Analysis of gas exchange between air capillaries and blood capillaries in avian lungs

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

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

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

Evolution of air breathing: oxygen homeostasis and the transitions from water to land and sky

Life originated in anoxia, but many organisms came to depend upon oxygen for survival, independently evolving diverse respiratory systems for acquiring oxygen from the environment. Ambient oxygen tension (PO2) fluctuated through the ages in correlation with biodiversity and body size, enabling organisms to migrate from water to land and air and sometimes in the opposite direction. Habitat expansion compels the use of different gas exchangers, for example, skin, gills, tracheae, lungs, and their intermediate stages, that may coexist within the same species; coexistence may be temporally disjunct (e.g., larval gills vs. adult lungs) or simultaneous (e.g., skin, gills, and lungs in some salamanders). Disparate systems exhibit similar directions of adaptation: toward larger diffusion interfaces, thinner barriers, finer dynamic regulation, and reduced cost of breathing. Efficient respiratory gas exchange, coupled to downstream convective and diffusive resistances, comprise the "oxygen cascade"-step-down of PO2 that balances supply against toxicity. Here, we review the origin of oxygen homeostasis, a primal selection factor for all respiratory systems, which in turn function as gatekeepers of the cascade. Within an organism's lifespan, the respiratory apparatus adapts in various ways to upregulate oxygen uptake in hypoxia and restrict uptake in hyperoxia. In an evolutionary context, certain species also become adapted to environmental conditions or habitual organismic demands. We, therefore, survey the comparative anatomy and physiology of respiratory systems from invertebrates to vertebrates, water to air breathers, and terrestrial to aerial inhabitants. Through the evolutionary directions and variety of gas exchangers, their shared features and individual compromises may be appreciated.

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.

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.

Effects of hypobaria on parabronchial gas exchange in normoxic and hypoxic ducks

Cardio-respiratory parameters and air sac and blood gases were measured in the unrestrained, unanesthetized duck during exposure to normobaric (PB = 746 Torr) or hypobaric (PB = 253 Torr) normoxia (PIO2 = 143 Torr) and hypoxia (PIO2 = 41.5 Torr). Compared with normobaria at the same PIO2, hypobaria caused a statistically significant increase in ventilation during both normoxia and hypoxia, resulting in elevated PO2 and diminished PCO2 in the caudal thoracic and clavicular air sac, and in increased PaO2 and decreased PaCO2. Similarly, lactic acid production was elevated in hypobaria, and the resulting decrease in arterial pH may be responsible for the increase in ventilation. Despite these changes, there was no evidence for altered gas exchange efficiency during hypobaria. This suggests that no significant diffusion limitation is present in the air capillary gas phase in normobaria, that could have been diminished with hypobaria. It also suggests that the aerodynamic valving efficiency, present during inspiration at the level of the medioventral bronchi, is not affected by hypobaria. Although the mechanisms underlying the increased lactic acid production and ventilation are not understood, they may exert an advantageous effect on high altitude tolerance of the bird.

Effects of normobaric and hypobaric hypoxia on ventilation and arterial blood gases in ducks

We measured ventilation (V1) and arterial blood gases in awake Pekin ducks exposed to normoxia at sea level, normobaric hypoxia achieved by lowering FIO2 at normal barometric pressure (NORMO), and hypobaric hypoxia achieved with a low pressure chamber and 21% O2 (HYPO). Average normoxic values were: V1 = 0.46 L . (kg.min)-1, PaO2 = 99.7 Torr, PaCO2 = 30.1 Torr. At PIO2 = 90 Torr, NORMO and HYPO measurements were not significantly different (P greater than 0.05). At PO2 = 46 Torr, NORMO V1 was less than HYPO V1 but blood gases were not significantly different: VI = 1.00 vs 1.45 L . (kg.min)-1; PaO2 = 31.3 vs 33.0 Torr; PaCO2 = 11.5 vs 10.6 Torr. Although both tidal volume (VT) and respiratory frequency (fR) were greater in HYPO, similar blood gases with NORMO and HYPO suggest similar parabronchial ventilation. The results suggest increased physiologic dead space, caused by reduced efficacy of aerodynamic valving, with reduced gas density in hypobaria.

Scanning electron microscope study of the spatial organization of the air and blood conducting components of the avian lung (Gallus gallus variant domesticus)

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 perpendicularly 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.(ABSTRACT TRUNCATED AT 250 WORDS)

Gas exchange during exercise in hypoxic ducks

We quantitatively assessed pulmonary gas exchange in Pekin ducks (Anas platyrhynchos) during running exercise (1.44 km X h-1 at 3 degrees incline) while the ducks spontaneously breathed either air (FIO2 = 0.21) or a hypoxic gas mixture (FIO2 = 0.12). During exercise, oxygen consumption increased 3 times above the resting value in normoxia and 3.6 times above rest in hypoxia. The convection requirement rose 34% and 20% in running normoxic and hypoxic ducks, respectively. The O2 extraction coefficient was the same in resting normoxic and hypoxic ducks (0.19 vs 0.18) and decreased by the same amount under exercise conditions (0.14 vs 0.15). Arterial PO2 was maintained during exercise in normoxia but increased slightly during exercise in hypoxia. Cardiac output increased by 73% and 111% during exercise in normoxic and hypoxic ducks, respectively. Calculations indicate that both the O2-diffusing capacity and the total conductance for O2 of the gas exchange system increased markedly during exercise in normoxia and hypoxia. We conclude that at this level of exercise, there was no apparent limitation to gas exchange in either the normoxic or hypoxic Pekin duck.

Receptive fields of intrapulmonary chemoreceptors in the Pekin duck

Reflex experiments indicate a uniform distribution of CO2 chemosensitivity in avian lungs, but neural recording experiments suggest a non-uniform distribution of intrapulmonary chemoreceptor (IPC) endings. To reconcile these observations, blood gases and PECO2 were measured while recording discharge frequencies of 32 IPC innervating the unidirectionally ventilated lungs of 14 Pekin ducks. IPC discharge frequencies, recorded from the left vagus, were determined while ventilating the perfused left lung with caudocranial and craniocaudal flows of 1% CO2 in air, and then while ventilating the unperfused left lung with known levels of CO2 in air. Lung PCO2 profiles were predicted using an eight-compartment computer model of cross-current gas exchange with log-normal ventilation-perfusion inequality and shunt. The PCO2 profiles and IPC discharge frequencies were used to calculate receptor location. At the 99% confidence limit, estimates of IPC location changed significantly in all but 7 IPC when the direction of ventilation was reversed, indicating many IPC have multiple endings. Eighteen of 32 IPC had receptive fields extending at least 50% of the parabronchial length, which may explain the uniform reflex chemosensitivity to intrapulmonary CO2 noted by others.

Effects of increasing metabolism by 2,4-dinitrophenol on respiration and pulmonary gas exchange in the duck

The effects of pharmacologically elevated metabolism on respiration and parabronchial gas exchange were studied in the anesthetized, spontaneously breathing duck using 2,4-dinitrophenol (DNP), injected in successive single doses of 1.2-2.5 mg per kg body mass. Oxygen uptake, MO2, increased with the cumulative amount of DNP, reaching a sevenfold resting level at the highest DNP level tolerated, 15 mg/kg. Ventilation increased nearly as much as MO2, mainly by an increase in respiratory frequency, fresp. Cardiac output increased somewhat less than MO2, mediated by increases in both cardiac frequency and stroke volume. Arterial blood-gases showed little change; however, mixed venous PO2 dropped significantly, and PCO2 increased significantly, with stimulated metabolism. Pulmonary diffusing capacity, DO2, showed a significant rise with MO2, beyond that expected from a reduction of functional lung heterogeneity. The results show that pharmacological stimulation of metabolism can evoke responses in the respiratory and circulatory systems that are comparable to those observed with exercise. The mechanism by which parabronchial diffusing capacity increases during elevated metabolism remains to be investigated.

Morphometrics of rapidly frozen goose lungs

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.

Measurement of continuous distributions of ventilation-perfusion in non-alveolar lungs

We modified the multiple inert gas elimination technique, which was originally developed for alveolar lungs, to measure continuous distributions of V/Q in the cross-current lungs of birds. In theory, the method is also applicable to counter-current gas exchangers. The algorithms for inferring essentially continuous V/Q distributions from a limited number of measurements and the least-squares approach for dealing with experimental error are independent of the model of gas exchanger being studied. A Monte-Carlo procedure was used to predict the expected frequency of occurrence of given magnitudes of experimental error for each model. If the observed frequency distribution of error exceeds the predicted, then this indicates an incorrect choice of model (analogous to chi-square tests). Thirty-four data sets from 8 geese indicate that: (1) the assumptions of the technique are adequately met; (2) the alveolar model is not appropriate for birds, but the cross-current model is; and therefore; (3) the cross-current modification of the multiple inert gas elimination technique can be used to assess V/Q inequality in avian lungs.

A method for localizing intrapulmonary chemoreceptors in the parabronchial mantle of the duck

1. A method is presented for estimating the location of avian intrapulmonary chemoreceptors within the parabronchial mantle. 2. By determining the discharge frequency of a receptor at known receptor site Pco2's in a nonventilated but perfused lung, the receptor discharge could be calibrated to indicate the receptor site Pco2 during both ventilation and perfusion. 3. The relation among receptor site Pco2, mixed venous Pco2 and inspired Pco2 may be compared with calculated Pco2 profiles along the contact between air capillaries and blood capillaries and the receptor location may be determined as the relative distance between the luminal and peripheral ends of the air capillaries. 4. Of four receptors at the caudal end of a parabronchus, two were located at the terminal end of the air capillary and two along the peripheral half of the air capillary.

Ventilatory response to the PCO2 profile in chicken lungs

We investigated the influence of intrapulmonary chemoreceptors (IPC) on ventilatory movements in anesthetized chickens when PCO2 profiles along the parabronchi were changed. In all experiments the right lung was denervated, both lungs unidirectionally ventilated, and PaCO2 kept constant. In series 1 (7 birds), gas flow and the PCO2 profile in the left lung were reversed. PaCO2, PECO2 and ventilatory movements did not change. In Series 2 (4 birds), PCO2 in caudal regions of the innervated lung was elevated by increasing gas flow and P1CO2 from 0 to 21 Torr. Ventilatory movements did not change. In Series 3 (4 birds), either lung was over-ventilated with 7 or 49 Torr P1CO2, alternating the gases between lungs every 100 sec. Ventilatory movements changes with P1CO2 but much less than predicted from P1CO2 effects in the non-perfused, innervated lung. From the longitudinal distribution of IPC and PCO2 profiles in the lung we predicted moderate to large changes in ventilatory movements in all series. The discrepancy between predicted and observed results in Series 1 and 2 indicates that IPC in caudal regions of the lung have little effect on ventilation under the conditions examined. In Series 3, observed ventilatory movements were less sensitive to P1CO2 than predicted, indicating that IPC sense a different PCO2 than the PCO2 profile in the parabronchial lumen and that IPC have a significant sensitivity to pulmonary blood PCO2.

Functional localization of avian intrapulmonary CO2 receptors within the parabronchial mantle

To determine the location of avian intrapulmonary CO2 receptors, we changed the CO2 stimulus at different regions within the parabronchial mantle and measured the resulting changes in breathing pattern. Three procedures were used to vary the CO2 stimulus: (1) reverse the direction of pulmonary perfusion; (2) stop pulmonary ventilation while maintaining perfusion; and (3) stop pulmonary perfusion while maintaining ventilation. Right and left lungs of adult, anesthetized White Leghorn type chickens were independently, unidirectionally ventilated. The right lung was used to maintain the bird while the left pulmonary artery and vein were cannulated and connected to an extracorporeal gas exchanger, thereby isolating this lung's perfusion. The innervation to both lungs remained intact. When left pulmonary perfusion was reversed, the bird's breathing pattern remained unchanged. The change in breathing pattern that resulted from stopping left pulmonary ventilation was the same during forward perfusion (pulmonary artery to pulmonary vein) as during backward perfusion (pulmonary vein to pulmonary artery). The change in breathing pattern that resulted from stopping forward perfusion was the same as that resulting from stopping backward perfusion. The results indicate that CO2 receptors are not concentrated on the peripheral side of the parabronchial mantle, where venous blood would influence tham, or on the luminal side of the mantle, where arterialized blood would influence them. The CO2 receptors are either distributed symmetrically between the peripheral and luminal sides of the mantle or located in the epithelial lining of the parabronchial lumen.

Gas exchange in the parabronchial lung of birds: experiments in unidirectionally ventilated ducks

Pulmonary exchange of O2 and CO2 was measured in unidirectionally ventilated ducks in an attempt to determine lung O2 diffusing capacity, DO2. Perfusion shunt (= venous admixture) was estimated from O2 exchange in hyperoxia, and the ventilation shunt (ventilation of non-perfused parallel lung units) was estimated from exchange of the highly soluble inert gas, chloroform. Differences in the ventilation/perfusion ratio of parallel lung units were assessed from measurement of CO2 exchange using a parallel two-compartment model. DO2 values were calculated accounting for ventilation shunt, perfusion shunt, and inhomogeneity. Perfusion shunt averaged 2.7% and ventilation shunt, 9.4%. The ventilation/perfusion ratio in the two compartments differed on the average by a factor of 2.6. The uncorrected values of DO2, not accounting for lung inhomogeneities, progressively declined with increasing inspired PO2, but this dependence was less pronounced after correcting for lung inhomogeneities. The corrected value of DO2 averaged 100 mumol . min-1 . torr-1 for ducks of 1.8 kg mean body weight. DO2 did not differ when nitrogen was replaced by helium in the ventilatory gas indicating that diffusion within the air capillaries did not contribute a significant resistance to O2 uptake. The results suggest that neither functional inhomogeneities nor diffusion between lung gas and blood limit O2 uptake of the resting duck. Under conditions of elevated metabolism, however, these parameters may become rate-limiting for O2 supply.