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Summary: For many organisms, respiratory gas exchange is a vital activity and different types of gas-exchange apparatus have evolved to meet individual needs. They include not only skin, gills, tracheal systems and lungs but also transient structures such as the chorioallantois of avian eggs and the placenta of eutherian mammals. The ability of these structures to allow passage of oxygen by passive diffusion can be expressed as a diffusive conductance (units: cm3 O2 min−1 kPa−1). Occasionally, the ability to estimate diffusive conductance by physiological techniques is compromised by the difficulty of obtaining O2 partial pressures on opposite sides of the tissue interface between the delivery medium (air, water, blood) and uptake medium (usually blood). An alternative strategy is to estimate a morphometric diffusive conductance by combining stereological estimates of key structural quantities (volumes, surface areas, membrane thicknesses) with complementary physicochemical data (O2-haemoglobin chemical reaction rates and Krogh's permeability coefficients). This approach has proved valuable in a variety of comparative studies on respiratory organs from diverse species. The underlying principles were formulated in pioneering studies on the pulmonary lung but are illustrated here by taking the human placenta as the gas exchanger. [ABSTRACT FROM AUTHOR]

Pore network modeling (PNM) based on networks extracted from tomograms is a well-established tool for simulating pore-scale transport behavior in porous media. A key element of this approach is the accurate determination of pore-to-pore conductance values, which is a complex task that greatly affects the accuracy of flow and diffusive mass transport studies. Classic methods of conductance estimation based on analytical solutions and shape factors only apply to simple pore geometries, whereas real porous media contain irregular-shaped pores. Although direct numerical simulations (DNS) can accurately estimate conductance considering pores' real morphology, it has a high computational cost that becomes infeasible for large tomograms. The present work remedies this problem using a deep learning (DL) approach, with a specific focus on diffusional transport which has received less attention than hydraulic conductance. A convolutional neural network (CNN) model was trained to estimate diffusive conductance of PNM elements from volumetric images of porous media. The developed framework estimates the diffusive conductance by analyzing individual pore-to-pore 3D images isolated from the tomogram to fully capture the topology and shapes. A key outcome of the present work is that only images of the pore regions are used as input data, avoiding excessive preprocessing time for data preparation. The results of the diffusive conductance prediction show good agreement with the test data obtained by DNS method, with 0.94 R 2 prediction accuracy and a speedup of 500x in prediction runtime. • A deep learning model was developed to estimate diffusive conductance of pore network elements. • Direct numerical simulations of diffusive transport was used as ground truth for training. • The deep learning model uses only images of pore to pore regions as input. • The trained model can predict conductances in seconds within a high accuracy. • Pore networks using the deep learning-based conductance predict formation factor twice as accurately. [ABSTRACT FROM AUTHOR]

Several species of the stem succulent Caralluma (Apocynaceae) are abundant perennials in arid regions of the Arabian Peninsula. These arid regions have a short wet season with erratic rainfall and are characterized by harsh climatic conditions of high temperature, high evaporation and sand storms. Work presented in this paper aimed at investigating importance of Crassulacean Acid Metabolism (CAM) for survival of three Caralluma species in their natural habitat. Investigations involved studying stomatal characteristics, stomatal diffusive conductance, chlorophyll fluorescence, and CAM in three species of Caralluma, namely C. acutangula (Decne.) N.E.Br., C. edulis (Edgew.) Benth. ex Hook.f., and C. subulata (Forssk.) Decne. Microscopic examination revealed a pattern of stomatal characteristics typical of CAM plants in these three Caralluma species. Results showed that these three Caralluma species were obligate CAM plants exhibiting this mode of photosynthesis during both the wet and the dry seasons. Under protracted water stress during the long dry season very low values of stomatal diffusive conductance and dampening of CAM acidification-deacidification cycles denoted the tendency of these three Caralluma species to shift from the obligate CAM physiotype to CAM-idling mode. Chlorophyll fluorescence measurements indicated that protracted water stress induced a reduction in Photosystem II (PSII) antenna efficiency and quantum yield in the three studied Caralluma species. This reduction of PSII activity occurred in concomitance with a marked rise in non-photochemical quenching of chlorophyll fluorescence denoting operation of non-photochemical energy dissipating mechanisms known to be important for photoprotection of the photosynthetic apparatus. [ABSTRACT FROM AUTHOR]

Background and Aims The rate of photosynthesis in paddy rice often decreases at noon on sunny days because of water stress, even under submerged conditions. Maintenance of higher rates of photosynthesis during the day might improve both yield and dry matter production in paddy rice. A high-yielding indica variety, ‘Habataki’, maintains a high rate of leaf photosynthesis during the daytime because of the higher hydraulic conductance from roots to leaves than in the standard japonica variety ‘Sasanishiki’. This research was conducted to characterize the trait responsible for the higher hydraulic conductance in ‘Habataki’ and identified a chromosome region for the high hydraulic conductance. Methods Hydraulic conductance to passive water transport and to osmotic water transport was determined for plants under intense transpiration and for plants without transpiration, respectively. The varietal difference in hydraulic conductance was examined with respect to root surface area and hydraulic conductivity (hydraulic conductance per root surface area, Lp). To identify the chromosome region responsible for higher hydraulic conductance, chromosome segment substitution lines (CSSLs) derived from a cross between ‘Sasanishiki’ and ‘Habataki’ were used. Key Results The significantly higher hydraulic conductance resulted from the larger root surface area not from Lp in ‘Habataki’. A chromosome region associated with the elevated hydraulic conductance was detected between RM3916 and RM2431 on the long arm of chromosome 4. The CSSL, in which this region was substituted with the ‘Habataki’ chromosome segment in the ‘Sasanishiki’ background, had a larger root mass than ‘Sasanishiki’. Conclusions The trait for increasing plant hydraulic conductance and, therefore, maintaining the higher rate of leaf photosynthesis under the conditions of intense transpiration in ‘Habataki’ was identified, and it was estimated that there is at least one chromosome region for the trait located on chromosome 4. [ABSTRACT FROM PUBLISHER]

Untrained rats selectively bred for either high (HCR) or low (LCR) treadmill running capacity previously demonstrated divergent physiological traits as early as the seventh generation (07). We asked whether continued selective breeding to generation 15 (G15) would further increase the divergence in skeletal muscle capillarity, morphometry, and oxidative capacity seen previously at G7. At G15, mean body weight was significantly lower (P < 0.001) in the HCR rats (n = 11; 194 ± 3 g) than in LCR (n = 12; 259 ± 9 g) while relative medial gastrocnemius muscle mass was not different (0.23 ± 0.01 vs. 0.22 ± 0.01% total body weight). Normoxic (FIO2 = 0.21) VO2max was 50% greater (P < 0.001) in HCR despite the lower absolute muscle mass, and skeletal muscle O2 conductance (measured in hypoxia; FIO2 = 0.10) was 49% higher in HCR (P <0.001). Muscle oxidative enzyme activities were significantly higher in HCR (citrate synthase: 16.4 ± 0.4 vs. 14.0 ± 0.6; β-hydroxyacyl-CoA dehydrogenase: 5.2 ± 0.2 vs. 4.2 ± 0.2 mmol∙kg-1∙min-1). HCR rats had -36% more. total muscle fibers and also 36% more capillaries in the medial gastrocnemius. Because average muscle fiber area was 35% smaller, capillary density was 36% higher in HCR, but capillary-to- fiber ratio was the same. Compared with G7, G15 HCR animals showed 38% greater total fiber number with an additional 25% decrease in mean fiber area: These data suggest that many of the skeletal muscle structural and functional adaptations enabling greater O2 utilization in HCR at G7 continue to progress following additional selective breeding for endurance capacity. However, the largest changes at G15 relate to O2 delivery to skeletal muscle and not to the capacity of skeletal muscle to use O2. [ABSTRACT FROM AUTHOR]

Abstract: We test the null hypothesis that the morphometric diffusive conductance of the placental villous membrane does not alter in pregnancies complicated by intrauterine growth restriction (IUGR) or pre-eclampsia (PE). Placentas were collected from cases of normotensive IUGR, pure PE, PE+IUGR and from control pregnancies. Microscopical fields on formalin-fixed, trichrome-stained histological sections were randomly sampled for location and orientation. Using stereological methods, the exchange surface areas of peripheral (terminal and intermediate) villi and their fetal capillaries and the arithmetic and harmonic mean thicknesses of the villous membrane (maternal aspect of trophoblast to luminal aspect of vascular endothelium) were estimated. An index of the variability in thickness of this membrane, and an estimate of its oxygen diffusive conductance, was derived secondarily. Group comparisons were drawn using two-way analysis of variance to identify main effects (of PE or IUGR) and interaction effects (between PE and IUGR). PE did not have significant effects on placental morphology and there were no significant effects of PE or IUGR on membrane thickness or its variability. In contrast, IUGR (with or without PE) was associated with reduced surface areas and this was the principal factor leading to a smaller membrane diffusive conductance in these placentas. When account was taken of fetal mass, specific conductance showed no effects of PE or IUGR despite the mass-specific conductance in pure IUGR placentas appearing to be smaller than that in controls. The decline in total conductances is indicative of perturbations operating at the levels of villous trophoblast and fetal vasculature and these may contribute to fetal hypoxic stress. [Copyright &y& Elsevier]

Conclusion. These results hold promise that morphometric analysis can be used to generate transMEM (middle ear mucosa) gas conductance estimates for MEM geometries representative of the shift from healthy to pathologic states (e.g. increased MEM thickness and capillary density). Objectives. Novel strategies to treat otitis media with effusion require a better understanding of how MEM geometry affects gas transport. Earlier studies developed techniques to empirically measure transMEM gas conductance and to estimate conductance using morphometric models of MEM geometry. We used chinchillas to determine the correspondence between experimentally measured transMEM CO2 conductance and that predicted by morphometric study of the MEM. Materials and methods. TransMEM CO2 conductance was measured unilaterally in 10 chinchillas; the animals were killed and the ME was removed and processed for morphometric analyses of MEM geometry. Results. The average measured and estimated transMEM CO2 conductances were 4.87±2.30×10-10 and 1.75±0.29×10-10 mol/s/mmHg, respectively. The magnitude and direction of the estimate error were similar for all ears, suggesting a fixed, negative bias to the estimate. A theoretically consistent source for this bias was identified as the representation of the true diffusional length within a 3-D geometry using a 2-D modeling platform. Best estimate correction for this effect based on available data significantly reduced the estimate bias. [ABSTRACT FROM AUTHOR]

Correlations between placental size and fetal mass during gestation fail to account for changes in composition that accompany placental growth and maturation. This study uses stereological data on the sizes of different tissue compartments in human placentas from 10 weeks of gestation to term and relates them to placental volume and to fetal mass by means of allometric analysis. In addition, tissue dimensions are used to calculate a physiological transport measure (diffusive conductance) for the villous membrane. Histological sections randomly sampled from placentas and analysed stereologically provided estimates of structural quantities (volumes, exchange surface areas, lengths, numbers of nuclei, diffusion distances). These data were combined with a physicochemical quantity (Krogh's diffusion coefficient) in order to estimate oxygen diffusive conductances for the villous membrane and its two components (trophoblast and stroma). Allometric relationships between these quantities and placental volume or fetal mass were obtained by linear regression analyses after log-transformation. Placental tissues had different growth trajectories: most grew more rapidly than placental volume and all grew more slowly than fetal mass. Diffusion distances were inversely related to placental and fetal size. Differential growth impacted on diffusive conductances, which, again, did not improve commensurately with placental volume but did match exactly growth of the fetus. Findings show that successful integration between supply and demand can be achieved by differential tissue growth. Allometric analysis of results from recent studies on the murine placenta suggest further that diffusive conductances may also be matched to fetal mass during gestation and to fetal mass at term across species. [ABSTRACT FROM AUTHOR]

Background: Morphometric oxygen diffusive conductance (Dp) was estimated to assess the potential efficiency of oxygen transfer across the materno–fetal interface in placentae obtained from victims of sudden infant death syndrome (SIDS). Study design: SIDS placentae were retrieved from archived storage and classified into normal birth weight (NBW, n=16), or small for gestational age (SGA, n=9) and compared against control placentae (n=40) or SGA (n=24) placentae. A combination of stereological techniques and physiological constants were used to estimate total Dp. Results: SIDS NBW cases showed a crucial reduction in fetal capillary surface area when compared with control placentae. SIDS SGA showed a number of deficiencies in basic volumetric and surface area parameters. Values for total and specific Dp in placentae in both SIDS groups were maintained at levels comparable with control and SGA cases, respectively. Conclusion: Since more reductions were observed in SIDS SGA group, this suggests that factors responsible for these reductions maybe associated with SGA rather than being SIDS-specific factors. [Copyright &y& Elsevier]

Diurnal and seasonal changes of net photosynthetic rate (Pn) and the efficiency of photosystem 2 (Fv/Fm) were measured on two perennial species growing on a soil catena in semi-arid south-east Spain. Stipa tenacissima, a tussock grass, grows on shallow soil at the top of the catena and Retama sphaerocarpa, a leguminous shrub, grows in the valley bottom. A linear relationship was found between light saturated photosynthetic rates (Pmax) and diffusive leaf conductance (gl) in both Retama and Stipa indicating that the intercellular CO2 concentration (ci) was maintained constant in both species diurnally. Relatively high values of calculated ci in Retama cladodes suggested that was not the primary limitation to carbon assimilation. Fv/Fm for the two species when well watered was around 0.8. Although Retama cladodes maintained this value throughout the year, Fv/Fm decreased to a minimum of 0.43 in Stipa leaves, at the end of the dry season. Our data suggest that plants in the Rambla Honda can substantially reduce transpiration without reducing photosynthetic rates to the same extent by closing their stomata, because Pn is reduced primarily by high respiration, decreased mesophyll conductance and by photoinhibition or permanent damage of photosystem 2. [ABSTRACT FROM AUTHOR]

Stomatal density and diffusive conductance were characterized for both leaf surfaces in three amphistomatous hybrid cultivars of poplar. Consistent differences in stomatal density were observed between cultivars and between leaf surfaces within a cultivar. Mean stomatal density in the three cultivars ranged from 150 to 330 stomata mm-2 for abaxial leaf surfaces and from 75 to 100 stomata mm-2 for adaxial surfaces. The density of stomata on the abaxial versus adaxial surface was related to the spatial orientation of leaves with respect to the horizon and this stomatal ratio ranged from 1.4 to 4.0 in the three clones. Also, stomatal density was greater in leaves at higher rather than lower nodal positions. Differences in diffusive conductance between cultivars and leaf surfaces were observed on intact and detached leaves in the light and dark. Within each cultivar mean abaxial conductance (kab) was greater than adaxial conductance (kad). Mean conductances in the light for the three cultivars ranged from 0.22 to 0.62 cm s-1 for abaxial, and from 0.15 to 0.17 cm s-1 for adaxial surfaces, and in the dark they were between 0.06 and 0.26 cm s-1 for abaxial, and from 0.04 to 0.06 for adaxial surfaces. The differences in conductance between cultivars and between leaf surfaces were correlated with their respective stomatal densities. Stomatal response to light and to leaf excision also varied between cultivars and between the two leaf surfaces. [ABSTRACT FROM AUTHOR]

Leaf surfaces of Festuca arundinacea Schreb. were subjected to controlled polishing and abrasion on a test-bed designed to stimulate components of abrasive wind damage. Both treatments substantially increased the leaf surface conductance, particularly the polishing treatment. Scanning electron micrographs of cryo-fixed leaf surfaces showed displacement and smoothing of the epicuticular waxes, damage to the cuticle, collapse of epidermal cells and fracture of trichomes. The importance of the epicuticular waxes in determining leaf diffusivity and permeability is discussed. [ABSTRACT FROM AUTHOR]