Recent advances into understanding some aspects of the structure and function of mammalian and avian lungs

Recent findings are reported about certain aspects of the structure and function of the mammalian and avian lungs that include (a) the architecture of the air capillaries (ACs) and the blood capillaries (BCs); (b) the pulmonary blood capillary circulatory dynamics; (c) the adaptive molecular, cellular, biochemical, compositional, and developmental characteristics of the surfactant system; (d) the mechanisms of the translocation of fine and ultrafine particles across the airway epithelial barrier; and (e) the particle-cell interactions in the pulmonary airways. In the lung of the Muscovy duck Cairina moschata, at least, the ACs are rotund structures that are interconnected by narrow cylindrical sections, while the BCs comprise segments that are almost as long as they are wide. In contrast to the mammalian pulmonary BCs, which are highly compliant, those of birds practically behave like rigid tubes. Diving pressure has been a very powerful directional selection force that has influenced phenotypic changes in surfactant composition and function in lungs of marine mammals. After nanosized particulates are deposited on the respiratory tract of healthy human subjects, some reach organs such as the brain with potentially serious health implications. Finally, in the mammalian lung, dendritic cells of the pulmonary airways are powerful agents in engulfing deposited particles, and in birds, macrophages and erythrocytes are ardent phagocytizing cellular agents. The morphology of the lung that allows it to perform different functions-including gas exchange, ventilation of the lung by being compliant, defense, and secretion of important pharmacological factors-is reflected in its "compromise design."

Implicit mechanistic role of the collagen, smooth muscle, and elastic tissue components in strengthening the air and blood capillaries of the avian lung

To identify the forces that may exist in the parabronchus of the avian lung and that which may explain the reported strengths of the terminal respiratory units, the air capillaries and the blood capillaries, the arrangement of the parabronchial collagen fibers (CF) of the lung of the domestic fowl, Gallus gallus variant domesticus was investigated by discriminatory staining, selective alkali digestion, and vascular casting followed by alkali digestion. On the luminal circumference, the atrial and the infundibular CF are directly connected to the smooth muscle fibers and the elastic tissue fibers. The CF in this part of the parabronchus form the internal column (the axial scaffold), whereas the CF in the interparabronchial septa and those associated with the walls of the interparabronchial blood vessels form the external, i.e. the peripheral, parabronchial CF scaffold. Thin CF penetrate the exchange tissue directly from the interparabronchial septa and indirectly by accompanying the intraparabronchial blood vessels. Forming a dense network that supports the air and blood capillaries, the CF weave through the exchange tissue. The exchange tissue, specifically the air and blood capillaries, is effectively suspended between CF pillars by an intricate system of thin CF, elastic and smooth muscle fibers. The CF course through the basement membranes of the walls of the blood and air capillaries. Based on the architecture of the smooth muscle fibers, the CF, the elastic muscle fibers, and structures like the interparabronchial septa and their associated blood vessels, it is envisaged that dynamic tensional, resistive, and compressive forces exist in the parabronchus, forming a tensegrity (tension integrity) system that gives the lung rigidity while strengthening the air and blood capillaries.

Comparative in vitro study of interactions between particles and respiratory surface macrophages, erythrocytes, and epithelial cells of the chicken and the rat

In mammals, surface macrophages (SMs) play a foremost role in protecting the respiratory system by engulfing and destroying inhaled pathogens and harmful particulates. However, in birds, the direct defense role(s) that SMs perform remains ambiguous. Paucity and even lack of SMs have been reported in the avian respiratory system. It has been speculated that the pulmonary defenses in birds are inadequate and that birds are exceptionally susceptible to pulmonary diseases. In an endeavour to resolve the existing controversy, the phagocytic capacities of the respiratory SMs of the domestic fowl and the rat were compared under similar experimental conditions by exposure to polystyrene particles. In cells of equivalent diameters (8.5 microm in the chicken and 9.0 microm in the rat) and hence volumes, with the volume density of the engulfed polystyrene particles, i.e. the volume of the particles per unit volume of the cell (SM) of 23% in the chicken and 5% in the rat cells, the avian cells engulfed substantially more particles. Furthermore, the avian SMs phagocytized the particles more efficiently, i.e. at a faster rate. The chicken erythrocytes and the epithelial cells of the airways showed noteworthy phagocytic activity. In contrast to the rat cells that did not, 22% of the chicken erythrocytes phagocytized one to six particles. In birds, the phagocytic efficiencies of the SMs, erythrocytes, and epithelial cells may consolidate pulmonary defense. The assorted cellular defenses may explain how and why scarcity of SMs may not directly lead to a weak pulmonary defense. The perceived susceptibility of birds to respiratory diseases may stem from the human interventions that have included extreme genetic manipulation and intensive management for maximum productivity. The stress involved and the structural-functional disequilibria that have occurred from a 'directed evolutionary process', rather than weak immunological and cellular immunity, may explain the alleged vulnerability of the avian gas exchanger to diseases.

The anatomy, physics, and physiology of gas exchange surfaces: is there a universal function for pulmonary surfactant in animal respiratory structures?

(Orgeig and Daniels) This surfactant symposium reflects the integrative and multidisciplinary aims of the 1st ICRB, by encompassing in vitro and in vivo research, studies of vertebrates and invertebrates, and research across multiple disciplines. We explore the physical and structural challenges that face gas exchange surfaces in vertebrates and insects, by focusing on the role of the surfactant system. Pulmonary surfactant is a complex mixture of lipids and proteins that lines the air-liquid interface of the lungs of all air-breathing vertebrates, where it functions to vary surface tension with changing lung volume. We begin with a discussion of the extraordinary conservation of the blood-gas barrier among vertebrate respiratory organs, which has evolved to be extremely thin, thereby maximizing gas exchange, but simultaneously strong enough to withstand significant distension forces. The principal components of pulmonary surfactant are highly conserved, with a mixed phospholipid and neutral lipid interfacial film that is established, maintained and dynamically regulated by surfactant proteins (SP). A wide variation in the concentrations of individual components exists, however, and highlights lipidomic as well as proteomic adaptations to different physiological needs. As SP-B deficiency in mammals is lethal, oxidative stress to SP-B is detrimental to the biophysical function of pulmonary surfactant and SP-B is evolutionarily conserved across the vertebrates. It is likely that SP-B was essential for the evolutionary origin of pulmonary surfactant. We discuss three specific issues of the surfactant system to illustrate the diversity of function in animal respiratory structures. (1) Temperature: In vitro analyses of the behavior of different model surfactant films under dynamic conditions of surface tension and temperature suggest that, contrary to previous beliefs, the alveolar film may not have to be substantially enriched in the disaturated phospholipid, dipalmitoylphosphatidylcholine (DPPC), but that similar properties of rate of film formation can be achieved with more fluid films. Using an in vivo model of temperature change, a mammal that enters torpor, we show that film structure and function varies between surfactants isolated from torpid and active animals. (2) Spheres versus tubes: Surfactant is essential for lung stabilization in vertebrates, but its function is not restricted to the spherical alveolus. Instead, surfactant is also important in narrow tubular respiratory structures such as the terminal airways of mammals and the air capillaries of birds. (3). Insect tracheoles: We investigate the structure and function of the insect tracheal system and ask whether pulmonary surfactant also has a role in stabilizing these minute tubules. Our theoretical analysis suggests that a surfactant system may be required, in order to cope with surface tension during processes, such as molting, when the tracheae collapse and fill with water. Hence, despite observations by Wigglesworth in the 1930s of fluid-filled tracheoles, the challenge persists into the 21st century to determine whether this fluid is associated with a pulmonary-type surfactant system. Finally, we summarize the current status of the field and provide ideas for future research.

The morphology of the pecten oculi of the ostrich, Struthio camelus

The pecten oculi is a structure peculiar to the avian eye. Three morphological types of pecten oculi are recognized: conical type, vaned type and pleated type. The pleated type has been well studied. However, there exists only scanty data on the morphology of the latter two types of pectens. The structure of the vaned type of pecten of the ostrich, Struthio camelus was investigated with light and electron microscope. The pecten of this species consists of a vertical primary lamella that arises from the optic disc and supports 16-19 laterally located secondary lamellae, which run from the base and confluence at the apex. Some of the secondary lamellae give rise to 2 or 3 tertiary lamellae. The lamellae provide a wide surface, which supports 2-3 Layers of blood capillaries. Pigmentation is highest at the distal ends of the secondary and tertiary Lamella where blood capillaries are concentrated and very scanty on the primary and the proximal ends of the secondary lamella where the presence of capillaries is much reduced. In contrast to the capillaries of the pleated pecten, the endothelium of the capillaries in the pecten of the ostrich exhibits very few microvilli. These observations suggest that the morphology of the pecten of the ostrich, a flightless ratite bird is unique to the pleated pecten and is designed to meet the balance between optimal vision and large surface area for blood supply and yet ensuring it is kept firmly erect within the vitreous.

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

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.

Spectacularly robust! Tensegrity principle explains the mechanical strength of the avian lung

Among the air-breathing vertebrates, the respiratory system of birds, the lung-air sac system, is remarkably complex and singularly efficient. The most perplexing structural property of the avian lung pertains to its exceptional mechanical strength, especially that of the minuscule terminal respiratory units, the air- and the blood capillaries. In different species of birds, the air capillaries range in diameter from 3 to 20 micro m: the blood capillaries are in all cases relatively smaller. Over and above their capacity to withstand enormous surface tension forces at the air-tissue interface, the air capillaries resist mechanical compression (parabronchial distending pressure) as high as 20 cm H(2)O (2 kPa). The blood capillaries tolerate a pulmonary arterial vascular pressure of 24.1 mmHg (3.2 kPa) and vascular resistance of 22.5 mmHg (3 kPa) without distending. The design of the avian respiratory system fundamentally stems from the rigidity (strength) of the lung. The gas exchanger (the lung) is uncoupled from the ventilator (the air sacs), allowing the lung (the paleopulmonic parabronchi) to be ventilated continuously and unidirectionally by synchronized bellows like action of the air sacs. Since during the ventilation of the lung the air capillaries do not have to be distended (dilated), i.e., surface tension force does not have to be overcome (as would be the case if the lung was compliant), extremely intense subdivision of the exchange tissue was possible. Minuscule terminal respiratory units developed, producing a vast respiratory surface area in a limited lung volume. I make a case that a firm (rigid) rib cage, a lung tightly held by the ribs and the horizontal septum, a lung directly attached to the trunk, specially formed and compactly arranged parabronchi, intertwined atrial muscles, and tightly set air capillaries and blood capillaries form an integrated hierarchy of discrete network system of tension and compression, a tensegrity (tensional integrity) array, which absorbs, transmits, and dissipates stress, stabilizing (strengthening) the lung and its various structural components.

Thin and strong! The bioengineering dilemma in the structural and functional design of the blood-gas barrier

In gas exchangers, the tissue barrier, the partition that separates the respiratory media (water/air and hemolymph/blood), is exceptional for its remarkable thinness, striking strength, and vast surface area. These properties formed to meet conflicting roles: thinness was essential for efficient flux of oxygen by passive diffusion, and strength was crucial for maintaining structural integrity. What we have designated as "three-ply" or "laminated tripartite" architecture of the barrier appeared very early in the evolution of the vertebrate gas exchanger. The design is conspicuous in the water-blood barrier of the fish gills through the lungs of air-breathing vertebrates, where the plan first appeared in lungfishes (Dipnoi) some 400 million years ago. The similarity of the structural design of the barrier in respiratory organs of animals that remarkably differ phylogenetically, behaviorally, and ecologically shows that the construction has been highly conserved both vertically and horizontally, i.e., along and across the evolutionary continuum. It is conceivable that the blueprint may have been the only practical construction that could simultaneously grant satisfactory strength and promote gas exchange. In view of the very narrow allometric range of the thickness of the blood-gas barrier in the lungs of different-sized vertebrate groups, the measurement has seemingly been optimized. There is convincing, though indirect, evidence that the extracellular matrix and particularly the type IV collagen in the lamina densa of the basement membrane is the main stress-bearing component of the blood-gas barrier. Under extreme conditions of operation and in some disease states, the barrier fails with serious consequences. The lamina densa which in many parts of the blood-gas barrier is <50 nm thin is a lifeline in the true sense of the word.

A 3D digital reconstruction of the components of the gas exchange tissue of the lung of the muscovy duck, Cairina moschata

To elucidate the shape, size, and spatial arrangement and association of the terminal respiratory units of the avian lung, a three-dimensional (3D) computer-aided voxel reconstruction was generated from serial plastic sections of the lung of the adult muscovy duck, Cairina moschata. The air capillaries (ACs) are rather rotund structures that interconnect via short, narrow passageways, and the blood capillaries (BCs) comprise proliferative segments of rather uniform dimensions. The ACs and BCs anastomose profusely and closely intertwine with each other, forming a complex network. The two sets of respiratory units are, however, absolutely not mirror images of each other, as has been claimed by some investigators. Historically, the terms 'air capillaries' and 'blood capillaries' were derived from observations that the exchange tissue of the avian lung mainly consisted of a network of minuscule air- and vascular units. The entrenched notion that the ACs are straight (non-branching), blind-ending tubules that project outwards from the parabronchial lumen and that the BCs are direct tubules that run inwards parallel to and in contact with the ACs is overly simplistic, misleading and incorrect. The exact architectural properties of the respiratory units of the avian lung need to be documented and applied in formulating reliable physiological models. A few ostensibly isolated ACs were identified. The mechanism through which such units form and their functional significance, if any, are currently unclear.

Systematic analysis of hematopoietic, vasculogenetic, and angiogenetic phases in the developing embryonic avian lung, Gallus gallus variant domesticus

In the embryonic lung of the domestic fowl, Gallus gallus variant domesticus, hematogenetic and vasculogenetic cells become ultrastructurally clear from day 4 of development. In the former group of cells, filopodial extensions coalesce, cytoplasm thickens, and accumulating hemoglobin displaces the nucleus peripherally while in the latter, conspicuous filopodial extensions and large nuclei develop as the cells assume a rather stellate appearance. From day 5, erythrocytes and granular leukocytes begin forming from cytoarchitecturally cognate hematogenetic cells. The cells become distinguishable when hemoglobin starts to accumulate in the erythroblasts and electron dense bodies form in the leukoblasts. Vasculogenesis begins from day 7 in different areas of the developing lung: erthrocytes (but not granular leukocytes) appear to attract committed vasculogenetic cells (angioblasts) that form an endothelial lining and vessel wall. Arrangement of angioblasts around forming blood vessels sets the direction along which the vessels sprout (angiogenesis). In some areas of the developing lung, through what seems like an inductive erythropoietic process, arcades of erythrocytes organize. Once endothelial cells surround such continuities, discrete vascular units organize. By day 10, the major parts of the in-built (intrinsic) pulmonary vasculature are assembled. Complete pulmonary circulation (i.e., through the exchange tissue) is not established until after day 18 when the blood capillaries start to develop. Since the precursory erythrocytes do not have a respiratory role, it is imperative that de novo erythropoiesis is essential for vasculogenesis. Diffuse (fragmentary) development and subsequent piecemeal assembly of the pulmonary vascular system may explicate the fabrication of a complex circulatory architecture that grants cross-current, counter-current, and multicapillary serial arterialization designs in the exchange tissue of the avian lung. The exceptional respiratory efficiency of the avian lung is largely attributable to the geometries (physical interfacing) between the bronchial and vascular elements at different levels of morphological organization.

Morphogenesis of the laminated, tripartite cytoarchitectural design of the blood–gas barrier of the avian lung: a systematic electron microscopic study on the domestic fowl, Gallus gallus variant domesticus

Formation of a thin blood-gas barrier in the respiratory (gas exchange) tissue of the lung of the domestic fowl, Gallus gallus variant domesticus commences on day 18 of embryogenesis. Developing from infundibulae, air capillaries radiate outwards into the surrounding mesenchymal (periparabronchial) tissue, progressively separating and interdigitating with the blood capillaries. Thinning of the blood-gas barrier occurs by growth and extension of the air capillaries and by extensive disintegration of mesenchymal cells that constitute transient septa that divide the lengthening and anastomosing air capillaries. After they contact, the epithelial and endothelial cells deposit intercellular matrix that cements them back-to-back. At hatching (day 21), with a thin blood-gas barrier and a large respiratory surface area, the lung is well prepared for gas exchange. In sites where air capillaries lie adjacent to each other, epithelial cells contact directly: intercellular matrix is lacking.

Expression of fibroblast growth factor-2 (FGF-2) in early stages (days 3-11) of the development of the avian lung, Gallus gallus variant domesticus: an immunocytochemical study

In the avian lung, the bronchial system forms from epithelial (endodermal) cells. The intrapulmonary primary bronchus is the focal point of airway development. It originates secondary bronchi (SB) along its proximal-distal extent and parabronchi (tertiary bronchi) arise from and connect the SB. From as early as day 3.5, fibroblast growth factor-2 (FGF-2) is diffusely expressed in the epithelial and mesenchymal cells. Up-regulation of FGF-2 in discrete areas of the developing lung seem to set the growth rate, trajectories followed, areas appropriated, three-dimensional symmetry and coupling of the airways. Expressed early in development and persisting into the incubation period, FGF-2 may be involved in the formation of the avian lung. Morphogenetic differences between the avian and the mammalian lungs may explain the existing structural contrarieties.

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.

Developmental dynamics of the bronchial (airway) and air sac systems of the avian respiratory system from day 3 to day 26 of life: a scanning electron microscopic study of the domestic fowl, Gallus gallus variant domesticus

The lung buds were first conspicuous on day 3 of embryogenesis. They fused on day 4 and the common growth divided into left and right primordial lungs on day 5. Progressively, the lungs elongated, diverged, and advanced towards the respective dorsolateral aspects of the body wall, reaching their definitive topographical locations in the coelomic cavity on day 6. On day 7, they rotated, attached onto the ribs, gradually started to slide into them, and were deeply inserted by day 8. The primary bronchus (PB) first appeared as a solid cord of epithelial cells (day 4) that successively canalized as it invaded the surrounding mesenchyme, extending along the proximal-distal axis of the lung. From day 8, the secondary bronchi (SB) begun to sprout from the PB in a craniocaudal sequence. On day 9, the parabronchi (PR) started to bud from the SB, projecting into the adjacent mesenchyme. They commenced to canalize on day 10 and greatly increased in length, number, and diameter. By day 13, the PR had anastomosed profusely and totally masked the SB. The luminal surface of the PR was lined by a columnar epithelium from which the atria (day 15), infundibulae (day 16), and air capillaries (ACs) (day 18) developed. At hatching (day 21), the ACs were well developed and had anastomosed profusely with the blood capillaries. Of the air sacs (ASs), the abdominal ones appeared earliest (day 5) followed by the cervical ones on day 6. In quick succession, the other ASs were well formed by day 10. After hatching, no further consequential structures formed: only shifts in topographical locations and an increase in size and number occurred. Morphogenetically, the avian respiratory system differs from the mammalian one in certain key aspects: besides the ASs that are unique to it, the lung is exceptionally complex in structure and is essentially mature at the end of the embryonic life.

Structure, function and evolution of the gas exchangers: comparative perspectives

Over the evolutionary continuum, animals have faced similar fundamental challenges of acquiring molecular oxygen for aerobic metabolism. Under limitations and constraints imposed by factors such as phylogeny, behaviour, body size and environment, they have responded differently in founding optimal respiratory structures. A quintessence of the aphorism that 'necessity is the mother of invention', gas exchangers have been inaugurated through stiff cost-benefit analyses that have evoked transaction of trade-offs and compromises. Cogent structural-functional correlations occur in constructions of gas exchangers: within and between taxa, morphological complexity and respiratory efficiency increase with metabolic capacities and oxygen needs. Highly active, small endotherms have relatively better-refined gas exchangers compared with large, inactive ectotherms. Respiratory structures have developed from the plain cell membrane of the primeval prokaryotic unicells to complex multifunctional ones of the modern Metazoa. Regarding the respiratory medium used to extract oxygen from, animal life has had only two choices--water or air--within the biological range of temperature and pressure the only naturally occurring respirable fluids. In rarer cases, certain animals have adapted to using both media. Gills (evaginated gas exchangers) are the primordial respiratory organs: they are the archetypal water breathing organs. Lungs (invaginated gas exchangers) are the model air breathing organs. Bimodal (transitional) breathers occupy the water-air interface. Presentation and exposure of external (water/air) and internal (haemolymph/blood) respiratory media, features determined by geometric arrangement of the conduits, are important features for gas exchange efficiency: counter-current, cross-current, uniform pool and infinite pool designs have variably developed.

Composite cellular defence stratagem in the avian respiratory system: functional morphology of the free (surface) macrophages and specialized pulmonary epithelia

Qualitative and quantitative attributes of the free respiratory macrophages (FRMs) of the lung--air sac systems of the domestic fowl (Gallus gallus variant domesticus) and the muscovy duck (Cairina moschata) were compared with those of the alveolar macrophages of the lung of the black rat (Rattus rattus). The birds had significantly fewer FRMs compared to the rat. In the birds, the FRMs were found both in the lungs and in the air sacs. Under similar experimental conditions, the most robust FRMs were those of the domestic fowl followed by those of the rat and the duck. Flux of macrophages onto the respiratory surface from the subepithelial compartment and probably also from the pulmonary vasculature was observed in the birds but not in the rat. In the duck and the domestic fowl, a phagocytic epithelium that constituted over 70% of the surface area of the blood-gas (tissue) barrier lines the atrial muscles, the atria and the infundibulae. The epithelial cells of the upper respiratory airways contain abundant lysosomes, suggesting a high lytic capacity. By inference, the various defence strategies in the avian lung may explain the dearth of FRMs on the respiratory surface. We counter-propose that rather than arising directly from paucity of FRMs, an aspect that has been over-stressed by most investigators, the purported high susceptibility of birds (particularly table birds) to respiratory ailments and afflictions may be explained by factors such as inadequate management and husbandry practices and severe genetic manipulation for fast growth and high productivity, manipulations that may have weakened cellular and immunological defences.

Fundamental structural aspects and features in the bioengineering of the gas exchangers: comparative perspectives

Over its life, an organism's survival and success are determined by the inventory of vital adaptations that its progenitors have creatively appropriated, devised and harnessed along the evolutionary pathway. Such conserved attributes provide the armamentarium necessary for withstanding the adverse effects of natural selection. Refinements of the designs of the respiratory organs have been critical for survival and phylogenetic advancement of animal life. Gas exchangers have changed in direct response to the respiratory needs of whole organisms in different environmental states and conditions. Nowhere else is the dictum that in biology 'there are no rules but only necessities' more manifest than in the evolutionary biology of the gas exchangers. The constructions have been continually fashioned and refined to meet specific needs. Solutions to common respiratory needs have been typified by profound structural convergence. Over the evolutionary continuum, as shifts in environmental situations occurred, infinitely many designs should theoretically have emerged. Moreover, without specific selective pressures and preference for certain designs, considering that there are only two naturally occurring respirable fluid media (air and water), air-lungs, water-lungs, air-gills and water-gills would have formed to similar extents. Factors such as body size, phylogenetic level of development, respiratory medium utilized and habitats occupied have permutatively prescribed the design of the gas exchangers. The construction of the modern gas exchangers has eventuated through painstaking cost-benefit analysis. Trade-offs and compromises have decreed only a limited number of structurally feasible and functionally competent outcomes. The morphological congruity (analogy) of the gas exchangers indicates that similar selective pressures have compelled the designs. Solutions to metabolic demands for molecular O2 have only differed in details. Passive physical diffusion, for example, is the ubiquitous method of transfer of O2 across biological tissues. Gills, evaginated gas exchangers, were the primordial respiratory organs that evolved for water breathing, whereas lungs (invaginated gas exchangers) developed for terrestrial (air) breathing. Transitional (= bimodal = amphibious) breathing has evolved in animals with specialized organs that extract O2 from both water and air. Lungs are tidally (= bidirectionally) ventilated, while gills are unidirectionally ventilated, a feature that allows the highly efficient counter-current disposition between blood and water. Since animals occupy inconstant environmental milieus and their metabolic states vary, gas exchangers are designed to operate optimally across a spectrum of conditions that range from resting to exercise and even under hypoxia. Inbuilt structural and functional flexibility provides the requisite safety factors that allow adjustments to modest pressures. The fundamental structural features that determine the respiratory function of a gas exchanger are respiratory surface area, thickness of the blood-water/gas (tissue) barrier and volume of the pulmonary capillary blood. The diffusing capacity of a gas exchanger correlates directly with the surface area and inversely with the thickness of the blood-water/gas (tissue) barrier. An extensive surface area is generated in gills by extensive stratification of the gas exchanger and in lungs by profuse internal subdivision. Compartmentalization yields small terminal gas exchange compartments that compel greater commitment of energy to ventilate. The surfactant, a phospholipid lining, reduces the forces of surface tension at the air-water interface. This attenuates the propensity of physical collapse of the minute gas exchange units and minimizes the cost of ventilation. The surfactant characterizes all the gas exchangers derived from the piscine air bladder. In the lower air-breathing vertebrates, such as the lungfishes (Dipnoi), amphibians and certain reptiles, the pneumocytes are not differentiated into type I and II cells, as is the case in the lungs of the higher vertebrates-birds and mammals. It is envisaged that in endotherms, the overall numerical density of the pneumocytes and hence the O2 consumption of the gas exchangers may be reduced and a thin blood-gas (tissue) barrier generated, factors that enhance respiratory efficiency. The thin blood-gas (tissue) barriers, for example, those of the mammalian (in the respiratory sections of the interalveolar septum) and avian lungs, consist of an epithelial cell and an endothelial cell with a common basement membrane. An interstitial space occurs in the blood-air/water (tissue) barriers of the gas exchangers of fish gills and lungs of lungfishes, amphibians, reptiles and in the supportive parts of the interalveolar septum of the mammalian lung. Collagen, elastic tissue, nerves, lymphatic vessels and smooth muscle elements are found in the interstitial space. The thickness of the blood-air/water (tissue) barrier allometrically changes very little. This suggests that the thicknesses of the blood-water/air (tissue) barriers have been optimized. The presentation and exposure to the gas exchange media (water/air to blood), features dictated by the geometry and arrangements of the structural components of the gas exchangers, contribute greatly to respiratory efficiency. The countercurrent presentation between water and blood in fish gills is the most efficient design in the evolved gas exchangers: It was imperative for survival in water, a medium that contains relatively less O2 and is more expensive to breathe. In the evolved vertebrate gas exchangers, the exposure of blood to air is best manifested in the diffuse design of the avian lung, where the capillary blood is literally suspended in a three-dimensional air space, the blood being exposed to air virtually across the entire blood-gas (tissue) barrier. A double capillary design occurs in the lungs of amphibians and generally those of reptiles, whereas a single capillary design commonly occurs in those of adult mammals. The capillary loading (the ratio of the volume of the capillary blood to the surface area across which blood is exposed to air) in lungs with a double capillary arrangement is high and manifests a poor design. On the other hand, the low capillary loading that characterizes the single capillary system indicates better exposure of blood to air and greater respiratory capacity. Fractal geometry features in the construction of the gas exchangers. The highly versatile design allows the gas exchangers to function optimally under different conditions and circumstances and to maintain congruent morphologies over a wide range of body size, shape and metabolic capacities. At the gas exchange level, sheet-flow design preponderates in the evolved gas exchangers; blood is efficiently exposed to the external respiratory medium. The respiratory capacity of a gas exchanger is comprehensively granted by refinements of structural features and functional processes. Modelling, mathematical integration of structural and functional parameters, provides a holistic view of the essence of the design of a gas exchanger.

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.

Morphometric characterization of the airway and vascular systems of the lung of the domestic pig, Sus scrofa: comparison of the airway, arterial and venous systems

The bronchial system (BS), the pulmonary artery (PA) and the pulmonary vein (PV) of the lung of the domestic pig, Sus scrofa were simultaneously cast with silicone rubber and studied. Asymmetrical dichotomous bifurcation preponderated in the tree-like arrangement of the three conducting systems. Lengths and diameters of the various generations were measured. At the extremities of the BS and the PA, alveoli and blood capillaries related very closely. In the cranial and middle lobes of the right and left lungs, topographically, the PA and the PV closely followed the BS, but in the accessory and the caudal (diaphragmatic) lobes, only the PA accompanied the BS: the PV run intersegmentally. Certain similarities and differences were observed between the diameters and lengths of the various generations of the three conducting systems. The strong correlations between some of the structural parameters indicated a high level of structural optimization. While morphometric variations suggest that the air and the blood flow dynamics may somewhat differ between the three conducting systems, they may also register structural features unique to the lung of the domestic pig, an animal that has been highly genetically exploited for fast growth and now leads an indolent lifestyle in captivity.