A theoretical model for studying the rate of oxygenation of blood in pulmonary capillaries

A mathematical analysis of the process of gas exchange in the lung is presented taking into account the transport mechanisms of molecular diffusion, convection and facilitated diffusion of the species due to haemoglobin. Since the rate at which blood gets oxygenated in the pulmonary capillaries is very fast, it is difficult to set up an experimental study to determine the effects of various parameters on equilibration rate. The proposed study is aimed at determining the effects of various physiological parameters on equilibration rate in pathological conditions.Among the significant results are that 1. dissolved oxygen takes longer to achieve equilibration across the pulmonary membrane and carbon dioxide attains equilibration faster, 2. the equilibration length increases with increase in blood velocity, haemoglobin concentration, calibre of pulmonary capillaries and fall in alveolar PO₂, 3. the alveolar PCO₂ and forward and backward reaction rates of haemoglobin with CO₂ do not materially affect the equilibration rate or length. 4. At complete equilibration, by the end of the pulmonary capillary 92% of the total haemoglobin has combined with oxygen and 8% free pigment is left which is present as carbamino haemoglobin, met haemoglobin, carboxy haemoglobin etc.These results are of some importance for anaemic conditions, muscular exercise, meditation, altitude physiology, hypo-ventilation, hyperventilation, etc.Symbols H⁺ hydrogen ion - O₂ oxygen - CO₂ carbondioxide - HbO₂ oxyhaemoglobin - HbCO₂ carbaminohaemoglobin - PO₂ partial pressure of O₂ - PCO₂ partial pressure of CO₂ - PaO₂ O₂ tension in arterial blood - PaCO₂ CO₂ tension in arterial blood - k₁ forward rate constant for Eq. (1) - k₂ backward rate constant for Eq. (1) - m₁ forward rate constant for Eq. (2) - m₂ backward rate constant for Eq. (2) - k equilibration rate - a radius of the capillary - Q velocity of blood - L length of the capillary - D₀ diffusion coefficient of O₂ - D_c diffusion coefficient of CO₂ - D_H diffusion coefficient of Hb - H total haemoglobin concentration - A matrix - c₁ concentration of dissolved O₂ in blood - c₂ concentration of HbO₂ in blood - c₃ concentration of dissolved CO₂ in blood - c₄ concentration of HbCO₂ in blood - c₅ concentration of haemoglobin - c1alv concentration of O₂ in the alveolar region - c_3alv concentration of CO₂ in the alveolar region - c_iven concentration of the ith species in venous blood - c_iart concentrations of the ith species in arterial blood - F _is concentrations of the species in dimensionless form - J₀, I₀ Bessel's functions - P_alvO₂ tension of O₂ in alveolar region - P_alvCO₂ tension of CO₂ in alveolar region.     

Pre-and post-branchial blood respiratory status during acute hypercapnia or hypoxia in rainbow trout,Oncorhynchus mykiss

Simultaneous venous (pre-branchial) and arterial (post-branchial) extracorporeal blood circulations were utilized to monitor continuously the rapid and progressive effects of acute environmental hypercapnia (water partial pressure of CO₂ 4.8±0.2 torr) or hypoxia (water partial pressure of O₂ 25±2 torr) on oxygen and carbon dioxide tensions and pH in the blood of rainbow trout (Oncorhynchus mykiss). During hypercapnia, the CO₂ tension in the arterial blood increased from 1.7±0.1 to 6.2±0.2 torr within 20 min and this was associated with a decrease of arterial extracellular pH from 7.95±0.03 to 7.38±0.03; the acid-base status of the mixed venous blood changed in a similar fashion. The decrease in blood pH in vivo was greater than in blood equilibrated in vitro with a similar CO₂ tension indicating a significant metabolic component to the acidosis in vivo. Under normocapnic conditions, venous blood CO₂ tension was slightly higher than arterial blood CO₂ tension difference was abolished or reversed during the initial 25 min of hypercapnia indicating that CO₂ was absorbed from the water during this period. Arterial O₂ tension remained constant during hypercapnia; however, venous blood O₂ tension decreased significantly (from 22.0±2.6 to 9.0±1.0 torr) during the initial 10 min. Hypercapnia elicited the release of catecholamines (adrenaline and noradrenaline) into the blood. The adrenaline concentration increased from 6±3 to 418±141 nmol · l^-1 within 25 min; noradrenaline concentration increased from 3±0.5 to 50±21 nmol · l^-1 within 15 min. During hypoxia arterial blood O₂ tension declined progressively from 108.4±9.9 to 12.8±1.7 torr within 30 min. Venous blood O₂ tension initially was stable but then decreased abruptly as catecholamines were released into the circulation. The release of catecholamines occurred concomitantly with a sudden metabolic acidosis in both blood compartments and a rise in CO₂ tension in the mixed venous blood only.Abbreviations CCO₂ plasmatotal carbondioxide - CtO₂ blood oxygen content - PO₂ partial pressure of oxygen - PCO₂ partial pressure of carbon dioxide - PaO₂ arterial bloodPO₂ - PaCO₂ arterial bloodPCO₂ - PvCO₂ venous bloodPCO₂ - PwO₂ waterPO₂ - PwCO₂ waterPCO₂ - Hb haemoglobin - SHbO₂ haemoglobin oxygen saturation - HPLC high-performance liquid chromatography - rbc red blood cell(s) - Hct haematocrit     

Metabolism of absorbed aspartate,asparagine, and arginine by rat small intestine in vivo

l-[U-¹⁴C]aspartate, l-[U-¹⁴C]asparagine, and l-[U-¹⁴C]arginine were administered luminally into isolated segments of rat jejunum in situ, and the radioactive products appearing in venous blood from the segment were identified and quantified, in a continuation of similar studies with l-glutamate and l-glutamine (Windmueller H.G. and Spaeth, A. E. (1975) Arch. Biochem. Biophys. 171, 662–672). Aspartate, administered alone (6 mm) or with 18 other amino acids plus glucose, was absorbed more rapidly than glutamate, but, as with glutamate, less than 1% was recovered intact in intestinal venous blood. More than 50% of aspartate carbon was recovered in CO₂, 24% in organic acids, mostly lactate, 12% in other amino acids (alanine, glutamate, proline, ornithine, and citrulline), and 10% in glucose, apparently the first demonstration of gluconeogenesis by intestine in vivo. In contrast to aspartate and glutamine, nearly all asparagine was absorbed intact, less than 1% being catabolized. About 4% of the absorbed dose was incorporated into the acid-insoluble fraction of intestine, as was the case with all the amino acids studied. In conventional or germ-free rats, only 60% of arginine was absorbed intact, while 33% was hydrolyzed to ornithine and urea. The urea and 38% of the ornithine were released into the blood; the remaining ornithine was metabolized further by intestine to citrulline, proline, glutamate, organic acids, and CO₂. Catabolism of several amino acids from the lumen plus glutamine from arterial blood may provide an important energy source in small intestine.     

Effects of glyoxylate on photosynthesis by intact chloroplasts

Because glyoxylate inhibits CO₂ fixation by intact chloroplasts and purified ribulose bisphosphate carboxylase/oxygenase, glyoxylate might be expected to exert some regulatory effect on photosynthesis. However, ribulose bisphosphate carboxylase activity and activation in intact chloroplasts from Spinacia oleracea L. leaves were not substantially inhibited by 10 millimolar glyoxylate. In the light, the ribulose bisphosphate pool decreased to half when 10 millimolar glyoxylate was present, whereas this pool doubled in the control. When 10 millimolar glyoxylate or formate was present during photosynthesis, the fructose bisphosphate pool in the chloroplasts doubled. Thus, glyoxylate appeared to inhibit the regeneration of ribulose bisphosphate, but not its utilization.

The fixation of CO₂ by intact chloroplasts was inhibited by salts of several weak acids, and the inhibition was more severe at pH 6.0 than at pH 8.0. At pH 6.0, glyoxylate inhibited CO₂ fixation by 50% at 50 micromolar, and glycolate caused 50% inhibition at 150 micromolar. This inhibition of CO₂ fixation seems to be a general effect of salts of weak acids.

Radioactive glyoxylate was reduced to glycolate by chloroplasts more rapidly in the light than in the dark. Glyoxylate reductase (NADP⁺) from intact chloroplast preparations had an apparent K_m (glyoxylate) of 140 micromolar and a V_max of 3 micromoles per minute per milligram chlorophyll.

     

Gas exchange and blood gases of Puna teal (Anas versicolor puna) embryos in the Peruvian Andes

Oxygen consumption, air cell gases, hematology, blood gases and pH of Puna teal (Anas versicolor puna) embryos were measured at the altitude at which the eggs were laid (4150 m) in the Peruvian Andes. In contrast to the metabolic depression described by other studies on avian embryos incubated above 3700 m, O₂ consumption of Puna teal embryos was higher than even that of some lowland avian embryos at equivalent body masses. Air cell O₂ tensions dropped from about 80 toor in eggs with small embryos to about 45 toor in eggs containing a 14-g embryo; simultaneously air cell CO₂ tension rose from virtually negligible amounts to around 26 torr. Arterial and venous O₂ tensions (32–38 and 10–12 toor, respectively, in 12- to 14-g embryos) were lower than described previously in similarly-sized lowland wild avian embryos or chicken embryos incubated in shells with restricted gas exchange. The difference between air cell and arterial O₂ tensions dropped significantly during incubation to a minimum of 11 torr, the lowest value recorded in any avian egg. Blood pH (mean 7.49) did not vary significantly during incubation. Hemoglobin concentration and hematocrits rose steadily throughout incubation to 11.5 g · 100 ml^-1 and 39.9%, respectively, in 14-g embryos.Abbreviations PO₂ partial pressure gradient of O₂ - BM body mass - D diffusion coefficient - G gas conductance (cm³·s^-1·torr^-1) - conductance to water vapor - IP internal pipping of embryos - P _ACO₂ partial pressure of carbon dioxide in air cell - P _AO₂ partial pressure of oxygen in air cell - P _aCO₂ partial pressure of carbon dioxide in arterial blood - P _aCO₂ partial pressure of oxygen in arteries - P _H barometric pressure (torr) - PCO₂ partial pressure of carbon dioxide - P _IO₂ partial pressure in ambiant air - PO₂ partial pressure of oxygen - P _VCO₂ venous carbon dioxide partial pressure - P _VO₂ mixed venous oxygen partial pressure - SE standard error - VO ₂ oxygen consumption     

Compartmentation and export of 14CO2 fixation products in mesophyll protoplasts from the C4-plant Digitaria sanguinalis

Isolated mesophyll protoplasts, and protoplast extracts containing intact chloroplasts, from the C₄ species Digitaria sanguinalis have been used to study Compartmentation and export of C₄ acids, using different C₃ precursors as substrate for ¹⁴CO₂ fixation. Mg²⁺ was necessary for maximum ¹⁴CO₂ fixation rates with both protoplasts and protoplast extracts, whereas Mg²⁺ was inhibitory for oxaloacetate and phosphoglycerate reduction. This inhibition could be overcome by preincubating the materials in the light with excess of EDTA before addition of Mg²⁺. Under these conditions pyruvate as substrate for ¹⁴CO₂ fixation induced mainly malate formation, whereas phosphoglycerate as substrate induced oxaloacetate formation, indicating competition for available NADPH between oxaloacetate and phosphoglycerate reduction. Oxaloacetate could be exported from the protoplasts at rates comparable to the rates of ¹⁴CO₂ fixation in intact leaves (200 μmol/mg Chl × h). This product probably passed the plasma membrane by simple diffusion, whereas the export of malate and aspartate seemed to be regulated, with the size of the intraprotoplast pool being relatively independent of the export rate. It is concluded that transport via the plasma membrane-cell wall path may play a role in metabolite flow during photosynthesis in C₄ plants.     

Algorithm for computing oxygen dissociation curve with pH,PCO2, and CO in sheep blood

By extending the study of Samaja and Gattinoni¹, an algorithm is described for computing the oxygen dissociation curve with variations in pH, PCO₂, and CO in homozygous HbB sheep blood. The difference in the values of O₂ pressure at 50% saturation in presence of CO computed from the present algorithm and Hill's equation does not exceed 0.5%. It is shown that O₂ affinity increases as the concentration of CO or pH increases or PCO₂ decreases. The algorithm is convenient for representing the oxygen dissociation curve with variation in pH, PCO₂ and the concentration of CO in modelling oxygen transport in sheep blood even under hypoxic conditions.     

Effect of Mineral and Organic Soil Constituents on Microbial Mineralization of Organic Compounds in a Natural Soil

This research addressed the effect of mineral and organic soil constituents on the fate of organic compounds in soils. Specifically, it sought to determine how the associations between organic chemicals and different soil constituents affect their subsequent biodegradation in soil. Four ¹⁴C-labeled surfactants were aseptically adsorbed to montmorillonite, kaolinite, illite, sand, and humic acids. These complexes were mixed with a woodlot soil, and ¹⁴CO₂ production was measured over time. The mineralization data were fitted to various production models by nonlinear regression, and a mixed (3/2)-order model was found to most accurately describe the mineralization patterns. Different mineralization patterns were observed as a function of the chemical and soil constituents. Surfactants that had been preadsorbed to sand or kaolinite usually showed similar mineralization kinetics to the control treatments, in which the surfactants were added to the soil as an aqueous solution. Surfactants that had been bound to illite or montmorillonite were typically degraded to lesser extents than the other forms, while surfactant-humic acid complexes were degraded more slowly than the other forms. The desorption coefficients (K_d) of the soil constituent-bound surfactants were negatively correlated with the initial rates of degradation (k₁) and estimates of ¹⁴CO₂ yield (P_o) as well as actual total yields of ¹⁴CO₂. However, there was no relationship between K_d and second-stage zero-order rates of mineralization (k_o). Microbial community characteristics (biomass and activity) were not correlated with any of the mineralization kinetic parameters. Overall, this study showed that environmental form had a profound effect on the ultimate fate of biodegradable chemicals in soil. This form is defined by the physicochemical characteristics of the chemical, the composition and mineralogy of the soil, and the mode of entry of the chemical into the soil environment.     

Influence of Temperature and Seed Ripening on the in-vivo Incorporation of CO(2) into the Lipids of Oat Grains (Avena sativa L.)

To elucidate the influence of growth temperature and of stage of maturity on lipid synthesis in seeds, oat plants (Avena sativa nuda L., variety NOS) were fed with ¹⁴CO₂ at different stages after flowering, and the ¹⁴C-incorporation into the grain lipids was determined at 2, 24, and 48 hours after the end of ¹⁴CO₂-application. By changing growth temperature from 12 C to 28 C after the application of ¹⁴CO₂ to intact plants, a higher ¹⁴C-labeling of saturated fatty acids was found at the higher temperature. At 28 C, palmitic and stearic acids contained 23% and 9% respectively of total fatty acid-¹⁴C shortly after the ¹⁴CO₂-application, whereas at 12 C the corresponding values were 19% and 4%, respectively. Within 2 days ¹⁴C-activity of saturated fatty acids decreased at both temperatures, but to a lesser degree at 28 C. The higher ¹⁴C-labeling of saturated fatty acids and its lower decrease within 2 days at 28 C clearly show a direct influence of temperature on fatty acid biosynthesis in oat grains.     

Quantitative evaluation of ruminal methane and carbon dioxide formation from formate through C-13 stable isotope analysis in a batch culture system

Methane produced from formate is one of the important methanogensis pathways in the rumen. However, quantitative information of CH₄ production from formate has been rarely reported. The aim of this study was to characterize the conversion rate (CR) of formic acid into CH₄ and CO₂ by rumen microorganisms. Ground lucerne hay was incubated with buffered ruminal fluid for 6, 12, 24 and 48 h. Before the incubation, ¹³C-labeled H¹³COOH was also supplied into the incubation bottle at a dose of 0, 1.5, 2.2 or 2.9 mg/g of DM substrate. There were no interactions (P>0.05) between dose and incubation time for all variables evaluated. When expressed as an absolute amount (ml in gas sample) or a relative CR (%), both ¹³CH₄ and ¹³CO₂ production quadratically increased (P<0.01) with the addition of H¹³COOH. The total ¹³C (¹³CH₄ and ¹³CO₂) CR was also quadratically increased (P<0.01) when H¹³COOH was added. Moreover, formate addition linearly decreased (P<0.031) the concentrations of NH₃-N, total and individual volatile fatty acids (acetate, propionate and butyrate), and quadratically decreased (P<0.014) the populations of protozoa, total methanogens, Methanosphaera stadtmanae, Methanobrevibacter ruminantium M1, Methanobrevibacter smithii and Methanosarcina barkeri. In summary, formate affects ruminal fermentation and methanogenesis, as well as the rumen microbiome, in particular microorganisms which are directly or indirectly involved in ruminal methanogenesis. This study provides quantitative verification for the rapid dissimilation of formate into CH₄ and CO₂ by rumen microorganisms.     

Biochemical components of the photosynthetic CO2 compensation point of higher plants.

Barley, Panicum milioides and Panicum maximum were exposed to ¹⁴CO₂ near their photosynthetic CO₂ compensation points and their respective ¹⁴C-products were determined. In short exposure times Panicum maximum had 100% of its ¹⁴C in malate and aspartate whereas Panicum milioides and barley had 16 and 3% of their respective ¹⁴C in C₄ organic acids. Near the respective CO₂ compensation points a linear relationship occurs in plotting the ratio of glycine, serine, and glycerate to C₄ organic acids. The ratio of ribulose 1,5-bisphosphate oxygenase to phosphoenolpyruvate carboxylase is linear with their CO₂ compensation points. The photosynthetic CO₂ compensation point apparently is controlled by the activity of enzymes producing photorespiration metabolites and the activity of phospheonolpyruvate carboxylase.     

Regulation of ribulose-1,5-bisphosphate carboxylase from tobacco: changes in pH response and affinity for CO2 and Mg2+ induced by chloroplast intermediates

Ribulose-1,5-bisphosphate caryboxylase-oxygenase is activated by CO₂ and Mg²⁺ in a process distinct from catalysis. The effect of chloroplast metabolites as they separately influenced either activation or catalysis of tobacco carboxylase was examined. Of the 28 metabolites examined, 13 effected activation of the carboxylase. The strongest positive effectors were NADPH, gluconate-6-P, glycerate-2-P, and glycerate-3-P. Negative effectors included ribose-5-P, fructose-6-P, glucose-6-P, and pyrophosphate. The concentration of CO₂ or Mg²⁺ necessary to produce half-maximal activation is defined as K_act. NADPH and gluconate-6-P decreased the K_act(CO₂) from 43 to 7.4 and 3.5 μm, respectively (pH 8.0, 5 mm MgCl₂). They also decreased the K_act(M.g²⁺), but had little affect on the affinity of the enzyme for CO₂ during the catalytic process. Increasing Mg²⁺ concentration decreased the K_act(CO₂) and increasing CO₂ concentration decreased the K_act-(Mg²⁺). NADP⁺ and gluconate-6-P also affected the pH profile of activation, shifting it toward lower pH values. Changes in activation had no effect on the pH profile for catalysis of CO₂ fixation. Effectors influenced ribulose-1,5-bisphosphate oxygenase in a manner analogous to the carboxylase. At air levels of O₂ and CO₂, the ratio of carboxylase to oxygenase activity was not changed by the presence of effectors, including hydroxylamine.     

Boron Deficiency Induced Changes in Translocation of 14CO2-Photosynthate Into Primary Metabolites in Relation to Essential Oil and Curcumin Accumulation in Turmeric (Curcuma longa L.)

Changes in leaf growth, net photosynthetic rate (P _N), incorporation pattern of photosynthetically fixed ¹⁴CO₂ in leaves 1–4 from top, roots, and rhizome, and in essential oil and curcumin contents were studied in turmeric plants grown in nutrient solution at boron (B) concentrations of 0 and 0.5 g m^-3. B deficiency resulted in decrease in leaf area, fresh and dry mass, chlorophyll (Chl) content, and P _N and total ¹⁴CO₂ incorporated at all leaf positions, the maximum effect being in young growing leaves. The incorporation of ¹⁴CO₂ declined with leaf position being maximal in the youngest leaf. B deficiency resulted in reduced accumulation of sugars, amino acids, and organic acids at all leaf positions. Translocation of the metabolites towards rhizome and roots decreased. In rhizome, the amount of amino acids increased but content of organic acids did not show any change, whereas in roots there was decrease in contents of these metabolites as a result of B deficiency. Photoassimilate partitioning to essential oil in leaf and to curcumin in rhizome decreased. Although the curcumin content of rhizome increased due to B deficiency, the overall rhizome yield and curcumin yield decreased. The influence of B deficiency on leaf area, fresh and dry masses, CO₂ exchange rate, oil content, and rhizome and curcumin yields can be ascribed to reduced photosynthate formation and translocation.     

Gas exchange and air flow in the lung of the snake,Pituophis melanoleucus

Summary Gas samples from various regions of the lung were obtained throughout the breathing cycle inPituophis melanoleucus. Changes in CO₂ concentration during the interbreath period differed markedly along the length of the lung. In general, the largest and most rapid increases in CO₂ tension were measured at the cranial end of the vascular lung. Further caudad in the vascular lung, the increase was slower and did not reach mixed venous CO₂ tension before exhalation. In animals exhibiting the lowest breathing frequencies and presumably larger tidal volumes, the region of gas exchange extended into the cranial portion of the air sac. There was little or no change in gas tensions within the remaining caudal regions of the air sac. Measurement of exhaled CO₂ and O₂ tensions at the nares confirmed the longitudinal gradient in gas exchange and also demonstrated the sequential emptying of the lung. Large regional differences in the ratio of blood flow to alveolar volume are probably responsible for the gradients in lung gases.Interpretation of N₂ clearance curves in terms of two freely communicating compartments demonstrated the presence of a ventilation inequality. Consistent with this was the lack of body wall contractions between breaths while animals were resting. However, just prior to and during activity body wall contractions not associated with breathing often occurred and resulted in pressure excursions in the lung of ca. five mm H₂O. In addition, the heart beat results in a pressure change within the lung of ca. 0.2 mmH₂O which may be significant in gas mixing.     

The effect of feeding on CO2 production and energy expenditure in ponies measured by indirect calorimetry and the 13C-bicarbonate technique

Energy expenditure (EE) can be estimated based on respiratory gas exchange measurements, traditionally done in respiration chambers by indirect calorimetry (IC). However, the ¹³C-bicarbonate technique (¹³C-BT) might be an alternative minimal invasive method for estimation of CO₂ production and EE in the field. In this study, four Shetland ponies were used to explore the effect of feeding on CO₂ production and EE measured simultaneously by IC and ¹³C-BT. The ponies were individually housed in respiration chambers and received either a single oral or intravenous (IV) bolus dose of ¹³C-labelled sodium bicarbonate (NaH¹³CO₃). The ponies were fed haylage 3 h before (T₋₃), simultaneously with (T₀) or 3 h after (T₊₃) administration of ¹³C-bicarbonate. The CO₂ produced and O₂ consumed by the ponies were measured for 6 h with both administration routes of ¹³C-bicarbonate at the three different feeding times. Feeding time affected the CO₂ production (P<0.001) and O₂ consumption (P<0.001), but not the respiratory quotient (RQ) measured by IC. The recovery factor (RF) of ¹³C in breath CO₂ was affected by feeding time (P<0.01) and three different RF were used in the calculation of CO₂ production measured by ¹³C-BT. An average RQ was used for the calculations of EE. There was no difference between IC and ¹³C-BT for estimation of CO₂ production. An effect of feeding time (P<0.001) on the estimated EE was found, with higher EE when feed was offered (T₀ and T₊₃) compared with when no feed was available (T₋₃) during measurements. In conclusion, this study showed that feeding time affects the RF and measurements of CO₂ production and EE. This should be considered when the ¹³C-BT is used in the field. IV administration of ¹³C-bicarbonate is recommended in future studies with horses to avoid complex ¹³C enrichment-time curves with maxima and shoulders as observed in several experiments with oral administration of ¹³C-bicarbonate.     

Effect of ammonia on respiratory functions of blood of Tilapia zilli

The present paper attempts an examination of different changes of blood respiratory properties when Tilapia zilli is exposed to ammonia in three sublethal concentrations (1.1, 2.2 and 3.3 mg NH₃ l⁻¹) for 2 weeks. The results revealed that oxygen and carbon dioxide partial pressures (P_O2 and P_CO2) were changed differently and irregularly both in the caudal artery and in the heart. The acid–base status (pH, HCO⁻₃, TCO₂ and base excess) of arterial and venous blood changed towards alkalosis during the first week. These changes were exaggerated during the second week of ammonia exposure. O₂ saturation of arterial blood was decreased, while that of venous blood was increased due to the disturbances in blood gas transport and exchange mechanisms and in the acid–base status. The oxygen equilibrium curve was shifted to the left and P₅₀ was decreased during most of the experimental periods.     

Respiratory properties of the blood of the burrowing red band fish Cepola rubescens L.

The respiratory properties of the whole blood of the burrowing red band fish Cepola rubescens L. were investigated. Oxygen dissociation curves constructed at 15°C were found to be close to hyperbolic in shape with a mean value for the cooperativity coefficient at half-saturation (n₅₀) of 1.56. Half-saturation oxygen tension (P₅₀) for pH = 7.56 (mean in vivo pH of venous blood) was 27 Torr. The blood showed a marked Bohr effect ($\text{Δ log}\text{P}{}_{50}\text{ΔpH}\text{= ?1.19}$) and also a Root effect which at the in vivo pH reduced oxygen carrying capacity by 20%. The Pv_CO2 was 3.2 Torr and the buffering power of the blood was low, the buffer value of true plasma averaging 5.43 mmol · 1^?1 · pH^?1. It is suggested that the large Bohr effect coupled with the low buffer value confers on the haemoglobin a flexibility, in terms of oxygen affinity, to withstand changes which occur in environmental oxygen tensions.     

Predictability of PaO2 in different inert gas-oxygen environments

Recent experimentation with metabolic changes in rats exposed to thermally isoconductive environments has involved the use of inert gas-oxygen mixtures with different total pressures (PT) and inspired oxygen fractions (F₁O₂). To determine the F_IO₂ for each mixture that would result in similar arterial oxygen pressures (P _a O₂) and saturations (OS), arterial blood of dogs breathing the mixtures through a mask was analyzed for pH,^PO₂,^PCO₂, and OS. Using P_IO₂'s calculated from the alveolar gas equation as a theoretical basis, the oxygen partial pressure for the helium mixture had to be increased by 5.6% while that in argon decreased by 10.2% below the computed values to obtain P _a O₂'s acceptably similar to those resulting from air exposure. No consistent variation in pH, P _a CO₂, and OS were apparent. Based on the data presented, which were obtained under highly specialized conditions, it appears that the use of helium and argon as diluent gases may significantly affect arterial oxygen tension.     

Fish red blood cell carbon dioxide transport in vitro: A comparative study

Red blood cell (rbc) carbon dioxide transport was examined in vitro in three teleosts (Oncorhynchus mykiss, Anguilla anguilla, Scophthalmus maximus) and an elasmobranch (Scyliorhinus canicula) using a radioisotopic assay that measures the net conversion of plasma HCO₃⁻ to CO₂. The experiments were designed to compare the intrinsic rates of rbc CO₂ excretion and the impact of haemoglobin oxygenation/deoxygenation among the species.Under conditions simulating in vivo levels of plasma HCO₃⁻ and natural haematocrits, the rate of whole blood CO₂ excretion varied between 14.0 μmol ml⁻¹ h⁻¹ (S. canicula) and 17.6 μmol ml⁻¹ h⁻¹ (O. mykiss). The rate of CO₂ excretion in separated plasma was significantly greater in the dogfish, S. canicula. The contribution of the rbc to overall whole blood CO₂ excretion was low in the dogfish (46 ± 6%) compared to the teleosts (trout, 71 ± 4%; turbot, 64 ± 5%; eel, 55 ± 3%).To eliminate the naturally occurring differences in haematocrit and plasma [HCO₃⁻] as inter-specific variables, the rates of whole blood CO₂ excretion were determined in blood that had been resuspended to constant [HCO₃⁻] (5 mmol⁻¹) and haematocrit (20%) in appropriate teleost and elasmobranch Ringer solutions. Under such normalized conditions, the rate of whole blood CO₂ excretion was significantly higher in the turbot (22.4 ± 1.3 μmol ml⁻¹ h⁻¹) in comparison to the other species (16.4–18.4 μmol ml⁻¹ h⁻¹) and thus revealed a greater intrinsic rate of rbc CO₂ excretion in the turbot.To study the contribution of Bohr protons, the rates of whole blood CO₂ excretion were assessed in blood subjected to rapid oxygenation during the initial phase of the 3 min assay period. Rapid oxygenation significantly enhanced the rate of CO₂ excretion in the teleosts but not in the elasmobranch. The extent of the increase provided by the rapid oxygenation of haemoglobin was a linear function of the extent of the Haldane effect, as quantified in each species from in vitro CO₂ dissociation (combining) curves. Under steady-state conditions, deoxygenated blood exhibited greater rates of CO₂ excretion than oxygenated blood in the teleosts but not in the elasmobranch. As a consequence of the Haldane effect, rbc intracellular pH was increased in the teleosts by deoxygenation but was unaltered in the elasmobranch.The results, by extrapolation, suggest that the rates of CO₂ excretion in vivo are influenced by the magnitude of the Haldane effect and the extent of haemoglobin oxygenation during gill transit in addition to the intrinsic rate at which the rbc converts plasma HCO₃⁻ to CO₂.     

Quantitative evaluation of respiration induced metabolic oscillations in erythrocytes

The changes in the partial pressures of oxygen and carbon dioxide (P_O2 and P_CO2) during blood circulation alter erythrocyte metabolism, hereby causing flux changes between oxygenated and deoxygenated blood. In the study we have modeled this effect by extending the comprehensive kinetic model by Mulquiney and Kuchel [P.J. Mulquiney, and P.W. Kuchel. Model of 2,3-bisphosphoglycerate metabolism in the human erythrocyte based on detailed enzyme kinetic equations: equations and parameter refinement, Biochem. J. 1999, 342, 581–596.] with a kinetic model of hemoglobin oxy-/deoxygenation transition based on an oxygen dissociation model developed by Dash and Bassingthwaighte [R. Dash, and J. Bassingthwaighte. Blood HbO₂ and HbCO₂ dissociation curves at varied O₂, CO₂, pH, 2,3-DPG and temperature levels, Ann. Biomed. Eng., 2004, 32(12), 1676–1693.]. The system has been studied during transitions from the arterial to the venous phases by simply forcing P_O2 and P_CO2 to follow the physiological values of venous and arterial blood. The investigations show that the system passively follows a limit cycle driven by the forced oscillations of P_O2 and is thus inadequately described solely by steady state consideration. The metabolic system exhibits a broad distribution of time scales. Relaxations of modes with hemoglobin and Mg²⁺ binding reactions are very fast, while modes involving glycolytic, membrane transport and 2,3-BPG shunt reactions are much slower. Incomplete slow mode relaxations during the 60 s period of the forced transitions cause significant overshoots of important fluxes and metabolite concentrations – notably ATP, 2,3-BPG, and Mg²⁺. The overshoot phenomenon arises in consequence of a periodical forcing and is likely to be widespread in nature – warranting a special consideration for relevant systems.