Morphometries of the avian lung: The structural-functional correlations in the design of the lungs of birds

Anatomical and Functional Study of the Ostrich (Struthio camelus) Lung through Macroscopic Analysis in Combination with Optical and Electron Microscopy Techniques

The Ostrich occupies a unique position as the largest bird on the planet. Like other ratites, it has been reputed to have a phylogenetically primitive lung. We used macroscopy, light microscopy, transmission and scanning electron microscopy as well as silicon rubber casting to elucidate the functional design of its lung and compare it with what is already documented for the avian species. The neopulmonic region was very small and poorly developed. The categories of the secondary bronchi (SB) present and their respective numbers included laterodorsal (8-10), lateroventral (4-5), medioventral (4-6) and posterior (16-24). The lateral aspects of the laterodorsals were covered with a transparent collapsible membrane internally lined with a squamous to cuboidal epithelium. The bulk of these SB were in close proximity to intercostal spaces and the intercostal muscles and were thought to be important in the propulsion of gases. The lung parenchyma was rigid, with the atria well supported by septa containing smooth muscles, connective tissue interparabronchial septa were absent, and blood capillaries were supported by epithelial bridges. There were two categories of epithelia bridges: the homogenous squamous type comprising two leaflets of type I cells and the heterogeneous type consisting of a type I pneumocyte and type II cell. Additional type two cells were found at the atrial openings as well as the walls of the infundibulae and the air capillaries. The atria were shallow and opened either directly into several air capillaries or into a few infundibulae. The presence of numerous type II cells and the absence of interparabronchial connective tissue septa may imply that the ostrich lung could be capable of some degree of compliance.

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.

Morphometry of the extremely thin pulmonary blood-gas barrier in the chicken lung

The gas exchanging region in the avian lung, although proportionally smaller than that of the mammalian lung, efficiently manages respiration to meet the high energetic requirements of flapping flight. Gas exchange in the bird lung is enhanced, in part, by an extremely thin blood-gas barrier (BGB). We measured the arithmetic mean thickness of the different components (endothelium, interstitium, and epithelium) of the BGB in the domestic chicken lung and compared the results with three mammals. Morphometric analysis showed that the total BGB of the chicken lung was significantly thinner than that of the rabbit, dog, and horse (54, 66, and 70% thinner, respectively) and that all layers of the BGB were significantly thinner in the chicken compared with the mammals. The interstitial layer was strikingly thin in the chicken lung ( approximately 86% thinner than the dog and horse, and 75% thinner than rabbit) which is a paradox because the strength of the BGB is believed to come from the interstitium. In addition, the thickness of the interstitium was remarkably uniform, unlike the mammalian interstitium. The uniformity of the interstitial layer in the chicken is attributable to a lack of the supportive type I collagen cable that is found in mammalian alveolar lungs. We propose that the surrounding air capillaries provide additional structural support for the pulmonary capillaries in the bird lung, thus allowing the barrier to be both very thin and extremely uniform. The net result is to improve gas exchanging efficiency.

Development of maximum metabolic rate and pulmonary diffusing capacity in the superprecocial Australian Brush Turkey Alectura lathami: an allometric and morphometric study

The Australian Brush Turkey Alectura lathami is a member of the Megapodiidae, the mound-building birds that produce totally independent, "superprecocial" hatchlings. This study examined the post-hatching development of resting and maximal metabolic rates, and the morphometrically determined changes in pulmonary gas exchange anatomy, in chicks during 3.7 months of growth from hatchlings (122 g) to subadults (1.1 kg). Allometric equations of the form y=aM(b) related gas exchange variables (y) to body mass (M, g). Metabolic rates were measured with open-flow respirometry (mL O2 min(-1)) of chicks resting in the dark and running above the aerobic limit on a treadmill. Resting metabolic rate (RMR=0.02 M(0.99)) and maximal metabolic rate (MMR=0.05 M(1.07)) scaled with exponents significantly above those of interspecific allometries of adult birds. However MMR was below that expected for other species of adult birds in flapping flight, consistent with the Brush Turkey's ground-dwelling habits. Total lung volumes (mL) increased faster than isometrically (V(L)=0.0075 M(1.19)), as did the surface area (cm(2)) of the blood-gas barrier (S(t)=7.80 M(1.23)), but the data overlapped those of adult species. Harmonic mean thickness of the blood-gas barrier was independent of body size (mean tau(ht),=0.39 microm) and was about twice that expected for flying birds. Diffusing capacity (mL O2 min(-1) kPa(-1)) of the blood-gas tissue barrier increased faster than isometrically (Dto2=0.049 M(1.23)); in hatchling Brush Turkeys, it was about 30% expected for adult birds, but this difference disappeared when they became subadults. When compared to altricial Australian pelicans that hatch at similar body masses, superprecocial Brush Turkeys had higher MMR and higher Dto2 at the same body size. A parallel allometry between MMR and Dto2 in Brush Turkeys and pelicans is consistent with the concept of symmorphosis during development.

An allometric study of lung morphology during development in the Australian pelican, Pelicanus conspicillatus, from embryo to adult

Pelicans produce altricial chicks that develop into some of the largest birds capable of sustained flight. We traced pulmonary morphological development in the Australian pelican, Pelicanus conspicillatus, from third trimester embryos to adults. We described growth and development with allometric relationships between lung components and body mass or lung volume, according to the equation y = ax(b). Pelican lung volume increased faster than body mass (b = 1.07). Relative to lung volume, the airways and vascular spaces increased allometrically (b > 1) in embryos, but isometrically (b approximately 1) after hatching. Parabronchial mantle volume decreased (b < 1) prior to hatching and increased isometrically thereafter. Surface area of air capillaries, blood capillaries and the blood-gas barrier increased relative to lung volume (b > 0.67) before and after hatching. Barrier thickness decreased before hatching, remained constant in juveniles and decreased by adulthood. The anatomical diffusing capacity significantly increased before hatching (b = 4.44) and after hatching (b = 1.26). Although altricial pelicans developed pulmonary complexity later than precocial turkeys, the volume-specific characteristics were similar. However, lungs of volant adult pelicans became significantly larger, with a greater capacity for gas exchange, than lungs of terrestrial turkeys. Exchange characteristics of growing pelican lungs were inferior to those of adult birds of 26 other species, but converged with them at maturity.

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.

Developmental allometry of pulmonary structure and function in the altricial Australian pelican Pelecanus conspicillatus

Quantitative methods have been used to correlate maximal oxygen uptake with lung development in Australian pelicans. These birds produce the largest altricial neonates and become some of the largest birds capable of flight. During post-hatching growth to adults, body mass increases by two orders of magnitude (from 88 g to 8.8 kg). Oxygen consumption rates were measured at rest and during exposure to cold and during exercise. Then the lungs were quantitatively assessed using morphometric techniques. Allometric relationships between body mass (M) and gas exchange parameters(Y) were determined and evaluated by examining the exponents of the equation Y=aMb. This intraspecific study was compared to interspecific studies of adult birds reported in the literature. Total lung volume scales similarly in juvenile pelicans (b=1.05) as in adult birds (b=1.02). However, surface area of the blood–gas barrier greatly increases (b=1.25), and its harmonic mean thickness does not significantly change (b=0.02), in comparison to exponents from adult birds (b=0.86 and 0.07, respectively). As a result, the diffusing capacity of the blood–gas tissue barrier increases much more during development (b=1.23) than it does in adult birds of different sizes (b=0.79). It increases in parallel to maximal oxygen consumption rate (b=1.28), suggesting that the gas exchange system is either limited by lung development or possibly symmorphic. The capacity of the oxygen delivery system is theoretically sufficient for powered flight well in advance of the bird's need to use it.

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.