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We have previously reconstructed the gas exchange tissue of the adult muscovy duck, Cairina moschata using a method of manually aligning sections and tracing the contours of the components of the gas exchange tissue. This reconstruction method demonstrated that the air capillaries are comprised of an expanded globular part interconnected by narrow air channels. The blood capillaries completely surround the air capillaries forming an anastomosing meshwork of short segments. However, the resulting reconstruction was limited in scope because of the laborious process of tracing the profiles of each component through the sequence of micrographs. We have now reconstructed a larger proportion of the exchange tissue by using a cross-correlation based alignment strategy and have demonstrated that the staining intensity of each of the exchange tissue components is sufficiently different to allow them to be identified by simple filtering and thresholding. The resulting reconstructions sample a much larger proportion of the exchange tissue and demonstrate the heterogeneity of structures from different locations in the parabronchus. We have shown that a sheet-flow-type arrangement of blood capillaries surrounds the infundibulum; this represents an unexpected functional convergence with the arrangement of blood capillaries surrounding the mammalian alveoli. It is feasible, using this reconstruction strategy, to analyse the exchange tissue of a large number of avian species in order to determine structural correlates of function. The resulting reconstructions could be analysed in order to determine the basis of the functional efficiency and rigidity of the avian 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 (∼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.

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.

Airflow in an avian respiratory system was simulated to study why birds affected with airsacculitis have respiratory distress. The airflow in the avian lung was modeled with a 1D electrical circuit and simulated for investigating what effect an increase in wall thickness of air sacs caused by airsacculitis has on flow in lung. The results demonstrated that thickening of the air sac wall caused anti-synchronization between an elastic recoiling force of the air sac walls and an intra-pleural pressure, bringing difficulties in expansion of air sacs to draw in airs during an inspiration period and thereby decreasing air to be pumped out during an expiration period. This was reflected in a decrease in air flow volume in parabronchi where gas exchange takes place. Therefore, it was concluded that airsacculitis causes imbalance in air flow dynamics in the avian lung and thus impairs breathing ability of birds.

It is known that alterations in respiratory gases in birds can cause a nonhomogenous redistribution of pulmonary blood flow between the 2 separate gas-exchanging regions of the avian lung, the paleopulmo (PALEO) and neopulmo (NEO); however, the effect of alterations in respired gas content on the distribution of pulmonary blood flow in birds, such as the chicken, that possess a highly developed NEO is not known. This study used a colorimetric microsphere method to determine the effects of hypoxia and hypercapnia on the relative distribution of pulmonary blood flow in anesthetized chickens (Gallus domesticus) during control (normoxic) and experimental (hypoxic or hypercapnic) conditions, where the relative regional distribution of blood flow in the lung is expressed as the ratio NEO/PALEO. Administration of a hypoxic gas mixture (16.0% O2) produced a 13.4% increase in NEO/PALEO, and, administration of a hypercapnic gas mixture (5.0% CO2) resulted in a 27.8% increase in NEO/PALEO. Our results are consistent with a mechanism in which the regional redistribution of pulmonary blood flow is mediated by local intrapulmonary factors.

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.

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.

Scanning electron microscopy (SEM) was utilized to examine the interparabronchial septum as a potential site of lymphatic drainage in the lungs of anesthetized chickens (Gallus domesticus). Birds were subjected to extracellular fluid volume expansion in order to produce hydrostatic pulmonary edema via increased pulmonary capillary fluid flux into the interstitial spaces of the lung. Micrographs obtained from freeze-dried lungs of volume-loaded birds were compared with similarly prepared lungs from normal control chickens, which were not volume loaded. The adjacent parabronchi of the control lungs were closely opposed by a minimal septal space, whereas the interparabronchial septal space of the volume-loaded birds was measurably thickened and appeared to be engorged as a result of hydrostatic pulmonary edema. The results of this study are consistent with observations of the lungs of mammals subjected to hydrostatic pulmonary edema and suggest that the interparabronchial septum may be a potential route of lymphatic drainage in the avian lung.

Scanning electron microscopy was used to examine the interparabronchial septa of chickens as a potential site of lymphatic drainage in the avian lung. Anaesthetized chickens were subjected to extracellular fluid volume expansion to produce pulmonary oedema as a result of increased capillary fluid flux into the interstitial spaces of the lung. In normal (control) chickens, the adjacent parabronchi were separated by a minimal septal space. In the "volume-loaded" birds, however, the interparabronchial septal spaces were measurably thickened and engorged as a result of hydrostatic pulmonary oedema. These results, which were consistent with reports of the effect of hydrostatic pulmonary oedema in mammals, suggest that the interparabronchial septum is a potential route of lymphatic drainage in the avian lung.

The form, geometry and epithelial morphology of the extrapulmonary primary bronchi (EPPB) of the domestic fowl (Gallus gallus var. domesticus) and the rock dove (Columba livia) were studied microscopically and by three-dimensional computer reconstruction to determine the structural features that may be involved in the rectification of the inspired air past the openings of the medioventral secondary bronchi (MVSB), i.e. the inspiratory aerodynamic valving (IAV). In both species, the EPPB were intercalated between the clavicular and the cranial thoracic air-sacs. A notable difference between the morphology of the EPPB in G. g. domesticus and C. livia was that, in the former, the EPPB were constricted at the origin of the MVSB, while a dilatation occurred at the same site in the latter. In both species, a highly vascularized, dorsally located hemispherical epithelial swelling was observed cranial to the origin of the MVSB. The MVSB were narrow at their origin and variably angled relative to the longitudinal axis of the EPPB. Conspicuous epithelial tracts and folds were observed on the luminal aspect of the EPPB in both C. livia and G. g. domesticus. From their marked development and their orientation relative to the angled MVSB, these properties may influence the flow of the air in the EPPB. It was concluded that features such as syringeal constriction, an intimate topographic relationship between the EPPB and the cranial air-sacs, prominent epithelial tracts and folds, an epithelial swelling ahead of the origin of the first MVSB (corresponding to the ‘segmentun accelerans’), and narrowing and angulation of the MVSB at their origin, may together contribute to IAV to a variable extent. In as much as the mechanism of pulmonary ventilation and mode of airflow in the parabronchial lung are basically similar in all birds, the morphological differences observed between G. g. domesticus and C. livia suggest that either the mechanism of production of IAV or its functional efficiency may be different, at least in these two species of birds.

A search for pulmonary arteriovenous anastomoses was made in 15 adult domestic fowls using Lycopodium spores and microspheres. The diameter of the spores and microspheres ranged from about 10 to 33 μm. To dilate any pulmonary arteriovenous anastomoses, the birds were warmed to induce panting, killed with chloroform, or injected intravenously with papavarine. The spores or microspheres were injected either into the jugular vein under anaesthesia, or into the pulmonary artery after death. After the pulmonary arterial injections, the effluent from the pulmonary vein, and histologieal sections of the lungs, were examined for spores or microspheres. When injections were made into the jugular vein, blood smears from the pulmonary veins, left atrium, and the aorta, as well as histologieal sections of the lungs and other organs were inspected. The results of all these experiments showed that no spores or microspheres were ever found on the venous side of the pulmonary circulation, indicating absence of pulmonary arteriovenous anastomoses.

To understand the structural basis of avian gas exchange better, we made a morphometric study of domestic and Canada goose lungs. The volume of glutaraldehyde-fixed domestic goose lungs (30 cm3/kg body weight) was similar to that determined from silicone casts of Canada goose lungs by Duncker (33 cm3/kg). To examine finer structures, we rapidly froze goose lungs under physiologic conditions, fixed tissue samples by a freeze substitution procedure and analyzed samples with stereological methods. From light micrographs we determined that about 55% of the lung is parabronchi in both species. Volume densities of air capillaries, blood capillaries and tissue and surface:volume ratios of these same structures were determined from electron micrographs. Our measurements agree with those from glutaraldehyde-fixed Canada goose lungs from other laboratories. Gas exchange surface area was largest in the good flier (Canada goose) but both birds had larger surface areas than comparably sized mammals. The harmonic mean blood-gas barrier thickness is smaller in both species of birds (0.3 microns) than in mammals. Thus, membrane diffusing capacities for gases should be larger in birds than in mammals. Pulmonary blood capillary transit time, as calculated from blood capillary volume and normal levels of cardiac output, are longer in birds than in mammals and should allow more time for blood-gas equilibrium. Pleats and folds were frequently observed in air and blood capillaries, suggesting that the avian lung may not be as rigid as was previously thought and that capillary volumes and surface areas may change under physiologic conditions.

The non-reversing gas flow pattern in the avian lung has been attributed to ‘aerodynamic valves’. A fundamental property of all aerodynamic valves is their dependence on inertial forces in the gas stream: sufficient reduction of inertial forces will cause aerodynamic valves to fail. If valving in the avian lung is aerodynamic, it should fail when gas stream momentum is reduced. We tested the dependence of the inspiratory valves in the goose lung on gas density and gas flow velocity. A bolus of tracer gas was placed in the tracheal cannula during an end-expiratory pause. Tracer gas appearance in a cranial air sac during the following inspiration and pause was used to deduce failure of the ‘inspiratory valve’ in cyclically ventilated geese. Little or no tracer entered the sac under control conditions, which approximated resting breathing, indicating highly effective valving. Lower flow rate or lower gas density caused increased tracer appearance, indicating valve failure. These results demonstrate the importance of gas inertial forces to normal valve function, and are direct evidence for the aerodynamic nature of the avian inspiratory valve.

Oxygen consumption (Vo2, tidal volume (v t ), and respiratory frequency (f) were measured in the passeriform fish crow, Corvus ossifragus (mass 0.274 kg), in dry air at temperatures of 5–25 °C. Ventilation (V i ) was calculated as f · V t , and oxygen extraction as the fraction of available oxygen removed from respiratory air. Vo2, V t , f, and V i tended to increase with decreasing T a , while oxygen extraction seemed temperature independent. Mean Vo2, at 20 °C was similar to the value calculated for a passerine bird. but about twice the value for a nonpasserine bird or a mammal of the same mass. Mean V i at 20 C was more than twice as great as in a nonpasserine but only 65% greater than in a mammal. Thus mean O2 extraction at 20 °C was about the same as in the nonpasserine. but substantially higher than in the mammal. The highest measured 02 extraction, about 0.31 at 25 °C, was twice as high as the calculated value for a mammal. These observations confirm the expectation of increased effectiveness of gas exchange in the avian lung, made possible by its arrangement as a through-flow system.

Copper deficiency in the chick causes anatomical changes in lung that are characterized by an apparent thinning of the air-blood capillary network of tertiary bronchi. Although the net content of the elastin in lung was not changed significantly by nutritional copper deficiency, it was possible to demonstrate an increase in the lysine content and a decrease in the desmosine content of lung elastin from copper-deficient chicks compared to that from controls. Further, the content of soluble elastin was also increased two- to fourfold in lung from copper-deficient chicks. Subsequently, nutritional copper deficiency was used to facilitate the isolation of chick lung tropoelastin. This lung protein appeared to be similar in selected properties to tropoelastin isolated from aorta of copper-deficient chicks. In addition, data are presented that indicate elastin in avian lung is possibly subject to only limited turnover.

A study of the fine structure of the avian lung revealed the presence of a unique laminated membrane surface and associated osmiophilic inclusions in epithelial cells of the tertiary bronchi and atria. These structures were not found in the air-capillary epithelium. Each lamination of the membrane surface had the appearance and dimensions of the unit membrane. It is suggested that the laminated membrane surface is associated with the formation of the osmiophilic inclusions and that these inclusions compare with those described in mammalian alveolar epithelium. It is further suggested that the laminated membrane surface is lipoprotein or phospholipid in composition and is responsible for the surface-tension-reducing properties of avian lung extracts.

Intrapulmonic airflow was investigated directly using a flowmeter in the posterior dorsal secondary bronchi, and indirectly by measuring CO2 content of airstreams passed into the trachea and out of the various air sacs. Secondary bronchial flow was found to be unidirectional in voluntary breathing but bidirectional when air was passed experimentally into and out of the trachea and air sacs. CO2 analysis indicates that air, in the conditions of the experiment and probably also in eupnea, enters the anterior sacs, partially at least, by the tertiary bronchi and enters the posterior sacs via the direct non-exchanging connections. The role of the anterior air sacs in determining inspiratory airflow through the lung is discussed and an attempt is made at integrating the ideas of aerodynamics and lung-air-sac topology in the context of airflow in the avian lung.

THOUGH the avian lung has been studied for many years (Campana, 1875; Fisher, 1905; Locy and Larsell, 1916 a, b; Akester, 1960), the relationship of structure to function is still incompletely understood (Vos, 1934; Dotterweich, 1936; Hazelhoff, 1951; Biggs and King, 1957; Shepard et al., 1959). The presence and possible role of a pulmonary surfactant in avian lungs is contested (Miller and Bondurant, 1961; Pattle, 1963). Electron microscopy has revealed osmiophilic inclusions in the epithelium of tertiary bronchi and atria (Nagaishi et al., 1964; Tyler and Pangborn, 1964), and a unique laminated membrane surface of these epithelial cells (Tyler and Pangborn, 1964). Similar inclusions in the alveolar epithelium of mammals are thought to be the source of the pulmonary surfactant (Klaus et al., 1962). Histochemical methods, although mainly qualitative, permit precise localization of chemical activities among the various tissue components of an organ. This type of information is not obtainable …

For measurement of gas flow, a flowmeter consisting of a heated wire anemometer combined with a thermocouple was implanted into a dorsobronchus of the lungs in unanesthetized ducks. The direction of gas flow recorded in the dorsobronchus during spontaneous breathing and during artificial ventilation was from the mesobronchus to the parabronchi both in inspiration and expiration. This result is in accordance with the unidirectional flow hypothesis propounded by Bethe and by Hazelhoff, and is incompatible with the alternating respiratory flow hypothesis of Zeuthen.

In the mammalian lung, respiratory macrophages provide front line defense against invading pathogens and particulate matter. In birds, respiratory macrophages are known as free avian respiratory macrophages (FARM) and a dearth of the cells in the avian lung has been purported to foreordain a weak first line of pulmonary defense, a condition associated with high mortality of domestic birds occasioned by respiratory inflictions. Avian pulmonary mechanisms including a three tiered aerodynamic filtration system, tight epithelial junctions and an efficient mucociliary escalator system have been known to supplement FARM protective roles. Current studies, however, report FARM to exhibit an exceptionally efficient phagocytic capacity and are effective in elimination of invading pathogens. In this review, we also report on effects of selective synthetic peroxisome proliferator activated receptor gamma (PPAR γ) agonists on non phlogistic phagocytic properties in the FARM. To develop effective therapeutic interventions targeting FARM in treatment and management of respiratory disease conditions in the poultry, further studies are required to fully understand the role of FARM in innate and adaptive immune responses.

The avian lung has been considered to be rigid and to remain isovolumetric during the respiratory cycle. We tested this hypothesis by implanting radiopaque markers of tantalum on the dorsal pulmonary surfaces and ventral pulmonary aponeuroses of Pekin ducks (Anas platyrhynchos) and measuring changes in lung thickness during the respiratory cycle using high speed cineradiography. We found small but regular changes in lung thickness that were synchronous with respiratory phase. Lung thickness was greatest at mid-inspiration (0.6% greater than mean) and least at mid-expiration (0.8% less than mean). Measurements made on ostrich (Struthio camelus) respiratory structures suggest that the maximal force that could be generated by the muscles (Mm. costopulmonales) at the margins of the ventral pulmonary aponeurosis is more than two orders of magnitude greater than would be required to resist pressure-induced changes in lung volume during respiration at rest. The action of these muscles could account for the very small magnitute of the volume changes measured during the respiratory cycle.

The lungs of five charadriiform species of bird, two of which are good divers and three predominantly flyers (soarers and gliders) have been analysed by morphometric techniques. Largely the morphometric structural values in the divers significantly exceeded those of the flyers (gulls). The average weight specific surface area of the blood-gas (tissue) barrier in the divers (28.45 +/- 2.05 cm2 X g-1 SD) surpassed that of the flyers (23.5 +/- 3.61 cm2 X g-1 SD). The divers had a higher volume of the pulmonary capillary blood per unit body weight (4.42 +/- 0.11 cm3 X kg-1 SD) than the flyers (2.84 +/- 0.58 cm3 X kg-1 SD). The weight specific volume of the lung in the divers (34.90 +/- 3.11 cm3 X kg-1 SD) exceeded that of the flyers (26.94 +/- 3.15 cm3 X kg-1 SD). The total morphometric pulmonary diffusing capacity per unit body weight in the divers (4.73 +/- 0.05 ml O2 X (min X mm Hg X kg)-1 SD) was higher than that of the flyers (3.09 +/- 0.47 ml O2 X (min X mm Hg X kg)-1 SD). The divers, however, had a notably thicker blood-gas (tissue) barrier with a harmonic mean thickness of 0.212 +/- 0.03 micron SD compared to that of the flyers (0.138 +/- 0.02 micron SD). The data acquired here commensurate the modes of life exhibited by these two groups of bird. The divers, which are relatively energetic birds, expend a lot of energy to move and stay underwater, concomitantly undergoing prolonged asphyxia during submergence and may hence need to extract as much of the oxygen in the pulmonary air as possible to prolong a dive. These birds appear in general to have structurally better adapted lungs than those of the gulls, birds which to a large extent exhibit relatively less energetic soaring and gliding flights.

The presence of unidirectional flow in the avian lung is thought to be effected by aerodynamic 'valves'. First we review the history of this hypothesis and summarize existing evidence. Second, we present a semiquantitative treatment of the various fluid dynamic factors that may be involved in directing fluid flow. The resulting calculations show in some detail how the inspiratory valve may work, and upon what mechanisms it may depend. Our calculations suggest that gas convective inertial forces are sufficient to effect inspiratory valving. Finally, we give some heuristic arguments regarding the mechanisms of expiratory valving.

Single-unit extracellular potentials from CO2-sensitive intrapulmonary chemoreceptors (IPC) were recorded with tungsten microelectrodes from the nodose ganglia of unidirectionally, artificially ventilated White Leghorn cockerels. The responses of IPC to step and sinusoidal forcing functions of fractional concentration of carbon dioxide (FCo2) indicate that 1) about two-thirds of IPC are bidirectionally rate sensitive, 2) delay to forcing varies from essentially zero to several times larger than the time of nitrogen gas passage through the lung and, on the average, is significantly shorter to step decrease than step increase in fco2, and 3) IPC with irregular discharge intervals are more rate sensitive and respond to higher frequencies than IPC with regular interspike intervals. The physical and chemical properties of the "receptor-lung" complex are probably important in determining the dynamic responses of IPC by partitioning of CO2 from other gases within the lumen of the parabronchus. IPC are located along the length of the bronchial gas exchange areas of the avian lung and transmit information to the CNS concerning the mean level, rate, and direction of change in the partial pressure of carbon dioxide (PCo2).

A paraformaldehyde perfusion technique is useful in studying biogenic amine-containing cells in the avian lung. In comparison to the frequently used lyophilization technique, the time required to prepare the tissue is greatly reduced.

The ventilatory activity of the anterior and posterior groups of air sacs was simulated in unidirectionally-ventilated geese and the resultant flow of air in the mediodorsal secondary bronchi was used as an indicator of the route which air followed through the lung. The results were used to isolate the roles of the respective groups of air sacs in the shaping of the unidirectional pattern of airflow known to exist during normal respiration. Findings indicated that, in contrast to Hazelhoff's model, the anterior and not the posterior sacs are responsible for producing the caudo-cranial flow of air through the parabronchi during inspiration. The posterior sacs, as predicted by Hazelhoff's model, are preliminary responsible for driving the caudo-cranial current through the parabronchi during expiration.

Single unit activity was recorded in the cat vagus in order to detect possible receptors firing in response to changing lung CO 2 concentration. The cats were ventilated at a high rate (60–120 breaths per min) and inspired CO 2 concentration was altered between 0 and 8 % in a step-like fashion, each phase consisting of about 10 breaths. Thus the effects of changing intrapulmonary CO 2 concentration could be differentiated from the effects of stretch of lung tissue. Activity was recorded in 7 cats from 120 units firing in phase with ventilation. Many receptors showed some CO 2 sensitivity, but no fiber was found discharging in response to CO 2 exclusively. The results provide no evidence for the occurence of specific CO 2 receptors in the feline lung with vagal afferents functionally similar to those reported for the avian lung.

The lungs of the domestic fowl were prepared for scanning electron microscopy after vascular and airway latex rubber casting to demonstrate the spatial organization of the various structural components that are involved in the gas exchange that takes place in the parabronchial tissue mantle. The bulk of the intrapulmonary air flows through the parabronchial lumen and then centrifugally diffuses into the exchange tissue through the atria, the infundibula, and the air capillaries. The blood flows centripetally from the interparabronchial arteries, then into the intraparabronchial arterioles, and finally into the blood capillaries, which together with the air capillaries constitute the functional terminal gas exchange units. The relationship between the air flow in the parabronchial lumen and the incoming blood (into the exchange tissue) has been shown to be crosscurrent, where the directions of the flow of these two gas exchange media are essentially perpondicularly disposed to each other; whereas the relationship between the blood capillaries and the air capillaries is countercurrent, the blood flowing towards the parabronchial lumen and the air in the opposite direction, i.e., towards its periphery. Both these spatial structural relationships between the air and blood are significant factors that contribute to the remarkable efficiency of the avian lung in gas exchange. The crosscurrent system enhances the duration of interaction between the parabronchial air and the blood, the oxygen being extracted from a parabronchus along its whole length, whereas the overall contribution of the countercurrent relationship, which is superimposed on the crosscurrent system, is not expected to be very high, mainly because of the tortuosity and profuse interlacing of the blood and air capillaries over a relatively short distance, that of approximately the thickness of the parabronchial tissue mantle.

The activities of formation of lactate and pyruvate were studied along with the development of chickens, Gallus domestica. In the lung the activity of lactate formation was highest in the embryo at 13th to 16th days of incubation, then the activity decreased as the development went on and continued to decrease even after the hatching. The change of the activity of pyruvate formation was almost parallel with that of lactate formation. On the other hand, in the liver the activities of formation of lactate and pyruvate remained constant and did not show such changes as observed for the lung.

HISTOLOGICAL examination of birds’ lungs indicates a considerable variation in air and blood volumes (Olander et al., 1967). However, in order to obtain quantitative estimates of these changes, methods are needed which can be applied to the entire organ. Such information is of importance in evaluating respiratory function in conditions where the lung becomes anatomically altered. Previous measurements of lung blood volumes in birds (Altman and Dittmer, 1961; McFarland, 1964) have not been made so as to prevent postmortem changes in blood volume in the organ. Other estimates of air volumes in the avian lung (King and Payne, 1958, 1962; Zeuthen, 1942) have been made by filling the respiratory tract with some foreign substance, the difficulties of which have been recognized. Consequently, new methods have been devised to measure air volumes (by buoyancy) and blood volumes (by isotope dilution) and these as well as data on normal chickens are reported . . .

Preliminary allometric equations relating avian respiratory variables to body weight permit a series of tentative statements regarding avian respiration which were hitherto impossible. Comparisons of avian and mammalian equations reveal dimensional similarities and differences in the two distinct respiratory systems. The avian lung is similar in weight but has a proportionately smaller air space than its mammalian counterpart. The total volume of the avian respiratory system is much greater than that of mammals. Avian tidal volumes are larger than those of mammals, but avian respiratory rates are lower, and avian minute volumes are somewhat less than in comparable sized mammals. Each tidal volume in birds provides air equivalent to a complete turnover of the airspace of the lungs plus trachea. Avian respiration provides a more extensive CO 2 washout in the tertiary bronchi exchange areas than occurs in the mammalian alveoli. The amount of oxygen removed from respiratory air in birds is independent of body weight, as it is in mammals, and birds may remove a greater proportion of the oxygen than do mammals.

The pressure/flow relationship was determined across the air sac/lung system, and between different points within the system, in birds ventilated normally and artificially. Resistances were also calculated for pathways to individual sacs. In all cases resistance rose with increasing flow rates. Pathways supplying anterior sacs had lower resistances than those to the posterior sacs. During both voluntary and artificial bidirectional ventilation flow across the lung was caudo-craniad in each phase. Airstreams were introduced into various air sacs and resultant intrapulmonic flows monitored. Observations supported the idea that the anterior sacs during inspiration and the posterior sacs during expiration are primarily responsible for flow through the lung. Patterns measured in different circumstances, combined with a physical analysis of the pressure/flow relationship, enabled the formulation of a scheme for the anatomical and aerodynamical determinants of normal intrapulmonic flows.

Des examens au microscope optique on montre l'existence de terminaisons nerveuses, dont on supposait la presence, dans les structures des bronchioles tertiaires et des atrium des poumons de la poule domestique (G. domesticus). Les caracteres morphologiques en sont decrits. On pense qul'il s'agit de la premiere preuve de l'existence de terminaisons afferentes dans les «conduits» d'air, realisee a l'aide du microscope optique.

L'examen au microscope electronique du bronchus primaire intrapulmonaire deGallus domesticus a revele la presence d'un complexe neurite-recepteur tres semblable aux cellules du glomus carotidien des mammiferes, et aux cellules de Merkel qui se trouvent dans les structures epidermiques des vertebres. La fonction possible chemorecepteur ou mechanorecepteur de ces cellules est discutee.

Experiments have been performed in ducks in order to determine which of both previously proposed physical models, the counter-current or the cross-current system, is best suited for analysis of gas exchange in parabronchial lungs of birds. Mesobronchi of both sides of the body were blocked between the origins of ventrobronchi and dorsobronchi using inflatable catheters, and both postthoracic air sacs were opened by introduction of large bore tubing through the body wall into their lumen. In this preparation a continuous parabronchial air flow could be achieved in either of both directions, from ventrobronchi to dorsobronchi by blowing air into the trachea, or in the opposite direction, from dorsobronchi to ventrobronchi, by blowing air from the air sacs into the bronchial system. During steady state, the pulmonary gas flow and PCO2 and PO2 in air as well as in arterial and mixed venous blood were measured. The results showed no dependence of partial pressures on the direction of parabronchial air flow. In particular, arterial PCO2 was found to be lower than expired (=end-parabronchial) PCO2 with both directions of parabronchial air flow. These findings cannot be easily explained on the basis of a counter-current gas exchange system, which has been proposed by Schmidt-Nielsen (1971), but are in good agreement with the assumption of a cross-current gas exchange system operative in the avian lung.