Diffusive Gas Transport Research Articles

Role of hematocrit in the recruitment of pulmonary diffusing capacity: comparison of human and dog.

In dogs, maximal O2 uptake (VO2max) per kilogram of body weight is two- to threefold that in humans; the difference cannot be explained solely by differences in structural features between species. We compared the functional recruitment of pulmonary diffusing capacity (DLCO) during exercise in dogs with that in humans to determine whether pulmonary gas exchange is matched to VO2max or the size of the lungs and to define the potential role of exercise-induced polycythemia in producing the superior aerobic capacity of the dogs. We compared the relationships of DLCO, membrane diffusing capacity (DMCO), and pulmonary capillary blood volume (Vc) with respect to pulmonary blood flow (Qc) by a rebreathing method during steady-state exercise in adult male human subjects and in conditioned adult male foxhounds. The slopes and intercepts of the relationships of DLCO and DMCO to Qc are significantly greater in dogs than in humans; the slopes of the relationship of Vc to Qc are similar. In dogs diffusive pulmonary gas transport is matched to the higher VO2max. The enhanced recruitment of DLCO and DMCO in dogs during exercise could potentially be explained entirely by the exercise-induced polycythemia, which is seen in dogs but not in humans.

Influence of gas composition on convective and diffusive intrapulmonary gas transport.

The influence of gas composition on convective and diffusive gas transport in the lungs was assessed by studying the dispersion of combined particle and argon (Ar) boluses induced by the passage through the lungs filled with three different gas mixtures. Particles, as a "nondiffusing gas," served as a tracer for convective gas transport, while the significance of diffusive gas transport was inferred from the difference in the behavior of Ar and particles. The lungs of six anesthetized and mechanically ventilated beagle dogs were equilibrated with air or either of the test atmospheres, He-O2 or SF6-O2, where nitrogen was replaced by helium (He) or sulfur hexafluoride (SF6). Due to differences in gas density and gas viscosity Reynolds numbers varied by a factor of twelve and Ar diffusivity by a factor of four between He-O2 and SF6-O2, suggesting that both kinds of intrapulmonary gas transport, convection and diffusion, should be affected. Combined particle and Ar boluses were inhaled into various lung depths and the extent of gas transport was inferred from changes in bolus shape induced by the passage through the lungs. In air, convective bulk gas transport generally followed the symmetric first-in, last-out principle and acted at all tested lung depths. Within the conducting airways, gas transport to the lung periphery was primarily due to convection but beyond these airways diffusion became rapidly significant. Breathing test atmospheres affects intrapulmonary gas transport only slightly. The extent of convective mixing was increased by 4% in SF6-O2 (p < .01) and reduced by 5% in He-O2 (p < .01) as compared to air. The symmetry of convective lung filling and emptying was slightly disturbed. In SF6-O2 the mean of the exhaled bolus was shifted by 8% toward the lung periphery. In He-O2 it was shifted by 4% toward the airway opening. In both test atmospheres exhaled Ar boluses were similar, suggesting that diffusive gas transport overwhelms the small changes in convective gas transport. Hence, factors other than gas composition-related flow characteristics, e.g., nonreversibility of in- and expiratory flow profiles or features of lung geometry, are the major determinants of gas transport in the lungs.

Internal pressurization and convective gas flow in some emergent freshwater macrophytes

Internal pressurization and convective through‐flow are demonstrated to be common attributes of wetland plants with cylindrical culms or linear leaves. Eight of 14 species tested produced static internal gas pressure differentials of 200–1,300 Pa relative to ambient and internal convective airflows of 0.2 to &gt;10 cm3 min−1 culm−1, depending on species. Four species produced internal static pressure differentials of &lt;100 Pa. Two species did not pressurize. The driving forces are gradients in temperature and water vapor between the internal gas spaces of the plants and the ambient atmosphere (thermal transpiration and humidity‐induced pressurization).A clear diel variation in pressurization and convective flow was observed; rates were highest in the afternoon and lowest at night, responding to ambient changes in light, temperature, and humidity. The resistance to airflow at the stem‐rhizome junction was very high for some species, resulting in a low ability to convert internal pressurization into convective airflow through the rhizomes. Species with a high potential for internal pressurization and a low internal resistance to convective flow seem to have a competitive advantage over species that rely exclusively on diffusive gas transport, which allows them to grow in deeper waters.

Estimating the gas partial pressure due to diffusive outgassing

The operating pressure of a leak-tight vacuum system is usually limited by the outgassing of materials in the system. A major source of this outgassing is frequently water, carbon oxides, and hydrocarbons which are bound to the surface and which may be partially removed with a relatively low-temperature bakeout. Diffusive outgassing from the interior of various materials may then become the major residual gas source particularly if the system is to be operated at an elevated temperature. Hydrogen outgassing from iron-based metals such as stainless steel, and water outgassing from plastic or polymeric materials are typical examples where diffusive outgassing processes have been demonstrated to be major gas sources. The diffusive transport of gas through materials is governed by the second order differential equation Dδ2C/δX2=δC/δt, known as Fick’s Law [W. Jost, Diffusion in Solids, Liquids, Gases (Academic, New York, 1960); R. M. Barrier, Diffusion In and Through Solids (Cambridge University, Cambridge, 1941); H. S. Carslaw, and J. C. Jaeger, Conduction of Heat in Solids, 2nd ed. (Clarendon, Oxford, 1959)]. Two alternate solutions of this equation are available which provide for simplified approximations for both the short-term and the long-term behavior, as well as for the exact numerical answer. An important aspect of true diffusive outgassing is its excellent predictability as compared to experiment. This permits calculating the outgassing rate following a known time-temperature-environment history. A simple graphical solution is demonstrated to accomplish this for the example of hydrogen outgassing from a thick walled stainless steel vessel operating at an elevated temperature following various bakeout schedules.

Effects of capillary red cell density on gas conductance of frog skin.

We tested experimentally the hypothesis that decreasing capillary red blood cell (RBC) density (dRBC) reduces the tissue diffusing capacity of frog skin to CO (DtiCO) and O2 (DtiO2). The effects of dRBC on CO2 transport were also assessed. C18O, O2, and CO2 transport between the skin and a cutaneous sample chamber on the belly of anesthetized (halothane) frogs (Rana pipiens) was measured by mass spectrometry, and the cutaneous conductances to C18O (GCO), O2 (GO2), and CO2 (GCO2) were calculated. The dRBC of the planar cutaneous capillary bed was measured by intravital fluorescent video microscopy. DtiCO and DtiO2 were calculated from a modification of the Roughton-Foster equation: 1/G = 1/Dti + 1/(theta RBC.dRBC), where theta RBC values were estimated from literature values. In one group of animals (n = 6), measurements were made before hemodilution (dRBC = 630 +/- 56 cells/mm2), after one hemodilution (dRBC = 349 +/- 50 cells/mm2), and after a second hemodilution (dRBC = 150 +/- 31 cells/mm2). In controls, time had no effect on GCO, GO2, or GCO2 (P greater than 0.42). Before hemodilution, GCO, GO2, and GCO2 were 0.069 +/- 0.010, 0.088 +/- 0.0012, and 1.23 +/- 0.010 nmol.min-1.Torr-1.cm-2, respectively, and lowering dRBC by hemodilution decreased all these parameters (P less than 0.025). The mean slopes of GCO, GO2, and GCO2 vs. dRBC were 6.0 +/- 1.3 x 10(-7), 7.2 +/- 2.3 x 10(-7), and 7.8 +/- 3.0 x 10(-6) nmol.min-1.Torr-1.RBC-1, respectively. Lowering dRBC also decreased DtiCO and DtiO2 (P less than 0.034). DtiCO and DtiO2 were 0.080 +/- 0.012 and 0.096 +/- 0.013 nmol.min-1.Torr-1.cm-2, respectively, before hemodilution. The mean slopes of DtiCO and DtiO2 vs. dRBC were 4.9 +/- 2.1 x 10(-7) and 6.5 +/- 2.8 x 10(-7) nmol.min-1.Torr-1.RBC-1, respectively. Hemodilution had no effect on perfused capillary density (P = 0.38). These results indicate that tissue diffusive conductance is proportional to dRBC. Regulation of dRBC may be an important mechanism modulating diffusive gas transport in tissue.

Convective and diffusive gas transport in canine intrapulmonary airways.

The significance of convective and diffusive gas transport in the respiratory system was assessed from the response of combined inert gas and particle boluses inhaled into the conducting airways. Particles, considered as "nondiffusing gas," served as tracers for convection and two inert gases with widely different diffusive characteristics (He and SF6) as tracers for convection and diffusion. Six-milliliter boluses labeled with monodisperse di-2-ethylhexyl sebacate droplets of 0.86-microns aerodynamic diameter, 2% He, and 2% SF6 were inspired by three anesthetized mechanically ventilated beagle dogs to volumetric lung depths up to 170 ml. Mixing between inspired and residual air caused dispersion of the inspired bolus, which was quantified in terms of the bolus half-width. Dispersion of particles increased with increasing lung depth to which the boluses were inhaled. The increase followed a power law with exponents less than 0.5 (mean 0.39), indicating that the effect of convective mixing per unit volume was reduced with depth. Within the pulmonary dead space, the behavior of the inert gases He and SF6 was similar to that of the particles, suggesting that gas transport was almost solely due to convection. Beyond the dead space, dispersion of He and SF6 increased more rapidly than dispersion of particles, indicating that diffusion became significant. The gas and particle bolus technique offers a suitable approach to differential analysis of gas transport in intrapulmonary airways of lungs.

Atmospheric pumping: A mechanism causing vertical transport of contaminated gases through fractured permeable media

As the barometer falls, gases are drawn upward out of the permeable Earth into the atmosphere. Conversely, a rising barometer pushes air downward. In a homogeneous permeable medium, these cyclical gas motions are piston‐like and nearly reversible, so they contribute only modestly to the net transport of contaminant gases. In a fractured permeable medium, however, the fractures will generally serve as breathing passages for all of the gas‐filled porosity, greatly increasing the amplitude and nonuniformity of vertical motions. The resulting transport process may be orders of magnitude more significant than molecular diffusion, according to the theoretical analysis presented here. Analytical solutions are first derived for the sinusoidal pressure response of a medium containing identical vertical fractures equally spaced by slabs of permeable matrix material. These solutions are then used to constrain the relationship between fracture aperture and fracture spacing, based on field comparisons between surface and subsurface pressure variations. The final phase of the analysis addresses the diffusive and advective transport of an inert trace gas which is carried by an oscillatory flow along a fracture having permeable walls. A maximum rate of transport is predicted to occur for an intermediate fracture spacing which is typically a few meters.

Contributions of diffusion jet flow and cardiac activity to regional ventilation in CFV

The magnitude and regional distribution of local gas transport during constant-flow ventilation (CFV) were quantified by imaging the washout of nitrogen 13 (13NN) from anesthetized and paralyzed mongrel dogs with positron emission tomography. Equal jet flows, through two 2-mm-ID bronchial catheters 1 cm distal to the carina, were adjusted to provide eucapnic CFV (total flow = 57.6 ml.s-1.kg-1). Basal, midheart, and apical transverse sections were studied in supine and prone anesthetized dogs. The ventilation per unit volume (sV) of selected areas was computed from local 13NN concentration vs. time curves during washout. To separate the regional contributions of CFV and cardiogenic oscillation to enhanced molecular diffusion, additional supine dogs were also studied during unilateral CFV. In this protocol the CFV jet flow was delivered to a single lung while the contralateral lung was left apneic. For each lung, washout data were obtained under CFV and apnea both living and postmortem animals. The local contributions of diffusion, CFV jet effects, and cardiac activity to gas transport were evaluated and tested for additive and multiplicative synergistic interactions. The regional distribution of gas transport during CFV was found to be highly nonuniform and characterized by higher ventilation to regions located close to the main bronchi and those located in the direction in which the CFV jet pointed. No major differences were observed between supine and prone positions. This regional pattern of ventilation distribution was found to be the result of complementary and nearly multiplicative interaction between the regional effects of the CFV jet, concentrated in the central airways, and the preferential cardiogenic gas transport enhancement in ventral regions close to the heart. The data were also analyzed with a model that divides the regional diffusive gas transport resistance into a central component, affected by the CFV jet, and a peripheral component, affected only by cardiac activity. This analysis showed substantial regional heterogeneities in the effects of the different gas transport mechanisms, which are consistent with the geometry of the bronchial tree and the location of the heart in the dog. The results indicate that regional nonuniformities must be considered when modeling gas transport in CFV.

Assessment of human exposure to gaseous pollutants.

A mathematical model to aid in assessment of human environmental exposure to volatile organic substances is presented. The model simulates the convective and diffusive transport of gas from the ambient environment into the human body by way of the respiratory and circulatory systems. Data required include easily obtained physical and chemical properties of substances as well as several estimated or measured physiological parameters. Transient and steady-state tissue concentrations resulting from an input atmospheric partial pressure are predicted. From these concentrations, an effective dose may be calculated, allowing for the determination of an exposure-response relationship based upon independently obtained dose-response data. The model's results compare favorably to experimental data on oxygen and halothane. Steady-state conditions are reached very rapidly. These results suggest that uptake of these substances is limited by both ventilation and perfusion. rates are demonstrated to be essentially linear within the current neighborhoods. Conditions in which the primary processes of ventilation, diffusion, perfusion, and elimination limit uptake of gases are considered. Expressions describing the conditions necessary for a single process to limit gas uptake are derived. Accompanying equations for estimating tissue concentrations under these limiting conditions are presented.

Diffusive gas transport through capillaries: A solution to the intermediate case based upon the kinetic theory of gases

Diffusion of gases through porous media is often treated in the continuum approximation of fluid dynamics. However, when the pore diameter is of the same order of magnitude as the mean free path of the molecules in the gas, the continuum approximation is no longer valid because of incorrect collision counting. This paper presents the first theoretical solution of this problem by strict use of the kinetic theory of gases. The diffusion coefficient as calculated herein exhibits a coupling term not present in the continuous fluid treatment.

Transport by gas-phase diffusion: lessons learned from the hen's egg.

Diffusive gas transport obeys laws which differ from those of convective transport. Diffusive gas transport can be described as an elite transport system because it not only distinguishes between O2, CO2, and water vapor molecules, but it is also influenced by barometric pressure and the presence of particular inert gas species. In the presence of air binary diffusion coefficients are applicable, but in the presence of He or SF6 effective diffusion coefficients must be used. By contrast, convective transport is an egalitarian transport system which conveys O2, CO2, and water vapor without discrimination at any altitude or in the presence of any inert gas mixture. Experiments in progress offer the opportunity to delineate for the first time precise diffusion-perfusion ratios and their effects upon the gas-space O2 and CO2 tensions.

Model simulation of single-breath washout of insoluble gases from dog lungs.

Intrapulmonary gas mixing by convection and diffusion is analyzed on the basis of various analog lung models incorporating singly or in combination series inhomogeneity, parallel inhomogeneity, intercompartmental diffusion, sequential inspiration, and sequential expiration. The slope of the alveolar plateau of insoluble gases is used as an indicator for incomplete gas mixing. By use of the models, we have attempted to simulate experimental results obtained in artificially ventilated dog lungs with simultaneous single-breath washout of He and SF6 (Meyer et al., J. Appl. Physiol. 55: 1795-1802, 1983). Sufficient agreement with experimental data is obtained only when diffusion between serial-parallel compartments and sequential expiration are incorporated, the first being mainly responsible for separation of He and SF6, the latter for the slope of the alveolar plateau of these gases. A combined operational model comprising two parallel compartments connected to a mixing compartment, with both convective and diffusive gas transport between the compartments, could account for most of the experimental observations.

Frequency-dependent response of ZrO 2 oxygen sensors to square-wave pulses of gas composition

The frequency response of ZrO 2 oxygen sensors to square-wave pulses of gas composition was studied on a laboratory flow reactor at 600°C. Square-wave frequencies between 0.001 and 5 Hz were applied to solenoid devices that switched alternate flow streams past the sensor. At frequencies <0.1 Hz, the sensor voltage is able to follow changes in bulk gas composition: during each half-cycle, the voltage attains values corresponding to the oxygen partial pressure in the individual flow streams. The sensor output cannot follow composition changes at 5 Hz. Here the voltage approaches a constant value characteristic of the average oxygen partial pressure of both gas streams. This behavior is viewed as a relaxation process involving bulk gas-boundary layer diffusive gas transport. Unusual features are seen when alternate gas streams contain reacting species. Voltage response to square-wave composition pulses resembles a pulse train whose “duty cycle” depends both on frequency and on the symmetry of composition excursions about a stoichiometric composition. A kinetic model is developed to account for this. Finally, it is shown that external coatings on the sensor surface lengthen the experimental relaxation time.

What is effective ΔP O 2 in a gas-phase diffusion system?

A diffusive gas transport system for the avian eggshell is described. This model proposes that the effective ambient P O 2 , acting as the pressure head for the inward O 2 flux, must be calculated on the assumption of complete water vapor saturation regardless of the humidity of the ambient environment.

Isotope effect in gas transport. I. Measurement of the free volume element in four polymers above the glass transition temperature

AbstractThe experimental observation of an isotope effect in diffusive gas transport across polymeric films is reported. The differences in energies of transport between hydrogen and deuterium are used to estimate the effective dimensions of characteristic minima or “wells” in the potential energy surface of rubbery polymers. The size of a well, determined by assuming it to be a cubical cavity, is interpreted as the effective free volume element vf, as measured by the hydrogen molecule probe. Estimations are made of the entropy of activation and “jump distance” for the hydrogen diffusion process based on vf values and experimentally determined pre‐exponential factors.