F. Kreuzer

Facilitated diffusion of oxygen and its possible significance; a review

Abstract The experimental evidence for the facilitation of O2 diffusion in the presence of Hb or Mb was reviewed. The basic characteristics of facilitated O2 diffusion pointed out the diffusive nature of this process and excluded other mechanisms proposed (e.g., a “bucket-brigade” mechanism). A review of the earlier attempts at a quantitative treatment of facilitated O2 diffusion showed deficiencies concerning both the premises for the theoretical approach and the availability of adequate experimental data for the basic parameters. The situation with respect to the facilitated diffusion of CO in particular suggested the possible importance of the chemical reaction rates. It was also found that the problem of the simultaneous gradients of O2 and HbO2 in the diffusion layer remained unsolved. The more recent experimental data for the diffusion coefficients of O2, Hb and Mb were summarized as a basis for further quantitative treatment. The recent mathematical work by the author of the present review was briefly described and showed that a quantitative description of the experimental evidence was possible based on the facilitated O2 diffusion being interpreted as simultaneous diffusion of O2 and HbO2 if the chemical reaction rates were also taken into account. The possible physiological significance of facilitated O2 diffusion in vivo was discussed, including nonsteady-state conditions as present under most physiological circumstances, with particular reference to the situation concerning Hb in the red cells and concerning Mb in muscle. It was concluded that more circumstantial and direct experimental evidence is needed in order to arrive at a definite evaluation of the importance of facilitated O2 diffusion in physiological conditions.

Advantage or disadvantage of a decrease of blood oxygen affinity for tissue oxygen supply at hypoxia

SummaryA shift of the oxygen dissociation curve to the right is often interpreted as an adaptation to hypoxia favorable for tissue oxygen supply. However, animals native to high altitude tend to show a rather high oxygen affinity. In order to elucidate this apparent discrepancy we investigated by numerical computer studies 1. the effect of a shift of the dissociation curve to the right as reflected in the mixed venous oxygen pressure, and 2. the role of this displacement in pulmonary gas exchange with particular reference to the alveolar-arterial oxygen pressure difference and the pulmonary diffusing capacity for oxygen. A right shift had a favorable effect only in the range of moderate hypoxia (and of normoxia) whereas there was a detrimental effect with severe hypoxia. The most important criterion for this distinction was the direction of the change in steepness of the physiological dissociation curve (straight line between arterial and venous points). A favorable effect was associated with a steeper slope after the shift, an unfavorable effect with a less steep slope. There was only a minor influence of a right shift on the oxygen diffusion gradient in the lung. Comparison between man (higher affinity) and rat (lower affinity) suggests that animals of small size with high metabolic rate (high arteriovenous oxygen difference) living in normoxic or possibly exposed to moderately hypoxic condition only are better served by a relatively low oxygen affinity whereas animals native to high altitude are better adapted to severe hypoxia when having a high oxygen affinity.

Cardiac hypertrophy, capillary and muscle fiber density, muscle fiber diameter, capillary radius and diffusion distance in the myocardium of growing rats adapted to a simulated altitude of 3500 m.

SummaryIn growing rats adapted to a simulated altitude of 3500 m for about 4 weeks and in their controls the evolution of cardiac ventricular weight was followed. The increase of total ventricular weight found in the adapted animals can be attributed exclusively to the increase of right ventricular weight. In other adapted and control animals cardiac capillary densities, muscle fiber diameter and external capillary radius were estimated and fiber—capillary ratio and diffusion distance were calculated. There was an increase of capillary density together with a decrease of muscle fiber density, fiber-capillary ratio and diffusion distance in the right but not in the left ventricle of the adapted rats. The muscle fiber diameters, however, were larger in both heart ventricles of the rats exposed to a simulated high altitude, especially in the right ventricle. This indicates that true hypertrophy of the muscle fibers is mainly responsible for the increase of right ventricular weight. In the left ventricle, however, a hypertrophy of the muscle fibers together with a decrease of stroma components is demonstrated. The physiological importance of the shorter diffusion distance in the right ventricle of the high altitude adapted rats is discussed and it is suggested that the shorter diffusion distance may help to keep the tissue O2 partial pressure above the critical value, mainly also in extreme situations with high myocardial O2 consumption.

Rapid polarographic in vivo oxygen catheter electrodes

Abstract A theoretical analysis is presented concerning the characteristics of membrane-covered polarographic oxygen electrodes mounted on the tip of a catheter for application to continuous recording in vivo, particularly in regard to flow dependency and response time. The requirements for flow independency and fast response oppose each other, so that the best compromise has to be found for any particular application. The diffusion of oxygen from the medium to the oxygen-consuming cathode surface meets the resistances and capacitances of two layers in series, the diffusion boundary layer of the medium adjacent to the electrode and the membrane. Analysis of the diffusion across these two layers provides equations for computing flow dependency and response time. These computations were compared with experimental determinations obtained with a modified version of electrodes developed previously, showing a response time of about 0.25 sec for 95% deflection. Conclusions are presented for appropriate evaluation of electrode performance and construction of new electrodes with an optimum of the desired properties.

Facilitated diffusion of oxygen in the presence of hemoglobin

Abstract The basic equations for the simultaneous diffusion and chemical reactions of oxygen and emoglobin in a film at steady state were solved assuming that the total oxygen flux was the sum of the ux by plain diffusion and that by diffusion of oxyhemoglobin. After collecting and scrutinizing the ertinent numerical data, particularly for the diffusion coefficients of oxygen and hemoglobin, numeri solutions were obtained by computer for a variety of conditions. It appeared that the gradients of xygen and oxyhemoglobin across the slab were notably different from those stipulated for the andition of chemical equilibrium. In particular we found that there must be a minute step in the oxygen gradient at the low pressure side with a slope equal to that at the high pressure side because f the boundary condition that the two surfaces must be impermeable for hemoglobin, and that the saturation is higher at the low pressure side and lower at the high pressure side than at chemical equilibrium. When assuming mean values from available data for the diffusion coefficients of oxygen and hemoglobin we arrived at excellent agreement between the computed fluxes and those obtained experimentally by other authors. It is concluded that the facilitation of oxygen diffusion in the presence of hemoglobin can be described quantitatively when the chemical reactions are taken into account.

Effects of pulmonary gas embolism on circulation and respiration in the dog

SummaryMongrel dogs weighing 15–25 kg and anesthetized with thiopental-gamma-hydroxybutyric acid were used to investigate the effects of pulmonary gas embolism on pulmonary arterial pressure (Pap), systemic arterial pressure (Pa) and cardiac output (

Factors influencing facilitated diffusion of oxygen in the presence of hemoglobin and myoglobin.

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Cardiac output, arterial and mixed-venous O2 saturation, and blood O2 dissociation curve in growing rats adapted to a simulated altitude of 3500 m

). Pulmonary gas embolism was produced either by venous injection or by venous infusion. The most marked effect of pulmonary gas embolism on circulation was an increase inPap which returned to the original level after stopping the gas administration. 1.After gas injectionPap rose to a maximum within 30\2-60 s. The extent of this rise inPap showed a positive correlation with the volume of the injected gas. The kind of gas (oxygen, helium, neon, nitrogen, air), however, did not influence the extent of the rise inPap, but did influence the time of return ofPap to the original level. Carbon dioxide showed an exceptional behavior in that it had almost no effect onPap at all.Pa hardly changed with the volume of the gas injections (20\2-60 ml injected within 1 s);