Dependence of O2 transfer conductance of red blood cells on cellular dimensions

To estimate the significance of the dimensions of RBC on O2 transfer, the kinetics of O2 release from RBC into medium containing dithionite (40 mmol/l) was measured, by a stopped-flow technique, for nine different species with varying RBC size (man, llama, vicuna, alpaca, dromedary camel, pygmy goat, domestic hen, muscovy duck and turtle). The observed O2 transfer kinetics were found to be size-dependent, i.e. the O2 transfer conductance of the single RBC, gst, was lower, whereas the specific O2 transfer conductance of packed RBC, Gst, or of whole blood, theta st, was higher for smaller RBC. The ratio of surface area to effective diffusion path length which was found to be about one fourth of the mean cell thickness irrespective of cell size and cell shape, may be considered as the essential morphological factor determining O2 transfer efficiency of the single RBC.

Cardiorespiratory changes during HCl infusion unrelated to decreases in circulating blood pH

To test the hypothesis that infusion of HCl changes blood pressure and respiration independent of decreases in circulating blood pH, an extracorporeal arteriovenous shunt (20 ml/min) between the femoral artery and vein was installed in anesthetized cats. Into this loop, acid (0.25 M HCl) and, approximately 10 cm downstream, base (0.25 M NaOH) could be infused simultaneously. Likewise, either acid or base could be infused individually. Right ventricular (Prv) and arterial (Pa) blood pressure, tidal volume (VT), and respiratory frequency (fresp) were recorded as well as blood gases and pH in arterial, right ventricular, and shunt loop blood at the reentrance into the animal. When HCl and NaOH were infused simultaneously and at equimolar rates (0.2 mmol/min for 10 min), there was a large increase in Prv, with little change or decrease in Pa. Respiratory frequency was increased, but total ventilation was not elevated because of a concomitant fall in VT. The rise in Prv and increase in fresp were transient in that they could only be evoked during the first HCl-NaOH infusion in a given animal. Repetitive infusions of HCl-NaOH into the same animal failed to elicit the response. Similar transient acid effects were evoked when HCl was infused without NaOH but not when NaOH was infused without HCl. During the second and third infusion of HCl, ventilatory responses were elicited that were explainable by stimulation of known chemoreceptors. The transient rise in Prv and fresp evoked by acid infusion might be explained by release of an agent from blood elements at the tip of the HCl infusion catheter, which in turn would constrict pulmonary vessels and influence breathing.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of Na+/H+ exchange reduces Ca2+ mobilization without affecting the initial cleavage of phosphatidylinositol 4,5-bisphosphate in thrombin-stimulated platelets

Stimulation of human platelets increases cytoplasmic pH (pHi) via activation of Na+/H+ exchange. We have determined the effect of inhibiting Na+/H+ exchange on (i) thrombin-induced Ca2+ mobilization and (ii) turnover of 32P-labelled phospholipids. Blocking Na+/H+ exchange by removal of extracellular Na+ or by ethylisopropylamiloride (EIPA) inhibited Ca2+ mobilization induced by 0.2 U/ml thrombin, whereas increasing pHi by NH4Cl enhanced the thrombin-induced increase in cytosolic free Ca2+. The effect of EIPA was bypassed after increasing pHi by moneasin. The thrombin-induced cleavage of phosphatidylinositol 4,5-bisphosphate (PIP2) was unaffected by treatments that blocked Na+/H+ exchange or increased pHi. It is concluded that activation of Na+/H+ exchange is a prerequisite for Ca2+ mobilization in human platelets but not for the stimulus-induced hydrolysis of PIP2.

Oxygen transfer conductance of human red blood cells at varied pH and temperature

The influence of temperature (varied from 37 to 7 degrees C) and of medium pH (varied from 7.7 to 7.1) on the kinetics of O2 uptake and release by human red blood cells (RBC) under stopped-flow conditions was investigated by double-beam spectrophotometry. From the rate of O2 saturation change and the mean effective PO2 difference between the medium and the Hb of RBC, the specific transfer conductance for O2, G, was calculated. The temperature coefficient, Q10, of G for O2 uptake averaged 1.17, activation energy, Ea, 2.6 kcal/mol O2; the average values for O2 release were: Q10 = 1.32, Ea = 4.8 kcal/mol O2. The G value for release of O2 from oxyhemoglobin solution, Gsol, yielded Q10 = 2.25, Ea = 13.5 kcal/mol O2. Comparison of these Q10 and Ea values with those for diffusion of O2 and hemoglobin in aqueous media leads to the conclusion that in the stopped-flow condition the conductance for O2 transfer was mainly limited by diffusion of O2 and hemoglobin in the red cell interior and by diffusion of O2 in the medium, and to a lesser degree by chemical reaction kinetics. This was further supported by the lack of dependence of O2 transfer conductance values on pH.

Effects of temperature on oxygen transfer conductance of human red blood cells

The influence of temperature (varied from 37 to 7 degrees C; average pH = 7.4) on the kinetics of O2 uptake and release by human red blood cells under stopped-flow conditions was investigated by double-beam spectrophotometry. The kinetics were characterized by the specific transfer conductance for O2, G. The temperature coefficient of G, Q10(G), for O2 uptake averaged 1.17, and activation energy, Ea(G) = 2.9 kcal/mol O2. The average values for O2 release were: Q10(G) = 1.30, and Ea(G) = 4.8 kcal/mol O2. The G values for release of O2 from oxyhaemoglobin solution, Gsol, yielded Q10(Gsol) = 2.06, Ea(Gsol) = 13.4 kcal/mol O2. Comparison of these Q10 and Ea values with those for diffusion of O2 and haemoglobin in aqueous media leads to the conclusion that the kinetics of O2 uptake and release by red blood cells in the stopped-flow condition is mainly limited by diffusion of O2 and haemoglobin in the red cell interior and by diffusion of O2 in the medium, and to a lesser degree by chemical reaction kinetics.

Respiration and blood gases in the duck exposed to normocapnic and hypercapnic hypoxia

Cardio-respiratory parameters were measured in the unrestrained, unanesthetized duck by whole body plethysmography during exposure to varied levels of inspired hypoxia without or with (3.7%) CO2. At any level of inspired PO2 (PIO2), ventilation (VE) was larger with CO2 inhalation, leading, for PIO2 greater than 50 Torr, to higher levels of arterial PO2 (PaO2) and O2 content (CaO2). Below PIO2 of 50 Torr, the (PI-Pa)O2 difference without CO2 reached a value as low as 5 Torr which was not diminished by further stimulation of VE by inhaled CO2. Without CO2 inhalation at this deep hypoxic level the ensuing hypoxia-induced respiratory alkalosis was partly compensated by lactacidosis, whereas CO2 inhalation resulted in markedly lower blood pH leading to significantly lower arterial and venous O2 content (Bohr effect). As a result, the deepest level of hypoxia tolerated without CO2 inhalation, 30 Torr, is significantly deeper than that, 36 Torr, tolerated when CO2 is inhaled. The data suggest that a number of factors contribute to the high hypoxia tolerance in birds, e.g. the effectiveness of parabronchial ventilation and the tolerance of low arterial PCO2 levels, whereby part of the lactacidosis is compensated.

Activation of Na+/H+ exchange in human platelets stimulated by thrombin and a phorbol ester

We have investigated changes in cytoplasmic pH (pHi) in activated human platelets, using the fluorescent probe 2,7-biscarboxyethyl-5(6)-carboxyfluorescein. Stimulation of platelets by thrombin or 12-O-tetradecanoylphorbol 13-acetate increased pHi by about 0.11 pH unit above the resting value. This increase in pHi depended on the presence of external Na+ and was inhibited by ethylisopropylamiloride. The data suggest that protein kinase C mediates Na+/H+ exchange in human platelets.

A phorbol ester and 1-oleoyl-2-acetylglycerol induce Na+/H+ exchange in human platelets

This study aimed at investigating the mechanisms by which stimulation of human platelets results in activation of Na+/H+ exchange. Platelets were suspended in a slightly buffered medium and the stimulus-induced, amiloride-sensitive H+ release, reflecting Na+/H+ exchange, was estimated from changes in the medium pH. H+ release could be evoked by thrombin and by activators of protein kinase C such as 1-oleoyl-2-acetylglycerol (OAG) or 12-O-tetradecanoylphorbol-13-acetate (TPA). Both the thrombin-and the OAG-induced Na+/H+ exchange could be blocked by trifluoperazine, a protein kinase C inhibitor. The thrombin-induced H+ release was also sensitive to increased intracellular cAMP levels, probably due to inhibition of phospholipase C activation, whereas the OAG-induced activation of Na+/H+ exchange was unaffected. Our data suggest that activation of Na+/H+ exchange is mediated by protein kinase C.

Effects of altering dead space volume on respiration and air sac gases in geese

Dead space volume (VD) was altered in spontaneously breathing, anesthetized geese from values far above (about 115 ml) to those far below (about 3 ml) the normal VD (approximately 40 ml). Respiratory gases were measured in cranial (CrS) and caudal air sacs (CdS) and in blood. The major findings were as follows: Ventilation increased linearly with VD, by increases in tidal volume (VT) at constant breathing rate (fresp); effective parabronchial ventilation, (VT-VD) X fresp, remained constant and so did arterial blood gases. No changes occurred in CrS gas composition. CdS PCO2 declined with decreasing VD, and the respiratory exchange ratio increased, reaching values above unity at the lowest VD. The gas composition in CrS, and particularly its relation to end-expired gas composition, is in agreement with current models of the gas flow pattern in the avian lung. The PCO2 values in CdS are higher than expected by simple models, e.g. by dead space re-inhalation. Neopulmonic gas exchange and incomplete gas mixing are suggested to contribute significantly to the gas composition of CdS.

Kinetics of oxygen uptake and release by red blood cells of chicken and duck

The specific conductance (G) for O2 transfer by red blood cells (RBCs) of chicken and muscovy duck was measured using the experimental (stopped-flow) and analytical techniques (RBC model) previously applied to human RBC (Yamaguchi, Nguyen Phu, Scheid & Piiper, 1985). Avian RBCs behaved similarly to human RBCs: G values were of similar magnitude; G for O2 uptake decreased with time and increasing O2 saturation; G for O2 release at high levels of dithionite decreased slightly with decreasing O2 saturation; G for O2 release was higher than G for O2 uptake. The deoxygenation kinetics of oxyhaemoglobin in solution was similar for both avian species. The G measured for O2 release at high dithionite concentration, considered to represent a good approximation to intra-erythrocyte O2 diffusion conductance, averaged (in mmol min-1 Torr-1 ml-1 RBC) 0.33 for chicken and 0.25 for duck (at 41 degrees C, pH of the suspension = 7.5, O2 saturation range 0.4-0.8). These species differences can be explained by differences in cell size, the RBC volume averaging 104 micron3 in the chicken and 155 micron3 in the duck. Compared with human RBCs, the G estimates for avian RBCs are somewhat smaller than would be predicted from size differences, which can be explained by the discoid shape of mammalian RBCs which constitutes an advantage compared with the ovoid avian RBC.

Cross-sectional PO2 distributions in Krogh cylinder and solid cylinder models

Radial profiles, gradients and frequency distributions of PO2 are calculated for two cylindrical tissue models with uniform O2 diffusion properties and O2 consumption: Krogh's cylinder with O2 supplied from a central capillary (model A), and a solid cylinder with O2 supplied from the outer surface (model B). Because the O2 diffusion flux is divergent in model A, and convergent in model B, the PO2 gradient flattens with increasing distance from the supplying surface more rapidly in model A than in model B. In model A, the frequency distribution of PO2 with respect to unit cross-sectional area is highly asymmetric, being skewed to low PO2, whereas it is uniform in model B. Model A is applicable to parallel capillaries uniformly distributed across tissue cross-section. For Model B a large number of capillaries surrounding a cylindrical structure is required. Model A appears to be much more adequate than model B to describe the normal morphometry in skeletal muscle (capillary number/fiber number ratio of about 2, capillary-to-muscle fiber radius ratio of about 0.1). The experimental finding of relatively low PO2 and small PO2 gradients within muscle fiber cross-sections is in agreement with both models, but agrees better with model A than with model B.

Model for analysis of counter-current gas transfer in fish gills

The validity of previously used simplified models for the analysis of gas transfer in fish gills was tested using an integrated model which includes water flow and blood flow in counter-current arrangement. The model accounts for the resistance to diffusion of O2 both in the water-blood barrier and in the interlamellar water, which is assumed to flow with a parabolic velocity profile between the secondary lamellae. The O2 diffusing capacity (transfer factor) for this model (Dint) was compared to that (Dm + w) calculated from the diffusing capacity of the water-blood barrier (Dm), and from the effective diffusive conductance of the parabolically streaming interlamellar water (Dw) as 1/Dm + w = 1/Dm + 1/Dw. These diffusing capacities were compared with that (Dadd) calculated from Dm and diffusing capacity of a water layer of 1/4 thickness of the interlamellar space (Dw) as 1/Dadd = 1/Dm + 1/Dw. Calculations with morphometric and gas exchange parameters in the elasmobranch Scyliorhinus stellaris reveal the following features: (1) In physiological conditions, Dm + w and Dint are similar to within 10%, but Dint is always higher. (2) Dint and Dm + w increase with increasing ventilation; Dint increases with decreasing perfusion, while Dm + w remains constant. (3) Both Dint and Dm + w agree reasonably well with Dadd. In other anatomical and physiological conditions, particularly for relatively high Dm, Dw, and Dw and high ventilation, greater discrepancies between Dint and Dm + w may occur but Dm + w appears to represent a reasonable approximation of the effective O2 diffusing capacity, which is best modelled as Dint.

Inert gas wash-out from tissue: model analysis

Model simulations on variously arranged two-compartment models are performed to provide a basis for interpreting the observed non-monoexponential (or non-linear logarithmic) wash-out time courses of inert gases from tissue. The variables considered are: blood flow, tissue volume, solubility of gas in tissue and blood, and diffusive conductances (diffusing capacities) for tissue/blood gas transfer and for gas transfer between tissue compartments. The wash-out is studied in terms of both mean tissue partial pressure and effluent venous blood partial pressure. Diffusion limitation within a tissue-blood capillary unit is shown to produce logarithmic wash-out rates which increase or decrease during wash-out, depending on the functional structure of the unit. On the other hand, in a system consisting of dissimilar tissue-blood capillary units arranged in parallel, the logarithmic wash-out rate decreases with wash-out time. It is shown that the conventional analysis of nonlinear logarithmic wash-out may overestimate or underestimate tissue perfusion or the extent of its inhomogeneity.

Oxygen binding in blood of Xenopus laevis (Amphibia) and evidence against Root effect

Blood oxygen binding was examined in the amphibian, Xenopus laevis, with the particular aim of determining whether the O2 capacity is diminished when blood pH is lowered, which is known as the Root effect in blood of some fishes. Hemoglobin-bound O2 concentration, [O2Hb], was determined by the Lex-O2-Con technique, and both total hemoglobin, [Hb]tot, and Met-hemoglobin, [MetHb], contents were measured spectrophotometrically. From these measurements were calculated the oxygen capacity, O2cap, and the content of active hemoglobin, [Hb]act, i.e. the difference between [Hb]tot and [MetHb]. The main finding was the independence of the ratio of O2cap/[Hb]act on pH, when differences between samples in [Hb]tot and the presence of MetHb, which was particularly pronounced at low pH, where properly accounted for. It is concluded that the Root effect does not exist in blood of the amphibian Xenopus laevis.

Effects of inhibiting carbonic anhydrase on isometric contraction of frog skeletal muscle

Carbonic anhydrase (CA) activity was determined in a homogenate of frog skeletal muscle by measuring the kinetics of CO2 hydration in a pH stopped-flow apparatus. The results suggest that frog skeletal muscle contains a high-activity CA with properties similar to those of the isoenzyme CA II found in white skeletal muscle tissue of the rabbit. In an attempt to assess the functional significance of CA in skeletal muscle, the maximal isometric force of frog gastrocnemius muscle was measured in response to direct or indirect (ischiadic nerve) single-pulse electrical stimulation before (control) and after exposing the muscle to various concentrations of the specific carbonic anhydrase inhibitors, ethoxzolamide, acetazolamide, and methazolamide. In the range of ethoxzolamide concentration between 10(-9) and 10(-6) M, maximal isometric force with indirect supramaximal stimulation declined progressively with inhibitor concentration to less than 10% of the control value. Acetazolamide and methazolamide were less effective in that concentrations of above 10(-4) M were necessary to inhibit maximum isometric force by 50%. Even at the highest ethoxzolamide concentration used (10(-6) M), no effect was observed either on the amplitude of the compound nerve action potential or on the conduction velocity of group I fibres in the ischiadic nerve, suggesting that ethoxzolamide did not affect the mechanisms responsible for spike generation or conduction in the motor fibres. With direct supramaximal stimulation of the gastrocnemius muscle, no effects on maximal isometric force were observed of CA inhibition by any of the inhibitors used. The results suggest that CA acts on the neuromuscular transmission. The exact site and mechanism of action are unknown.

Kinetics of O2 uptake and release by human erythrocytes studied by a stopped-flow technique

The kinetics of O2 uptake into and release from human erythrocytes was investigated at 37 degrees C by a stopped-flow technique. From the time course of O2 saturation (SO2) change a specific transfer conductance of erythrocytes for O2 (GO2) was calculated. The following results were obtained: 1) GO2 decreased in the course of O2 uptake, but initial GO2 was nearly independent of SO2 at which uptake started; 2) addition of albumin to the medium reduced GO2; 3) increasing dithionite concentration in the medium in O2-release experiments progressively enhanced GO2, which became virtually constant for nearly the entire course of release; and 4) O2 uptake and O2 release (without dithoite) in the same SO2 range yielded very similar GO2. These results suggested that O2 uptake and release were importantly limited by diffusion through the external medium and that in the SO2 range between 0.3 and 0.8, chemical reaction exerted little limiting effect. Since O2 release at the highest dithionite concentration (40 mmol/l) appeared to be virtually unlimited by external diffusion, GO2 measured under these conditions, averaging 8.7 ml X min-1 X Torr-1 X ml erythrocytes-1, was considered to mainly reflect intracellular diffusion limitation. The corresponding specific transfer conductance for O2 transfer in whole blood (hematocrit, 0.45) is 3.9 ml X min-1 X Torr-1 X ml blood-1.

Kinetics of O2 uptake and release by red cells in stopped-flow apparatus: effects of unstirred layer

Using a stopped-flow apparatus, measurements were made of the velocity of uptake and release of oxygen by red cells of man, sheep and goat, three species of widely differing red cell size. The results were used to calculate resistances to O2 uptake provided by: (1) any unstirred layer (USL) outside the cells; (2) the cell membrane; and (3) the cell substance, in which the process of simultaneous diffusion and chemical reaction occurs. For O2 release, the USL was virtually abolished by using sufficient dithionite in the reactant buffer for it to diffuse up to the cell membrane and mop up O2 as it passes out of the cell. Hence, differences in the rate of O2 uptake and release allowed estimation of the resistance and thickness of the USL. Its thickness in the three species was between 0.7 and 0.9 micron; it provided at least 70% of the resistance to O2 uptake located outside the cell interior (cell membrane plus USL). Existence of the USL slows the uptake of O2 in the stopped-flow apparatus by a factor of at least 1.8 to 2.0.

Evidence for a role of NA+/H+ exchange in platelets activated with calcium-ionophore A 23187

We have investigated the release of protons from human platelets and platelet aggregation induced by the calcium ionophore, A 23187. Addition of the ionophore to suspensions of washed platelets resulted in fast liberation of H+. In the presence of 0.2 mM amiloride, a potent inhibitor of Na+/H+ countertransport, the amount of protons liberated was decreased by 50% and was further reduced to about 10% by 1 mM amiloride. Similar inhibition of H+-release was observed after decreasing Na+ in the incubation medium. Both results suggest that increasing internal Ca2+ by the ionophore induces Na+/H+ exchange in human platelets. Platelet aggregation could be induced by adding the ionophore to the platelet suspension. This aggregation was inhibited by amiloride, at least when induced by low ionophore concentrations. The results suggest that stimulation of Na+/H+ exchange, and the concomitant increase in intraplatelet pH, are important mechanisms in platelet activation.

Thrombin stimulates Na+-H+ exchange across the human platelet plasma membrane

We have investigated the release of protons from thrombin-stimulated platelets. Addition of thrombin to suspensions of washed platelets resulted in fast liberation of H+. In the presence of 0.1 mM amiloride, a potent inhibitor of the Na+/H+ transport system, the amount of protons liberated was decreased by about 50%, and was further reduced to about 15% by 1 mM amiloride. Similar inhibition of H+ release was observed after Na+ in the incubating medium had been replaced by choline. We conclude that one of the earliest events in thrombin-stimulated platelets consists of the activation of an Na+/H+ countertransport, which leads to an increase in intracellular pH.

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.

Gas concentration profiles along airways of dog lungs during high-frequency ventilation

Following equilibration with inert gases (He and SF6), dog lungs were partially washed out, either by high-frequency ventilation (HFV) or by conventional mechanical ventilation (CMV), to varied inert gas clearance levels, at which expirograms were recorded by mass spectrometry. Relative alveolar slopes were distinctly positive for HFV and tended to increase with lung clearance; they were, however, smaller than those of CMV and were smaller for He than for SF6 during both ventilatory modes. Fowler dead space was smaller for HFV than for CMV, with significant differences between test gases only during CMV. Plots of concentration against linear distance between measuring site and alveolar region showed that most of the total concentration drop during HFV occurred, with nearly linear slope, along the endotracheal tube and the upper airways, with no difference between He and SF6. In the alveolar region, on the other hand, relative concentration gradients were similar for HFV and CMV, both showing separation of He and SF6. The data suggest that gas transport in the upper airways during HFV is not diffusion limited. Gas mixing in alveolar regions, although more complete for HFV than for CMV, is limited by diffusion; however, this incomplete gas mixing does not appreciably limit overall gas transport during HFV.

Dual role of diffusion in tissue gas exchange: blood-tissue equilibration and diffusion shunt

The role of diffusion in tissue gas exchange is investigated using a simple mathematical model which incorporates both tissue-blood equilibration and gas transfer between arterial and venous vessels with counter-current flow, leading to 'diffusion shunt'. Both increasing the diffusion coefficient of the gas considered or decreasing the blood flow results in two antagonistic effects: (i) improvement of blood/tissue equilibration, (ii) increase in extent of diffusion shunt. The diffusion shunt retards inert gas wash-out (local tissue clearance) and leads thus, if not taken into account, to an underestimation of capillary blood flow calculated from the wash-out rate constant. For O2 (and CO2) the diffusion shunt reduces the efficacy of blood/tissue transfer, but its extent is expected to be only moderate because of the chemical combination of these gases in blood.

Dual role of diffusion in tissue gas exchange: blood-tissue equilibration and diffusion shunt

The role of diffusion in inert gas washout from tissue is investigated using simple mathematical models incorporating diffusive blood-tissue equilibration and diffusion shunt due to diffusive gas transfer between precapillary and postcapillary vessels with counter-current blood flow. With increasing diffusivity blood-tissue equilibration is improved, but simultaneously the diffusion shunt by veno-arterial back diffusion is increased. Similarly, with decreasing blood flow, the extent of diffusion limitation in blood-tissue transfer is diminished, but at the same time veno-arterial diffusive shunting is enhanced. Diffusion shunt slows inert gas washout (local tissue clearance) and thus, if not taken into account, leads to an underestimation of capillary blood flow calculated from the washout rate constant. Diffusion shunting of O2 diminishes the efficacy of blood-tissue transfer, but its extent is predicted to be smaller compared to that for inert gases, because the chemical combination of O2 in blood decreases diffusive shunting.