Oxygen Diffusion Model Research Articles

Effect of Acute Hyperglycemia on Oxygen and Oxidative Metabolism in the Intact Cat Retina

The Crabtree effect is the phenomenon of inhibition of respiration by glycolysis, as a result of elevated glucose levels. It is not certain whether the Crabtree effect occurs in the retina, which has a high glycolytic capacity. In the current study, in vivo photoreceptor oxygen consumption was examined during the normo- and hyperglycemic states in the dark-adapted cat retina to determine whether the Crabtree effect occurs in the outer retina. Spatial profiles of oxygen tension were obtained in the cat retina, in vivo, with the use of oxygen microelectrodes during control conditions and acute (5.19 +/- 0.83 hour) episodes of hyperglycemia (blood glucose, >350 mg/dL). The outer retinal portions of the profiles were fitted to a model of oxygen diffusion to quantify photoreceptor oxygen consumption. Photoreceptor oxygen consumption did not significantly change during hyperglycemia compared with control conditions. Choroidal PO(2) decreased during hyperglycemia by an average of 5.8 +/- 7.4 mm Hg. This led to an increase in the fraction of O(2) used by the photoreceptors that was derived from the inner retina. Choroidal PO(2) did not recover when blood glucose levels were returned to normal. Average inner retinal PO(2) was not affected by the episodes of hyperglycemia. The Crabtree effect does not occur to any significant degree in the outer retina, because hyperglycemia did not affect photoreceptor oxygen consumption. Choroidal PO(2) decreased during hyperglycemia, and the oxygen deficit was made up by the retinal circulation.

Modeling of oxygen diffusion and metabolism from capillary to muscle.

In the bioengineering and sports training field, there are interests in obtaining information on oxygen transport and metabolism during muscle exercise. The goal of our study was to develop a time-dependent model which can simulate the changes in oxygen diffusion and metabolism in muscle tissue, especially from capillary to mitochondria. We postulate that oxygen diffuses into the tissue at a rate proportional to the oxygen concentration in plasma when applied to a steady state condition. As we know, any build up of oxygen in the tissue is directly related to oxygen leaving the capillary into the tissue and the consumption of oxygen by the mitochondria, or the metabolic demand. We designed a model which consisted of four compartments: (1) capillary; (2) interstitial space between capillary and myofibril tissue cell; (3) parenchymal cell through myofibril to mitochondria in muscle tissue; (4) at mitochondria. Oxygen comes from arteriole to capillary, then diffuses to mitochondria through interstitial space and myofibril. Oxygen is reduced to water in mitochondria, and ATP is generated through proton transport. Our model simulates the measured oxygen uptake during rest and muscle exercise as the input oxygen uptake represented as H2O formation.KeywordsOxygen ConcentrationOxygen DiffusionInterstitial SpaceOxygen TransportDifferential Equation ModelThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Calibrated histochemistry applied to oxygen supply and demand in hypertrophied rat myocardium.

Oxygen supply and demand of individual cardiomyocytes during the development of myocardial hypertrophy is studied using calibrated histochemical methods. An oxygen diffusion model is used to calculate the critical extracellular oxygen tension (PO(2,crit)) required by cardiomyocytes to prevent hypoxia during hypertrophic growth, and determinants of PO(2,crit) are estimated using calibrated histochemical methods for succinate dehydrogenase activity, cardiomyocyte cross-sectional area, and myoglobin concentration. The model calculation demonstrates that it is essential to calibrate the histochemical methods, so that absolute values for the relevant parameters are obtained. The succinate dehydrogenase activity, which is proportional to the maximum rate of oxygen consumption, and the myoglobin concentration hardly change while the cardiomyocytes grow. The cross-sectional area of the cardiomyocytes, which increases up to threefold in the right ventricular wall due to pulmonary hypertension in monocrotaline-treated rats, is the most important determinant of PO(2,crit) in this model of myocardial hypertrophy. The relationship between oxygen supply and demand at the level of the cardiomyocyte can be investigated using paired determinations of spatially integrated succinate dehydrogenase activity and capillary density. Hypoxia-inducible factor 1alpha can be demonstrated by immunohistochemistry in cardiomyocytes with high PO(2,crit) and increased spatially integrated succinate dehydrogenase activity, indicating that limited oxygen supply affects gene expression in these cells. We conclude that a mismatch of oxygen supply and demand may develop during hypertrophic growth, which can play a role in the transition from myocardial hypertrophy to heart failure.

Electrochemical topotactic oxidation of nonstoichiometric perovskites at ambient temperature

Different approaches to describe nonstoichiometry in perovskite-related oxides, oxygen diffusion models and kinetic data on electrochemical intercalation of oxygen in perovskite-related oxides are considered in the present study. It is demonstrated that the formation of microheterogeneous systems is characteristic of nonstoichiometric perovskite-related oxides. The adequate models should be considered for the proper characterization of the low temperature oxygen transport in oxides. These models should account for the high concentration of the high diffusivity paths in the sample. For microdomain textured Ca0.5Sr0.5FeO2.5 perovskite as example, the kinetic analysis of electrochemical oxygen intercalation is carried out.  2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Radiation response and cure rate of human colon adenocarcinoma spheroids of different size: the significance of hypoxia on tumor control modelling

Purpose: To evaluate the adequacy of a Poisson tumor control probability ( tcp) model and the impact of hypoxia on tumor cure. Methods and Materials: A human colon adenocarcinoma cell line, WiDr, was grown as multicellular spheroids of different diameters. Measurements were made of cell survival and spheroid cure following 300-kV X-ray external beam irradiation in air and nitrogen. Cell survival data were fitted using a two-compartment and an oxygen diffusion model. Spheroid cure data were fitted using the tcp model. Results: Hypoxia was seen only for spheroids greater than 500 μm in diameter. For small spheroids tcp estimates of radiosensitivity and clonogenic number showed excellent agreement with experimentally derived values. For large spheroids, although tcp estimates of radiosensitivity were comparable with measurements, estimates of the clonogenic number were considerably lower than the experimental count. Reoxygenation of large spheroids before irradiation resulted in the tcp estimates of the number of clonogenic cells agreeing with measured values. Conclusions: When hypoxia was absent, the tcp model accurately predicted cure from measured radiosensitivity and clonogen number. When hypoxia was present, the number of cells capable of regrowth in situ was considerably lower than the number of clonogenic cells that initially survived irradiation. As this counteracted the decreased radiosensitivity, hypoxia was less important for cure than predicted from cell survival assays. This finding suggests that chronic hypoxia may not limit directly the success of radiation therapy.

Alpha case thickness modeling in investment castings

The alpha case thickness at the surface of a Ti-6Al-4V (wt pct) step wedge investment casting has been measured and successfully predicted. The prediction uses temperature-time results obtained from a heat flow simulation of the casting. The temperature-time results were coupled to a simple model for diffusion of oxygen into the beta phase during continuous cooling. Oxygen concentration and microhardness profiles were measured from the surface in contact with the ZrO2 face coat of the shell mold into the interior of the casting. The oxygen content in the metal at the shell mold interface was between 5 and 9.5 wt pct in general agreement with a thermodynamic calculation for bcc Ti in contact with ZrO2. At the limit of the alpha case region, as determined by standard metallographic technique, the oxygen concentration was found to be no more than 0.02 wt pct above the level of oxygen in the bulk alloy. Using this information and one particular literature value for the activation energy for diffusion of oxygen, a nearly linear relationship was obtained between the measured and predicted alpha case thicknesses at various positions on the casting surface. Reduction of the prefactor of this diffusion coefficient by a factor of 7.5 produces excellent agreement between predicted and measured alpha case thicknesses. Such a reduction is not inconsistent with the scatter of literature values for the diffusion coefficient.

Oxidation and hardness profile of V–Ti–Cr–Si–Al–Y alloys

Several alloys of composition V–4Ti–4Cr–(0–0.5)Si–(0–0.5)Al–(0–0.5)Y were selected for oxidation in air. In addition a V–4Ti–4Cr–0.5Si–0.5Al–0.5Y alloy was pre-implanted with helium to study the effect of helium on oxidation behavior. Rapid oxidation experiments in air were conducted on tensile specimens and disc bend specimens at 300°C, 500°C, 600°C and 700°C for 1 h. After oxidation, tensile specimens were mounted in cross-section, and micro-indentation hardness tests were conducted. The depths of increased hardness obtained by indentation tests and the thickness of cleavage fracture zones measured by using scanning electron microscopy (SEM) micrographs were similar. The hardness profile of the cross-section in the depth direction corresponded to a model of oxygen diffusion in vanadium. Three-point bend tests were conducted on helium-implanted and air- exposed disc specimens. The bending stress of the specimen with 700 appm helium was larger than that with 70 appm helium.

Oxygen permeation through an oil-encapsulating glassy food matrix studied by ESR line broadening using a nitroxyl spin probe

A non-invasive method based on the broadening of electron spin resonance (ESR) lines in the presence of oxygen (oximetry) has been developed to determine the rate of permeation of oxygen from head space into oil, encapsulated in a glassy matrix (a food model made from sucrose, maltodextrin and gelatine by freeze-drying). The lipophilic nitroxide 16-doxylstearic acid, 16-DSA, was used as a spin-probe, and it was found to be concentrated mainly in the oil phase in the glassy matrix. The concentrations of oxygen in the freshly made glasses were found to be similar to the concentration in atmospheric air, and the process of freeze-drying is apparently not able to remove oxygen before the glassy system solidifies. Storing the oil-encapsulating glasses under oxygen increased the oxygen concentration inside the matrices, and the rate of permeation was found to increase with temperature. A kinetic model for the oxygen permeation was established, based on the rate data obtained up to full saturation of the oil with oxygen below the glass transition temperature (Tg=65°C ), and on data for partial oxygen saturation above the glass transition temperature. The kinetic model includes a temperature independent master curve and allows for structural heterogeneity. The energy of activation for oxygen permeation was found to be 74±6 kJ/mol for the glassy matrix, and the large value is in favour of the molecular model for oxygen diffusion rather than the free volume model, and accords with the zeroth-order kinetics for oxidation of lipids encapsulated in a glassy matrix, which has previously been observed to be associated with oxygen permeation as the rate-determining step.

On the estimation of oxygen concentration in biological cells based on noisy boundary observations

The free boundary problem is one of the important nonlinear problems in various fields of engineering, including thermal, materials, and hydraulic engineering. In this paper, modeling of the free boundary problem in oxygen diffusion problems in biological cells and the estimation problem of oxygen concentration in biological cells are considered. To date, under the assumption that the rate of oxygen absorption in cells is constant, mathematical modeling of oxygen diffusion phenomena in biological cells has been undertaken. In a real situation, however, depending on the activity of each cell, the absorption rate of oxygen changes and the change is generally random. Thus, in this paper, considering the random change of the rate of oxygen absorption, we propose a mathematical model of oxygen diffusion in biological cells. By using the proposed model, we study the method of estimation of oxygen concentration in cells in the presence of noisy observed data at the outer layer of the skin. © 2000 Scripta Technica, Electron Comm Jpn Pt 3, 83(7): 53–61, 2000

Atmospheric oxidation of the pyritic waste rock in Maardu, Estonia. 1 field study and modelling

A field survey and modelling of the oxidation and carbonate buffering reactions inside the alum-shale-containing waste rock dumps located in Maardu, Estonia, was accomplished. In the slope areas, the shale has been altered at high temperatures due to the spontaneous combustion and the pyritic acidity has been eliminated through migration of SOx gases out from the dump. In the central parts of the waste rock plateaus, low temperature pyrite oxidation fronts develop towards the dump depth and towards the centres of individual shale lumps. The main secondary phases precipitating in the weathering profile are gypsum, ferric oxyhydroxide, K-jarosite and smectite. The respective field data made it possible to calibrate the two-stage oxygen diffusion model and the characteristic pyrite oxidation rate 0.06–0.08 mol of pyrite reacted per kg of available water (pyrox/H2O value) was estimated to describe the first tens of years of dump performance. The model is capable to compare different shale disposal strategies that are illustrated with two case scenarios. The buffering of sulphuric acid by Mg-calcite appears to be an incongruent reaction with gypsum precipitating that leads to the build-up of the high Mg/Ca ratio in the leachate. Application of the Mg/Ca method estimates the pyrox/H2O value in the range of 0.05–0.14 mol/kg.

Re-Evaluation of the Oxygen Diffusion Model for Predicting Minimum Contact Lens Dk/t Values Needed to Avoid Corneal Anoxia

(1) To update Fatt's mathematical model of the distribution of oxygen tension (pO2) across the cornea and contact lens (CL) to include the recent finding that corneal oxygen consumption increases with the acidification that occurs with CL wear. (2) To estimate the minimum transmissibility (CL Dk/t) to avoid epithelial anoxia or to avoid stromal anoxia. A five-layer static and one-dimensional mathematical model of oxygen diffusion through the cornea based on Fatt's models was used. The relationships between acidosis and increased QO2, and acidosis and CL Dk/t were used to estimate corneal QO2 for a given CL Dk/t. (1) Revised model predictions are in agreement with direct tear pO2 measurements beneath CLs in the rabbit. (2) For the human eye, the minimum CL Dk/t for oxygen delivery to the basal epithelial cells was determined to be 23 for the open eye and 89 for the closed eye. To prevent anoxia throughout the entire corneal thickness the Dk/t requirements are 35 for the open eye and 125 for the closed eye. (1) Model predictions of the oxygen distribution beneath contact lenses are significantly lower than previous models that did not include the effect of acidosis on corneal QO2. (2) Minimum Dk/t values that allow oxygen delivery to the basal epithelium are in agreement with the Dk/t needed to avoid corneal edema.

In situ electrochemical oxygen generation with an immunoisolation device.

The viability and function of transplanted tissue encapsulated in immunobarrier devices is subject to oxygen transport limitation. In this study, we have designed and used an in situ electrochemical oxygen generator which decomposes water electrolyticaly to provide oxygen to the adjacent planer immunobarrier diffusion chamber. The rate of oxygen generation, which increases linearly with electrical current, was accurately controlled. A theoretical model of oxygen diffusion was also developed and was used to calculate the oxygen profiles in some of the experimental systems. In vitro culture experiments were carried out with beta TC3 cells encapsulated in titanium ring devices. The growth and viability of cells with or without in situ oxygen generation was studied. We found that under otherwise similar culturing conditions, the thickness of the cell layer and the viability of cells was the highest in devices cultured in stirred media with oxygen generation, even though the thickness had not reached the theoretically predicted value, and lowest in those unstirred and without oxygen generation.

Attenuation of the Photobaric‐Photoacoustic Signal in Leaves by Oxygen‐Consuming Processes

AbstractCharland et al. (Biochim. Biophys. Acta 1992, 1098, 261–265) obtained photoacoustic data from sugar maple tree leaves, in which the photobaric part of the photoacoustic signal declined in time following a transition from high light to low light level, which they interpreted as indicating stromal and plasmal oxygen‐consuming processes. Here, a simple mathematical model of oxygen diffusion, which includes a continuous distribution of oxygen‐consuming sinks in the diffusion path from the photosynthetic membrane to the inner air phase, is presented. The model explains the main features of the dependence of the steady‐state signal on the modulation frequency and the light intensity, although the numerical agreement between the data and the results of the model is only semi‐quantitative, which is discussed. It turns out that at sufficiently high light intensity, or at a short time after a previous exposure to a high light level, the effect of oxygen consumption tends to zero because a high stromal oxygen concentration is built up which is saturating for the uptake process. Within this limitation, the merit of the photoacoustic signal as an indicator for photosynthetic oxygen evolution is preserved, answering recent doubts.

A finite-element model of oxygen diffusion in the pulmonary capillaries.

We determined the overall pulmonary diffusing capacity (DL) and the diffusing capacities of the alveolar membrane (Dm) and the red blood cell (RBC) segments (De) of the diffusional pathway for O2 by using a two-dimensional finite-element model developed to represent the sheet-flow characteristics of pulmonary capillaries. An axisymmetric model was also considered to assess the effect of geometric configuration. Results showed the membrane segment contributing the major resistance, with the RBC segment resistance increasing as O2 saturation (SO2) rises during the RBC transit: RBC contributed 7% of the total resistance at the capillary inlet (SO2 = 75%) and 30% toward the capillary end (SO2 = 95%) for a 45% hematocrit (Hct). Both Dm and DL increased as the Hct increased but began approaching a plateau near an Hct of 35%, due to competition between RBCs for O2 influx. Both Dm and DL were found to be relatively insensitive (2-4%) to changes in plasma protein concentration (28-45%). Axisymmetric results showed similar trends for all Hct and protein concentrations but consistently overestimated the diffusing capacities (approximately 2.2 times), primarily because of an exaggerated air-tissue barrier surface area. The two-dimensional model correlated reasonably well with experimental data and can better represent the O2 uptake of the pulmonary capillary bed.

Oxygen gas-sensing behaviour of V2O5-SnO-TeO2 glass

The d.c. conductivity, σ, and the oxygen gas-sensing behaviour of V2O5–SnO–TeO2 glass prepared by press-quenching were studied in argon and oxygen gas atmospheres at temperatures ranging from 303–473 K. The glass of 50V2O5·20SnO·30TeO2 (mol %) was n-type semiconducting. The high-temperature conductivity was lower in oxygen and higher in argon than that in air. This was explained by the V4+ ions in the glass being oxidized by oxygen which had diffused into the glass, resulting in an increase in V5+ with time. The experimental relationship between σ and oxygen partial pressure, PO2, agreed quantitatively with the theoretical relation σ ∝ PO2-1/4. Changes in conductivity by switching the atmospheres between oxygen and argon gases were found to be reproducible. From the data of these dynamic changes, the oxygen gas sensitivity, S, at 473 K was obtained to be 1.3 in oxygen atmosphere. The dynamic changes could be quantitatively explained by an oxygen diffusion model. Throughout these discussions, the present tellurite glass was found to possess a potential applicability as an oxygen gas sensor.

Myocardial oxygen tension and capillary density in the isolated perfused rat heart during pharmacological intervention.

Oxygen is essential for normal cardiac function and plays an important role in cardiac regulation. Electron paramagnetic resonance (EPR) oximetry appears to have some significant advantages for measuring oxygen tension (pO2) in the beating heart. This study presents the serial measurement of myocardial pO2 by EPR oximetry in the isolated crystalloid perfused heart during treatment with different cardioactive drugs: dobutamine, metoprolol, verapamil, vasopressin, and N omega-Nitro-L-Arginine Methyl Ester (L-NAME). Baseline myocardial pO2 was 176 +/- 14 mmHg (mean +/- S.E.). Myocardial capillary density in the intact contracting heart was calculated to be 2300 +/- 100 mm-2, using local myocardial pO2 and a cylindrical model for oxygen diffusion in tissue. Each drug had characteristic effects on myocardial pO2, myocardial oxygen consumption (MVO2), and capillary density. Metoprolol and verapamil increased myocardial pO2 by 51% and 18%, respectively, dobutamine decreased myocardial pO2 by 84% while vasopressin and L-NAME had no significant effect on myocardial pO2. Metoprolol and verpamil decreased MVO2 by 9% and 56%, respectively, while dobutamine increased MVO2 by 59%. A quantitative comparison of effects on the capillary bed based on changes in myocardial pO2 and MVO2 was made. Metoprolol and verapamil had opposite effects on the capillary bed. Verapamil decreased myocardial capillary density by 39%, while capillary density increased by 10% (n.s.) with metoprolol. Data following perfusion without drug is also presented. We conclude that: 1) The application of EPR oximetry with LiPc provides dynamic evaluation of local myocardial pO2 in the contracting heart. 2) Using a cylindrical model of oxygen delivery and diffusion in tissue, these data may be used to describe the changes of capillary density during pharmacological interventions.

Size Principle of Striated Muscle Cells

We have investigated the relationship between cross-sectional area (CSA) and maximum rate of oxygen consumption (VO2max, in nmol .mm-3 .s-1) of heart and skeletal muscle cells from different species. VO2max and CSA were determined for single muscle fibres of Xenopus laevis at 20°C and for cardiomyocytes in thin trabeculae dissected from the right ventricle of rats at 38°C. Succinate dehydrogenase activity was determined using a quantitative histochemical method to estimate VO2max in mammalian skeletal muscle fibres. Literature values of volume density of mitochondria were used to estimate VO2max in some mammalian cardiomyocytes. We found that an inverse relationship exists between VO2max (range 1.5 to 0.024 nmol . mm-3 . s-1) and cross-sectional area (0.0002 to 0.018 mm2, respectively): VO2max = constant/CSA, where the value of the constant equals 0.39±0.18 pmol . mm-1 .s-1 (mean ± S.D., n = 14). Such a relationship is predicted by a simple Hill-type model for oxygen diffusion in cilindrical cells, if it is assumed that muscle cells are evolved so that anoxic cores in muscle cells are prevented. This indicates that the product of endurance of muscle cells (which is proportional to VO2max) and force production (which is proportional to cross-sectional area) is limited by oxygen diffusion, and that adaptation of heart and skeletal muscle cells to increased workload is limited by the interstitial oxygen tension.

Oxygen gas-sensing properties of V2O5-Sb2O3-TeO2 glass

The oxygen gas-sensing behavior, as measured by the d.c. conductivity σ, of 62.3V2O5·4.3Sb2O3·33.3TeO2 glass (mol%) prepared by press-quenching was studied in Ar and O2 gas atmospheres at temperatures between 303 and 473 K. The glass was n-type semiconducting. The conductivity was lower in O2 and higher in Ar than that in air. The explanation for this was that V4+ ions in the glass were oxidized by the O2 gas which had diffused into the glass, resulting in an increase in V5+ with time. The experimental relationship between σ and the oxygen partial pressure PO2 agreed quantitatively with the theoretical relation σαPO2−1/4. Variations in conductivity on switching the atmospheres between O2 and Ar gases were found to be reproducible, and the O2 gas sensitivity S at 473 K was found to be 1.2. These dynamic changes could be explained quantitatively by an oxygen diffusion model. From discussions on the gas-sensing behavior, the present glass was revealed to be potentially applicable as an O2 gas sensor.

Phase separation kinetics in La2CuO4+? and inhomogeneous hole doping in the antiferromagnetic regime (0 <x < 0.02) of La2?x Sr x CuO4

The diffusion of the excess oxygen during phase separation in La2CuO4+δ was studied using thermal history-dependent normal state magnetic susceptibilityϰ(T, t) measurements versus temperatureT and timet as a probe. A large thermal hysteresis ofϰ(T) was observed for La2CuO4.044 between data obtained after quenching to 5 K and then warming, and data obtained while or after slowly cooling from 300 K. A model for the excess oxygen diffusion is presented, from which theϰ(T, t) data yield aT-independent activation energy of 0.24(3) eV for the diffusion coefficient of the excess oxygen from 150 to 220 K. In related work, we have used139La NQR andμSR measurements to probe the antiferromagnetic (AF) region (x<0.02) of the La2−xSr x CuO4 system below the Neel temperatureTN(x), from which we extract the Cu+2 staggered magnetizationM†(x, T). M†(x, T=0), extrapolated from above 30 K, was successfully modeled with spin-wave theory, assuming that the doped holes are mobile and are situated in walls in the CuO2 plane which uncouple undoped AF domains; these domains are coupled to those in adjacent CuO2 planes. This agreement supports the previous hypothesis that microsegregation of the (mobile) doped holes into domain walls occurs above ∼30 K, consistent with the phenomenology of Emery and Kivelson. Below ∼30 K, an anomalous increase inM†(x, T) is observed, such thatM†(x, T=0) is nearly independent ofx. We interpret this effect as arising from localization of the doped holes below ∼30 K.

Equilibrium oxygen vacancy concentrations and oxidant diffusion in germania, silica, and germania-silica glasses

Equilibrium oxygen vacancy concentrations in 100% GeO 2 , 87 mol% GeO 2 13 mol% SiO 2 , and 24 mol% GeO 2 76 mol% SiO 2 glasses were determined as a function of melting temperature and atmosphere, and the effects of oxygen and water in the atmosphere on the oxygen vacancy annihilation kinetics in these glasses were investigated. Equilibrium oxygen vacancy concentrations for a glass increased as the melting temperature increased, and decreased as the germania content of the glass decreased. The equilibrium vacancy concentration in 100% GeO 2 was inversely proportional to the square root of the oxygen partial pressure in the atmosphere. Oxygen vacancy formation energies were approximately 2.5 eV. When 100% GeO 2 and 87 mol% SiO 2 13 mol% SiO 2 glasses prepared by melting were heated, the glasses dehydrated, and the oxygen vacancy concentration was controlled by effective oxygen diffusion. Activation energies for effective oxygen diffusion in 100% GeO 2 and 87 mol% SiO 2 13 mol% SiO 2 glasses were 1.2 and 1.1 eV, respectively. A model for effective oxygen diffusion is presented, which shows a dependency of the effective oxygen diffusion coefficient on the oxygen vacancy concentration. When water-free 24 mol% GeO 2 76 mol% SiO 2 glass, prepared by CVD, was heated, the glass hydrated, and the oxygen vacancy concentrations were controlled by effective water diffusion. As the germania content of the glasses increased, the effective water diffusion coefficients increased. The presence of silica in the germania-silica glasses caused a kink to appear in the Arrhenius plots of effective water diffusion coefficients.