, the absence of nasal respiratory turbinates in these animals indicates that they were likely to have maintained reptile-like (ectothermic) metabolic rates during periods of rest or routine activity. Nevertheless, given the metabolic capacities of some extant reptiles during periods of elevated activity, early birds were probably capable of powered flight. Similarly, had, for example, theropod dinosaurs possessed aerobic metabolic capacities and habits equivalent to those of some large, modern tropical latitude lizards (e.g., Varanus), they may well have maintained significant home ranges and actively pursued and killed large prey. Additionally, this scenario of active, although ectothermic, theropod dinosaurs seems reinforced by the likely utilization of crocodilian-like, diaphragm breathing in this group. Finally, persistent in vivo burial of their nests and apparent lack of egg turning suggests that clutch incubation by dinosaurs was more reptile- than birdlike. Contrary to previous suggestions, there is little if any reliable evidence that some dinosaur young may have been helpless and nestbound (altricial) at hatching.

Some recent advances on the study and understanding of the functional design of the avian lung: morphological and morphometric perspectives

The small highly aerobic avian species have morphometrically superior lungs while the large flightless ones have less well-refined lungs. Two parabronchial systems, i.e. the paleopulmo and neopulmo, occur in the lungs of relatively advanced birds. Although their evolution and development are not clear, understanding their presence is physiologically important particularly since the air- and blood flow patterns in them are different. Geometrically, the bulk air flow in the parabronchial lumen, i.e. in the longitudinal direction, and the flow of deoxygenated blood from the periphery, i.e. in a centripetal direction, are perpendicularly arranged to produce a cross-current relationship. Functionally, the blood capillaries in the avian lung constitute a multicapillary serial arterialization system. The amount of oxygen and carbon dioxide exchanged arises from many modest transactions that occur where air- and blood capillaries interface along the parabronchial lengths, an additive process that greatly enhances the respiratory efficiency. In some species of birds, an epithelial tumescence occurs at the terminal part of the extrapulmonary primary bronchi (EPPB). The swelling narrows the EPPB, conceivably allowing the shunting of inspired air across the openings of the medioventral secondary bronchi, i.e. inspiratory aerodynamic valving. The defence stratagems in the avian lung differ from those of mammals: fewer surface (free) macrophages (SMs) occur, the epithelial cells that line the atria and infundibula are phagocytic, a large population of subepithelial macrophages is present and pulmonary intravascular macrophages exist. This complex defence inventory may explain the paucity of SMs in the avian lung.

A qualitative and quantitative study of the lung of an ostrich,Struthio camelus

The ostrich lung, with its lack of interparabronchial septa, the presence of very shallow atria and exceptional morphometric refinement, structurally resembles those of small, energetic flying birds, whereas it also displays features characteristic of the flightless ratites in which the neopulmo is relatively poorly developed and a segmentum accelerans may be generally lacking. The large size of the bronchial system of the ostrich may help explain the unique shifts in the airflow pathways that must occur from resting to panting breathing, explaining its insensitivity to acid–base imbalance of the blood during sustained panting under thermal stress. The mass-specific volume of the lung is 39.1 cm3kg−1 and the volume density of the exchange tissue is remarkably high (78.31%). The blood–gas (tissue) barrier is relatively thick (0.56μm) but the plasma layer is very thin (0.14μm). In this flightless ratite bird, the mass-specific surface area of the tissue barrier (30.1 cm2g−1), the mass-specific anatomical diffusing capacity of the tissue barrier for oxygen (0.0022mlO2s−1Pa−1kg−1), the mass-specific volume of pulmonary capillary blood (6.25 cm3kg−1) and the mass-specific total anatomical diffusing capacity for oxygen (0.00073mlO2s−1Pa−1kg−1) are equivalent to or exceed those of much smaller highly aerobic volant birds. The distinctive morphological and morphometric features that seem to occur in the ostrich lung may explain how it achieves and maintains high aerobic capacities and endures long thermal panting without experiencing respiratory alkalosis.