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PubMed1977

Diffusion of water vapor in binary and ternary gas mixtures at increased pressures.

Diffusion of water vapor was measured at 25 degrees C in the binary systems water vapor-He and water vapor-N2 at approximately 1, 4 10, 20 and 50 atm. Diffusion was also measured in the ternary system water vapor-He-O2 in both 2%O2-98%He, and at 1 and 20 atm. At 1 atm, comparison of binary diffusion coefficients (delta) shows that water vapor diffuses 3.3 times more readily in He than in N2. Values of delta at elevated pressures were reduced in proportion to 1/p, the absolute pressure, in qualitative agreement with Chapman-Enskog theory. However, the reduction in Delta was less than predicted at pressure above 4 atm in water vapor-N2 and above 20 atm in water vapor-He. The deviations from theory are more pronounced in N2 than in He, and become larger at higher pressures. When 26% O2 is present in the ternary system water vapor-He-O2, diffusion of water vapor is reduced to about 60% of its rate in pure He. In contrast, 2% O2 in He has little effect on diffusion of water vapor.

Diffusion↗

PubMed1980

Non-equimolar counter-diffusion in ternary gas systems.

Previous studies of multicomponent gas diffusion, using mathematical and physical models, have dealt with equimolar diffusion in closed systems. To simulate respiratory gas transport more faithfully, we have investigated ternary gas diffusion in open systems, which allow non-equimolar diffusion to occur. Theoretical results showed the effects of gas composition on the induced fluxes. A unique state in which the diffusion system satisfies both the equimolar and non-equimolar conditions was found for gas systems in which the net flux changes as the composition varies. To verify the non-equimolar film theory, we conducted experiments with a diffusion cell apparatus. Diffusional fluxes across a porous membrane were obtained for N2-O2-SF6, N2-O2-Ne, and N2-O2-He combinations. The experimental results agreed with the predicted values, validating the theoretical film approach.

Animals↗

PubMed1987

Alveolar-capillary diffusion of oxygen in dogs exercising in hypoxia.

To detect and quantify diffusion limitation in alveolar-capillary O2 transfer, measurements of pulmonary gas exchange were performed in 6 awake, chronically tracheostomized dogs (mean body weight 28.3 kg) breathing low O2 (arterial PO2 35-39 Torr), with or without CO2 added to inspired gas. From rest to exercise, with O2 uptake averaging 23 ml/(min X kg), the ideal-alveolar-to-arterial PO2 difference (PAiO2-PaO2) increased from 2.1 +/- 0.2 Torr (mean +/- SE) to 2.9 +/- 0.2 in hypoxia, and from 1.9 +/- 0.7 Torr to 3.0 +/- 0.5 in hypercapnic hypoxia. The apparent pulmonary O2 diffusing capacity (DLO2), calculated from O2 uptake, mixed-venous PO2, arterial PO2, and ideal-alveolar PO2, was found to be increased by hypercapnia and exercise. During exercise DLO2 averaged 74 ml/(min X Torr) in hypoxia and 76 ml/(min X Torr) in hypercapnic hypoxia. Because of the influence of inhomogeneity effects, these values should be considered as minimum values for the true pulmonary O2 diffusing capacity. When compared to the pulmonary CO diffusing capacity (DLCO) previously determined by C18O rebreathing in the same dogs in similar conditions, the DLO2/DLCO ratio averaged 1.2, thus being in accordance with the value predicted from the corresponding Krogh diffusion constant ratio. It is concluded that the DLO2 and DLCO values determined drug exercise in hypoxia may be considered to represent acceptable measures for alveolar-capillary diffusion conductance of lungs.

Animals↗

PubMed1990

CO2 back-diffusion in the rete aids O2 secretion in the swimbladder of the eel.

In order to estimate the relative importance of back-diffusion of O2 and CO2 for their partial pressure enhancement in the swimbladder, we have determined O2 and CO2 partial pressure and content and pH in microsamples collected non-obstructively from the afferent and efferent rete vessels in the European eel. 1. The PO2 increased significantly along the arterial vessels of the rete (from 33 to 303 Torr), with no change in O2 content, suggesting O2 not to be exchanged in the rete counter-current system. 2. A corresponding increase of PCO2 (from 4 to 35 Torr) was accompanied by a significant rise in CO2 content (from 8 to 15 mmol.L-1), suggesting significant CO2 back-diffusion in the rete. 3. Changes in PO2 during passage of blood through the swimbladder epithelium were variable and small, and the PO2 in rete venous blood was similar to that in rete arterial blood, explaining the lack of O2 back-diffusion. 4. Using blood CO2 dissociation data, about 70% of the rise in arterial CO2 content was estimated to derive from diffusion of CO2, the remaining 30% from diffusion of HCO3-, from venous to arterial rete capillaries, or from H+ transport in the opposite direction. The data indicate that CO2 back-diffusion in the rete does not only raise the rete arterial PCO2; it also reduces the O2 capacity (Root effect) and thus enhances the arterial PO2.

Air Sacs↗

PubMed1994

Ventilation-perfusion inequality and diffusion impairment in acutely injured lungs.

To assess the significant role of diffusion impairment and its unequal distribution in acutely injured lungs with alveolar flooding, oleic acid was intravenously injected into twenty-five mongrel dogs. The animals were divided into two groups, A and B. 0.1% CO in air was delivered, as an inspired gas, to the animals of group A. Simultaneously, saline containing a trace amount of six foreign inert gases was infused through a peripheral vein. While allowing the animals in group B to breathe air, saline containing ethylene, acetylene and freon 22 was infused. After injection of oleic acid, group A revealed increase in intrapulmonary shunt accompanied by a marked broadening of ventilation-perfusion (VA/Q) and diffusing capacity-perfusion (G/Q) distributions. A considerable amount of total cardiac output was received by the lung areas with low G/Q ratios where significant diffusion limitation was predicted to occur. Group B showed that excretion of freon 22 (gas with lower diffusivity) in injured lungs was considerably distorted as compared to those of ethylene and acetylene (gases with higher diffusivities), again ascertaining the importance of diffusion limitation in lungs with exudate in alveolar regions.

Animals↗

PubMed1996

Pulmonary transit time and diffusion limitation during heavy exercise in athletes.

To investigate relationships between pulmonary transit times (PTT) and pulmonary diffusion limitation during exercise, 10 high aerobic capacity athletes (VO2max = 5.15 +/- 0.52 l.min-1) who had multiple inert gas elimination analysis evidence suggestive of diffusion disequilibrium were studied at rest and maximal exercise. Diffusing capacity for oxygen (DLO2) was calculated from the inert gas data. First pass radionuclide angiography was performed using 99mTechnecium labeled erythrocytes and whole lung PTT and pulmonary blood volume (PBV) were calculated. PTT decreased from 9.32 +/- 1.41 sec at rest, to 2.91 +/- 0.30 sec during exercise and was correlated with diffusion limitation suggested by the inert gases (r = -0.58, P < 0.05). PBV increased during exercise to over 25% of whole blood volume and correlated with DLO2 (r = 0.82, P < 0.01). These data suggest that diffusion limitation is related to shortened PTT in athletes and that maximal recruitment of PBV may defend against diffusion limitation.

Adult↗

PubMed1992

Counter-diffusion of lactose and lactic acid in kappa-carrageenan/locust bean gum gel beads with or without entrapped lactic acid bacteria.

The effective diffusion coefficient (De) and equilibrium partition factor (Kp) for lactose and lactic acid in k-carrageenan (2.75% w/w)/locust bean gum (0.25% w/w) (LBG) gel beads (1.5-2.0 mm diameter), with or without entrapped Lactobacillus casei subsp. casei (L. casei), were determined at 40 degrees C. Results were obtained from transient concentration changes in well-stirred solutions of finite volume in which the beads were suspended. Mathematical models of unsteady-state diffusion into and/or from a sphere and appropriate boundary conditions were used to calculate effective diffusion coefficients of lactose and lactic acid from the best fit of the experimental solute concentration changes. The effective diffusivities of lactose and lactic acid were 5.73 x 10(-10) and 9.96 x 10(-10) m2 s-1, respectively. Furthermore, lactic acid was found to modify gel structure since lactose diffusion characteristics (De and Kp) differed significantly from an earlier study and in the literature. In gel beads heavily colonized with L. casei, the effective diffusion coefficients of lactose and lactic acid were respectively 17% and 24% lower than for cell-free beads. Partition coefficients also confirmed the obstruction effect due to the cells, and decreased from 0.89 to 0.79, and from 0.98 to 0.87, for lactose and lactic acid, respectively. External mass transfer was estimated by an unsteady-state model in infinite volume using the Biot number. The effect of external mass transfer resistance on De results and the data reported in the literature are discussed.

Carrageenan↗

PubMed1994

Brain maturation and high-energy phosphate diffusivity: alteration in cytosolic microenvironment and effective viscosity.

Maturational changes in intracellular brain phosphocreatine (PCr) transport were investigated using 31P-nuclear magnetic resonance diffusion spectroscopy. The diffusivities of PCr showed significant maturational facilitation in rat brain in vivo. Physicochemical analysis of the cytosol microenvironment as a multicomponent solution, where one of the components is a dilute polymer, indicated that the observed developmental facilitation of PCr diffusivity is likely to be due to a decline in the concentration of the free amino acid taurine. Changes in the concentrations of biopolymers (i.e., proteins or lipids) have only little effect, if any, on PCr diffusivity. PCr diffusivity values of rat brain measured in vivo showed excellent quantitative agreement with the predicted values estimated using a model for multicomponent diffusion. The study confirmed that the taurine/N-acetylaspartate exchange observed during postnatal development of rat brain plays a major, it not unique, role in maturational facilitation of intracellular high-energy phosphate transport.

Albumins↗

PubMed1993

Diffusion of minimally invasive therapy in The Netherlands.

In the context of the European project, the diffusion of 10 selected cases of minimally invasive therapy (MIT) in the Dutch health care system was documented and analyzed. In each case the diffusion pattern resulting from the balance of stimulating and impeding factors is described, and summarized in a matrix framework. Budgetary pressures, lack of evidence on effectiveness and physician conservatism are the most important factors impeding diffusion of MIT. Patient demand, media reporting and presence of medical innovators are important diffusion-stimulating factors. Finally, a judgment is made on the policy implications of these findings, based on a comparison of the extent and speed of the diffusion of each case, the quality of evidence on effectiveness and cost-effectiveness and, in a few cases, existing policies. In most cases, more active policy making, either stimulating or otherwise guiding diffusion, would be desirable.

Angioplasty, Balloon, Coronary↗

PubMedDate not supplied

Diffusion of monomers from bonding resin-resin composite combinations through dentine in vitro.

OBJECTIVES: Previous work has demonstrated diffusion of the monomer triethylene glycol dimethacrylate (TEGDMA) from resin composite through dentine in vitro. The objective of the present work was to examine monomer diffusion from bonding resins and resin composites used in combination. METHODS: Occlusal cavities were prepared in tooth crowns and restored with bonding resin-resin composite combinations. Aqueous samples from the 'pulpal chamber' of each tooth were removed at timed intervals for analysis by reversed phase-high performance liquid chromatography and mass spectrometry. RESULTS: Bonding resins contributed to monomer diffusion. A TEDGMA-containing bonding resin used in combination with a TEGDMA-containing resin composite hastened and increased TEGDMA diffusion through dentine. A 2-hydroxyethyl methacrylate (HEMA)-containing bonding resin used in combination with a TEGDMA-containing resin composite reduced TEGDMA diffusion only slightly compared with the resin composite alone and added substantial diffusion of HEMA. CONCLUSION: The bonding resins tested contributed to monomer passage to the pulp space and did not prevent movement of monomer from resin composites to the pulp.

Adolescent↗

PubMed1986

The effect of diffusion on the trapping of membrane-bound receptors by localized coated pits.

Localized coated pits are considered in the primary steps of receptor-mediated endocytosis. According to the pit reinsertion mechanism, we have modified our previous kinetic model and studied the effect of diffusion on the trapping rate constant (k+). Using experimental data for low density lipoprotein (LDL) receptors on fibroblast cells, we found that the binding of receptors to coated pits is not totally diffusion controlled. For example, the process is less than 78% diffusion controlled if receptors are not allowed to escape the coated pits. However, due to the large uncertainties in the experimental parameters, a diffusion-controlled process cannot be ruled out. The greatest differences between localized and random reinsertion were found when the escaping rate constant (k-) is much greater than the rate constant for invagination of the pits (lambda 1). Under this condition, k+ for localized reinsertion is no less than 39% diffusion controlled, while k+ for random reinsertion shows no diffusion effect at all.

Cell Membrane↗

PubMed1996

Diffusion of albumins in rat cortical slices and relevance to volume transmission.

The apparent diffusion coefficient, D*, was measured in rat cortical slices and compared to the free diffusion coefficient, D, for three negatively charged proteins, lactalbumin (mol. wt = 14,500), ovalbumin (45,000) and bovine serum albumin (66,000). The temporal evolution of the spatial distribution of albumin molecules labeled with the Texas Red fluorophore was determined using integrative optical imaging at intervals after a brief pressure injection from a micropipette in slices of adult rat cerebral cortex and dilute agarose gel. Diffusion coefficients were obtained by fitting appropriate equations to the data. In slices at 34 degrees C, the values of D* (10(-7) cm2/s, mean +/- S.E.M.) for lactalbumin, ovalbumin and bovine serum albumin were 2.37 +/- 0.10, 1.60 +/- 0.08 and 1.63 +/- 0.07, respectively. In agarose gel, values of D (10(-7) cm2/s) were 11.87 +/- 0.20, 10.02 +/- 0.25 and 8.29 +/- 0.17, respectively. From these data the tortuosity factors, (D/D*)0.5, were calculated, with 2.24 obtained for lactalbumin, 2.50 for ovalbumin and 2.26 for bovine serum albumin. Previous optical measurements using dextrans with mol. wts of 40,000 and 70,000 gave tortuosities of 2.16 and 2.25, but in contrast previous determinations with ion-selective microelectrodes using the small cation tetramethylammonium (mol. wt = 74.1) give tortuosities of about 1.6. The results show that proteins as large as bovine serum albumin diffuse through brain extracellular space but are more hindered than smaller molecules. A simple model compared the differences in diffusion properties of bovine serum albumin, dopamine and nitric oxide in brain tissue and discussed the implications for volume transmission of chemical information between cells. The results are also relevant to the behavior of diffusible factors in brain development and the delivery of therapeutic agents.

Albumins↗

PubMed1995

Quantitative magnetic resonance flow and diffusion imaging in porous media.

Quantitative flow and diffusion measurements have been made for water in model porous media, using magnetic resonance micro-imaging methods. The samples consisted of compacted glass beads of various sizes down to 1 mm diameter. Typical flow and diffusion images exhibited a spatial resolution of 117 microns x 117 microns and velocities in the range 1-2 mm/s. Comparison of volume flow rates calculated from the flow velocity maps with values measured directly yielded good agreement in all cases. There was also good agreement between the mean diffusion coefficient of water calculated from the diffusion maps and the bulk diffusion coefficient for pure water at the same temperature. In addition, the mean diffusion coefficient did not depend on the pore sizes in the bead diameter range of 1-3 mm. Our results also show that partial volume effects can be compensated by appropriate thresholding of the images prior to the final Fourier transformation in the flow-encoding dimension.

Diffusion↗

PubMed1993

Solid-state NMR studies of 1H spin diffusion in adsorbed organic molecules.

1H spin diffusion times of toluene (MB) and tetrahydrofuran (THF) adsorbed on a series of porous solids (charcoal, SiO2 and Al2O3) were measured by a selective inversion technique. The experimental results show that they cover a wide range (from less than one millisecond to several hundreds of milliseconds). For all samples, a tri-exponential behavior was observed in the magnetization recovery processes of the negative peaks. This is attributed to the existence of the two different kinds of spin diffusion processes in addition to the T1 relaxation. One is assigned to the intermolecular spin diffusion between the surface acidic protons of the adsorbent and the organic molecules adsorbed on the solid surface, the other to the intramolecular spin diffusion of adsorbed molecules. Due to hydrogen bonding between the surface hydroxyl groups and the adsorbate, the intermolecular spin diffusion of THF adsorbed on various solids is more effective compared to that of adsorbed MB. In addition, the intermolecular 1H spin diffusion between charcoal and adsorbed THF molecules was confirmed by indirect measurement suggested by Tekely et al.

Charcoal↗

PubMed2003

Dependence of plastoquinol diffusion on the shape, size, and density of integral thylakoid proteins.

The diffusion of plastoquinol in the chloroplast thylakoid membrane is modelled using Monte Carlo techniques. The integral proteins are seen as obstacles to diffusion, and features of percolation theory emerge. Thus, the diffusion coefficient diminishes with increasing distance and there is a critical threshold of protein concentration, above which the long-range diffusion coefficient is zero. The area occupied by proteins in vivo is assessed and appears to be around this threshold, as determined from calculations assuming randomly distributed noninteracting proteins. Slight changes in the protein arrangement lead to pronounced changes in diffusion behaviour under such conditions. Mobility of the proteins increases the protein occupancy threshold, while boundary lipids impermeable to PQ diffusion decrease it. Further, the obstruction of plastoquinone/plastoquinol binding sites in a random arrangement is evaluated.

Binding Sites↗

PubMed2006

Serial diffusion-weighted imaging of neonatal herpes encephalitis: a case report.

We reported a patient with neonatal herpes simplex encephalitis in whom diffusion-weighted imaging was performed repeatedly. Diffusion-weighted imaging at 20h after the onset of seizures revealed scattered small spotty high intensity lesions in both hemispheres and a high intensity area in the left fronto-temporal lobe. There was no abnormal finding on conventional magnetic resonance imaging. Second diffusion-weighted imaging 72h after the onset revealed expanded scattered high intensity lesions in the bilateral hemisphere, a high intensity area in the left fronto-temporal lobe, and a new high intensity area in the right temporal lobe. There was no report on neonatal herpes simplex encephalitis that showed scattered high intensities in diffusion-weighted imaging. Scattered small high intensities on diffusion-weighted imaging may suggest endothelial cell infection with swelling and small vessel necrosis. Early diffusion-weighted imaging will be valuable for early detection and diagnosis of neonatal herpes simplex encephalitis.

Brain↗

PubMed2004

Determination of time-dependent partition coefficients for several pesticides using diffusion theory.

Diffusion-retarded partitioning of pesticides with aggregated soils results in a time-dependent partition coefficient (Kd') which is different at equilibrium from the partition coefficient derived from conventional 24-h batch studies (Kd) measured on dispersed soil. An experiment was undertaken to determine the importance of Kd' for the prediction of pesticide concentrations in solutions bathing artificial soil aggregates and to determine whether diffusion theory could accurately predict the concentrations. Two clay soils were mixed with polyacrylamide to create artificial aggregates of 0.8, 1.4 and 1.7 cm diameter when dry. After saturation, the aggregates were immersed in solutions containing isoproturon or a mixture of isoproturon, chlorotoluron and triasulfuron. The decline with time of the pesticide concentrations in the bathing solution was monitored and the results were compared with predictions from a diffusion-based model. The effective diffusion coefficients of the compounds were obtained by either fitting the non-linear diffusion model to the data (D(ef)) or by independent calculations based on the properties of the compounds and of the aggregates (D(ec)). The diffusion model was able to predict the temporal variation in pesticide concentrations in the bathing solution reasonably well whether D(ef) or D(ec) values were used. However, equilibrium concentrations in solution were sometimes overestimated due to increased sorption with time at the particle scale. Overall, the ratio between D(ef) and D(ec) ranged from 0.23 to 0.95 which was a reasonable variation when compared to the range of aggregate sizes used in the experiments and of the Kd values of the compounds.

Acrylic Resins↗

PubMed2006

Parallel pore and surface diffusion of levulinic acid in basic polymeric adsorbents.

The equilibrium and kinetics of levulinic acid (LA) adsorption on two basic polymeric adsorbents, 335 (highly porous gel) and D315 (macroreticular), were investigated. Experimental adsorption rates in batch stirred vessels under a variety of operating conditions were described successfully by the parallel pore and surface diffusion model taking into account external mass transfer and nonlinear Toth isotherm. The film-pore diffusion model was matched with the rate data and the resulting apparent pore diffusivities were strongly concentration-dependent and approached to a constant value for 335 adsorbent. Thus, the constant value was taken as the accurate pore diffusivity, while the pore diffusivity in D315 was estimated from the particle porosity. The surface diffusivities decreased with increasing initial bulk concentration for both adsorbents. The inverse concentration dependence was correlated reasonably well to the change of isosteric heat of adsorption as amount adsorbed.

Adsorption↗