Gas Exchange Theory Research Articles

An Improved Conceptual Model Quantifying the Effect of Climate Change and Anthropogenic Activities on Vegetation Change in Arid Regions

Vegetation shows a greening trend on the global scale in the past decades, which has an important effect on the hydrological cycle, and thus quantitative interpretation of the causes for vegetation change is of great benefit to understanding changes in ecology, climate, and hydrology. Although the Donohue13 model, a simple conceptual model based on gas exchange theory, provides an effective tool to interpret the greening trend, it cannot be used to evaluate the impact from land use and land cover change (LULCC) on the regional scale, whose importance to vegetation change has been demonstrated in a large number of studies. Hence, we have improved the Donohue13 model by taking into account the change in vegetation cover ratio due to LULCC, and applied this model to the Yarkand Oasis in the arid region of northwest China. The estimated change trend in leaf area index (LAI) is 1.20%/year from 2001 to 2017, which accounts for approximately half of the observed (2.31%/year) by the moderate resolution imaging spectroradiometer (MODIS). Regarding the causes for vegetation greening, the contributions of: (1) LULCC; (2) atmospheric CO2 concentration; and (3) vapor pressure deficit were: (1) 88.3%; (2) 40.0%; and (3) −28.3%, respectively, which reveals that the largest contribution was from LULCC, which is probably driven by increased total water availability in whole oasis with a constant transpiration in vegetation area. The improved Donohue13 model, a simple but physics-based model, can partially explain the impact of factors related to climate change and anthropogenic activity on vegetation change in arid regions. It can be further combined with the Budyko hypothesis to establish a framework for quantifying the changes in coupled response of vegetation and hydrological processes to environment changes.

Anatomical backgrounds on gas exchange parameters in the lung

Many problems regarding structure-function relationships have remained unsolved in the field of respiratory physiology. In the present review, we highlighted these uncertain issues from a variety of anatomical and physiological viewpoints. Model A of Weibel in which dichotomously branching airways are incorporated should be used for analyzing gas mixing in conducting and acinar airways. Acinus of Loeschcke is taken as an anatomical gas-exchange unit. Although it is difficult to define functional gas-exchange unit in a way entirely consistent with anatomical structures, acinus of Aschoff may serve as a functional gas-exchange unit in a first approximation. Based on anatomical and physiological perspectives, the multiple inert-gas elimination technique is thought to be highly effective for predicting ventilation-perfusion heterogeneity between acini of Aschoff under steady-state condition. Changes in effective alveolar PO2, the most important parameter in classical gas-exchange theory, are coherent with those in mixed alveolar PO2 decided from the multiple inert-gas elimination technique. Therefore, effective alveolar-arterial PO2 difference is considered useful for assessing gas-exchange abnormalities in lung periphery. However, one should be aware that although alveolar-arterial PO2 difference sensitively detects moderately low ventilation-perfusion regions causing hypoxemia, it is insensitive to abnormal gas exchange evoked by very low and high ventilation-perfusion regions. Pulmonary diffusing capacity for CO (DLCO) and the value corrected for alveolar volume (VAV), i.e. DLCO/VAV (KCO), are thought to be crucial for diagnosing alveolar-wall damages. DLCO-related parameters have higher sensitivity to detecting abnormalities in pulmonary microcirculation than those in the alveolocapillary membrane. We would like to recommend four categories derived from combining behaviors of DLCO with those of KCO for differential diagnosis on anatomically morbid states in alveolar walls: type-1 abnormality defined by decrease in both DLCO and KCO; type-2 abnormality by decrease in DLCO but increase in KCO; type-3 abnormality by decrease in DLCO but restricted rise in KCO; and type-4 abnormality by increase in both DLCO and KCO.

Long‐term CO2 fertilization increases vegetation productivity and has little effect on hydrological partitioning in tropical rainforests

AbstractUnderstanding how tropical rainforests respond to elevated atmospheric CO2 concentration (eCO2) is essential for predicting Earth's carbon, water, and energy budgets under future climate change. Here we use long‐term (1982–2010) precipitation (P) and runoff (Q) measurements to infer runoff coefficient (Q/P) and evapotranspiration (E) trends across 18 unimpaired tropical rainforest catchments. We complement that analysis by using satellite observations coupled with ecosystem process modeling (using both “top‐down” and “bottom‐up” perspectives) to examine trends in carbon uptake and relate that to the observed changes in Q/P and E. Our results show there have been only minor changes in the satellite‐observed canopy leaf area over 1982–2010, suggesting that eCO2 has not increased vegetation leaf area in tropical rainforests and therefore any plant response to eCO2 occurs at the leaf level. Meanwhile, observed Q/P and E also remained relatively constant in the 18 catchments, implying an unchanged hydrological partitioning and thus approximately conserved transpiration under eCO2. For the same period, using a top‐down model based on gas exchange theory, we predict increases in plant assimilation (A) and light use efficiency (ε) at the leaf level under eCO2, the magnitude of which is essentially that of eCO2 (i.e., ~12% over 1982–2010). Simulations from 10 state‐of‐the‐art bottom‐up ecosystem models over the same catchments also show that the direct effect of eCO2 is to mostly increase A and ε with little impact on E. Our findings add to the current limited pool of knowledge regarding the long‐term eCO2 impacts in tropical rainforests.

Impact of CO2 fertilization on maximum foliage cover across the globe's warm, arid environments

Satellite observations reveal a greening of the globe over recent decades. The role in this greening of the “CO2 fertilization” effect—the enhancement of photosynthesis due to rising CO2 levels—is yet to be established. The direct CO2 effect on vegetation should be most clearly expressed in warm, arid environments where water is the dominant limit to vegetation growth. Using gas exchange theory, we predict that the 14% increase in atmospheric CO2 (1982–2010) led to a 5 to 10% increase in green foliage cover in warm, arid environments. Satellite observations, analyzed to remove the effect of variations in precipitation, show that cover across these environments has increased by 11%. Our results confirm that the anticipated CO2 fertilization effect is occurring alongside ongoing anthropogenic perturbations to the carbon cycle and that the fertilization effect is now a significant land surface process.

An interannual assessment of the relationship between the stable carbon isotopic composition of ecosystem respiration and climate in a high-elevation subalpine forest

[1] We measured the carbon isotopic composition (δ13C) of ecosystem respiration (δ13CR) in a subalpine forest across four growing seasons to examine whether patterns in δ13CR were consistent with those expected based on leaf-level gas-exchange theory, and in agreement with past studies of the relation between δ13CR and climate conducted across broad geographic regions. Conventional trends (i.e., less negative δ13CR with increased vapor pressure deficit (VPD) and air temperature (TAIR), and decreased soil moisture (θ)) were observed when we focused on the driest portions of average-wetness years and when δ13CR was positively correlated with nighttime ecosystem respiration (RE). Nonconventional trends (i.e., more negative δ13CR with decreased θ, and increased VPD and TAIR) were observed under specific climatic conditions (e.g., late snowmelt; extreme TAIR late in the growing season), and when δ13CR was negatively correlated with RE. These nonconventional trends were independently corroborated using δ13C of extracted sugars from needles of dominant tree species at the site. Our results clearly demonstrate that the commonly reported relations between δ13CR and climate may break down depending on the interactions among environmental conditions. Efforts to model and predict the variability of δ13CR under changing climatic variables must characterize and parameterize the effects of unique combinations of weather conditions and variable hydrologic regimes, in combination with the susceptibility of photosynthetic isotope discrimination to extreme air temperatures.

Flat beer vs. physiological improvement: effect of acetazolamide during hypoxic exercise

Flat beer <i>vs.</i> physiological improvement: effect of acetazolamide during hypoxic exercise

Gas Exchange, Carbon Isotope Discrimination, and Productivity in Winter Wheat

Leaf gas exchange theory indicates that carbon isotope discrimination (∆) is negatively associated with CER/g3 (leaf CO2 exchange rate/conductance). Discovery of genetic variation for ∆ in wheat (Triticum aestivum L.) and other species has generated interest in using carbon isotope discrimination in plant breeding. However, the hypothesized negative association between CER/g3 and &amp; has sometimes not been observed in field‐grown crops. In the present study, CER/g3, &amp; of peduncle tissue, plant productivity and yield were examined in eight winter wheat genotypes under both irrigated and nonirrigated field conditions. Water stress‐ and genotype‐induced variations in CER/g3 were more consistently correlated with variations in gs than with CER. However, low CER in ‘Bezostaya’ wheat resulted in a consistently low genotypic ranking for CER/g3. In 1988, values of CER/g3 increased with water stress, while ∆ values dropped from 18.17 to 17.48‰. These results imply a negative relationship between CER/g3 and ∆, consistent with leaf gas exchange theory; however, ∆ was both positively (1988; r = 0.83** [P ≤ 0.01]) and negatively (1989; r = ‐0.79* [P ≤ 0.05]) associated with CER/g3 in irrigated plots. Positive correlations were obtained between ∆ and both biomass productivity (1988, r = 0.54*; 1989, r = 0.45 [P = 0.08]) and grain yield (1988, r = 0.66*; 1989, r = 0.55*) when data were averaged across genotypes and irrigation treatments. Consistently low ∆ values were obtained for ‘Sturdy’, an early‐released, drought‐susceptible cultivar with low productivity. These results suggest that genotypic rankings for peduncle ∆ must be carefully interpreted, as low ∆ can be associated with low yield and stress susceptibility in wheat.

Effects of acetone in heparin on the multiple inert gas elimination technique.

We have detected acetone in several brands of heparin. If uncorrected, this leads to errors in measuring acetone in blood collected in heparinized syringes, as in the multiple inert gas elimination technique for measuring ventilation-perfusion ratio (VA/Q) distributions. Error for acetone retention [R = arterial partial pressure-to-mixed venous partial pressure (P-V) ratio] is usually small, because R is normally near 1.0, and the error is similar in arterial and mixed venous samples. However, acetone excretion [E = mixed expired partial pressure (P-E)-to-P-V ratio] will appear erroneously low, because P-E is accurately measured in dry syringes, but P-V is overestimated. A physical model of a homogeneous alveolar lung at room temperature and without dead space shows: the magnitude of acetone E error depends upon the ratio of blood sample to heparinized saline volumes and acetone partial pressures, without correction, acetone E can be less than that of less soluble gases like ether, a situation incompatible with conventional gas exchange theory, and acetone R and E can be correctly calculated using the principle of mass balance if the acetone partial pressure in heparinized saline is known. Published data from multiple inert gas elimination experiments with acetone-free heparin, in our labs and others, are within the limits of experimental error. Thus the hypothesis that acetone E is anomalously low because of physiological mechanisms involving dead space tissue capacitance for acetone remains to be tested.

On the theory of gas exchange between the ocean and the atmosphere : Savenko, V.S. and V.L. Lebedev, 1984. Dokl. Akad. Nauk SSSR, 275(5):1185–1189. (In Russian.)

On the theory of gas exchange between the ocean and the atmosphere : Savenko, V.S. and V.L. Lebedev, 1984. Dokl. Akad. Nauk SSSR, 275(5):1185–1189. (In Russian.)

Theory of gas exchange in the avian parabronchus

To understand the distribution of oxygen and carbon dioxide in the avian lung, a theoretical treatment of gas exchange in the parabronchus of the avian lung is described. The model is modified after Zeuthen (1942). In addition to bulk flor through the parabronchial lumen, diffusion through the air spaces of both the parabronchial lumen and air capillaries is treated. The relationship of P O 2 and P CO 2 within the blood capillaries, air capillaries, and parabronchial lumen to parabronchial blood flow and ventilation is graphically shown. The results indicate that the variations of P O 2 and P CO 2 along an air capillary are less than one torr under resting conditions. Removal of diffusion resistance within the air space of the air capillaries increases calculated parabronchial gas exchange by less than 0.1% at rest. At high or resting ventilation rates the partial pressure profile along the parabronchial lumen calculated considering bulk flow only agrees well with the profile calculated considering bulk flow and axial diffusion, but as the ventilation rate decreases there is increasingly large disagreement. Forward diffusion of O 2 toward the parabronchus reduces pre-parabronchial P O 2 and backward diffusion of CO 2 from the parabronchus increases P CO 2 . Neglecting diffusion within the air spaces of both the lumen and the air capillaries increases calculated parabronchial gas exchange by less than 2% (CO 2) or 6% (O 2) at rest.

Some Implications of Leaf Tearing in Musaceae

Leaf temperatures associated with torn and untorn leaves of Musaceae were taken in both dry and wet seasons at Barro Colorado Island, Canal Zone. Transpiration rates and leaf resistance to water—vapor diffusion were determined. Energy—budget analysis is used to describe the relationship of leaf dimension and leaf resistance to thermal survival and water usage. Gas—exchange theory is applied to predict the photosynthetic implications of leaf tearing. The analyses showed that leaves less than 10 cm wide are not subject to critical heat stress, have lower water loss, and higher ratios of photosynthesis to water expended than do leaves of widths greater than 10 cm. In wet season, leaves have lower resistance to the diffusion of water vapor and accordingly are less subject to excessive heating.

Theory of gas exchange in the functionally inhomogeneous human placenta

The effects of irregularities in the maternal blood flow Q̇ M , fetal blood flow Q̇ F , and O 2-diffusing capacity D P on the gas exchange in the human placenta are theoretically examined. In the process we find (1) uneven distribution of Q̇ M / Q̇ F (in this case, Q̇ M / Q̇ F must be up to 40% greater in the functionally inhomogeneous placenta than in a homogeneous placenta, depending on the statistical distribution curve on which it is based, in order to achieve the same arterialization effect); (2) uneven distribution of D P / Q̇ F (regional inhomogeneities in D P / Q̇ F also require greater D P / Q̇ F values than would be necessary for the same gas exchange in the homogeneous placenta); (3) combined distribution irregularities of Q̇ M / Q̇ F and D P / Q̇ F (on the basis of recent measurements, Q̇ M / Q̇ F must be expected to fall within the range 1.0–1.4 and D P / Q̇ F between the limits 5 × 10 −3 [1/ mm Hg] and 11 × 10 −3 [1/ mm Hg] in a placenta with an optional number of statistically distributed inhomogeneities).

INERT GAS-EXCHANGE THEORY USING AN ELECTRIC ANALOGUE.

When an inert gas of moderate or high solubility in blood is inhaled, the rate at which the alveolar concentration rises toward the inspired concentration increases as the inspired concentration is increased. The only previous systematic analysis of whole-body uptake of inert gases to allow for this effect was restricted to a single, artificial, respiratory pattern and the numerical calculations had to be made on a digital computer. This paper develops the theory for a range of respiratory patterns and shows how the computations may be made on a slightly modified form of a simple electric analogue. It is shown that the rate of saturation of the body increases less markedly with inspired concentration if the inspired alveolar ventilation, rather than the expired alveolar ventilation, is kept constant during the saturation process. Conversely, washout is more rapid with a constant inspired ventilation than with a constant expired ventilation. The theory is extended to show how the uptake of one inert gas may substantially affect the uptake of another, administered simultaneously. uptake, distribution and elimination; induction; recovery; drugs; inhaled anesthetics; nitrous oxide; diethyl ether; halothane; computers; ventilation; concentration effect; alveolar ventilation Submitted on February 13, 1964