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    Increased precipitation enhances soil respiration in a semi-arid grassland on the Loess Plateau, China
    (PeerJ Inc., 2021-02-02) Wang Y; Xie Y; Rapson G; Ma H; Jing L; Zhang Y; Zhang J; Li J; Zhu B
    BACKGROUND: Precipitation influences the vulnerability of grassland ecosystems, especially upland grasslands, and soil respiration is critical for carbon cycling in arid grassland ecosystems which typically experience more droughty conditions. METHODS: We used three precipitation treatments to understand the effect of precipitation on soil respiration of a typical arid steppe in the Loess Plateau in north-western China. Precipitation was captured and relocated to simulate precipitation rates of 50%, 100%, and 150% of ambient precipitation. RESULTS AND DISCUSSION: Soil moisture was influenced by all precipitation treatments. Shoot biomass was greater, though non-significantly, as precipitation increased. However, both increase and decrease of precipitation significantly reduced root biomass. There was a positive linear relationship between soil moisture and soil respiration in the study area during the summer (July and August), when most precipitation fell. Soil moisture, soil root biomass, pH, and fungal diversity were predictors of soil respiration based on partial least squares regression, and soil moisture was the best of these. CONCLUSION: Our study highlights the importance of increased precipitation on soil respiration in drylands. Precipitation changes can cause significant alterations in soil properties, microbial fungi, and root biomass, and any surplus or transpired moisture is fed back into the climate, thereby affecting the rate of soil respiration in the future.
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    The links between human breath methane, dietary fibre digestion, and the gut microbiota : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Nutrition Science at Massey University, Manawatu, New Zealand
    (Massey University, 2022) Payling, Laura Marie
    The concentration of methane that is exhaled in human breath has been associated with the composition and fibre-fermenting function of the human colonic microbiota. The current research aimed to investigate whether composition and fibre fermentation function of the colonic microbiota differ in individuals who are low breath methane emitters (LE) or high breath methane emitters (HE). Healthy adult individuals (18) were recruited and breath tested. Unexpectedly, they showed positive correlations between breath hydrogen and methane. Then, the highest and lowest breath methane emitters provided faecal samples used for shotgun metagenomic sequencing and as faecal inocula for in vitro colonic fermentations with dietary fibres (β-glucan and lignocellulose). Individuals who were LE reported higher dietary vitamin E, fibre, and fat intakes and a Bacteroides-driven microbiota composition compared to HE individuals who reported a greater starch intake and a Prevotella-driven microbiota composition. The faecal microbiota from individuals who were HE had a greater abundance of taxa from the Methanobrevibacter genus and more methane gas production during in vitro colonic fibre fermentation compared to the microbiota of individuals who were LE; however, the results were variable within the HE group. There was a greater rate and extent of dietary fibre fermentation in LE compared to HE individuals during in vitro colonic fermentation, which aligned with the greater fibre intakes of LE individuals. Furthermore, the faecal microbiota of LE individuals showed increased beneficial organic acid production and a greater abundance of functional pathways related to amino acid metabolism compared to HE during in vitro colonic fermentation. These results did not align with published research on human breath methane and the gut microbiota. However, there was consensus with emerging hypotheses suggesting that there are important pathways involved in hydrogen sulphide production and hydrogen utilisation that are largely unexplored. Further investigations in these areas could help redefine our understanding of fibre fermentation by the human colonic microbiota.
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    Mathematical modelling of the cardiovascular system to study the effects of respiratory sinus arrhythmia and heart failure : this dissertation is submitted for the degree of Doctor of Philosophy, School of Natural and Computational Science, Massey University
    (Massey University, 2021) Noreen, Shumaila
    This thesis presents the development of lumped parameter models of the cardiovascular system with a specific aim of simulating the system dynamics over a range of heart rates. The models contain several new modelling features that have been introduced progressively throughout the thesis starting with isolated models and continuing with closed loop models of the circulation. Specifically, the contraction of the cardiac chambers is modelled using a time-dependent muscle force with constant elasticity instead of time dependent elasticity. A new hypothesis about the mechanical contraction of the atria generates realistic pressure volume loops. The inter-ventricular interaction is modelled as well. Additionally, hysteresis is incorporated in the aortic valve to produce an end-systolic reverse (negative) flow. Most of the model parameters were taken from the literature and experimental data. Sensitivity analysis was performed on one of the models outputs by changing one parameter at a time; this analysis indicated that the total blood volume is the most influential parameter in the model. The developed models were used to study the effects of Respiratory Sinus Arrhythmia (RSA), variability in heart rate at the frequency of breathing. RSA is an indicator of good health but the mechanism that gives rise to RSA and its function are still debatable. Two potential sources of RSA were incorporated: periodic heart rate that mimics the central regulation of heart rate which originates in the brainstem, and periodic systemic veins resistance that mimics one possible effect of the pleural pressure which drives breathing. The effects of RSA on cardiac output were then studied. The simulations suggest that the mean cardiac output does not change significantly due to RSA at either low or high heart rates. Two types of heart failure were simulated using the new models by changing certain model parameters: systolic and diastolic. Both the systolic and diastolic heart failures caused an accumulation of blood in the lungs. The ejection fraction for diastolic heart failure remained within the normal physiological range while in the case of systolic heart failure the ejection fraction reduced rapidly. These results are consistent with physiological observations.
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    Is our breathing optimal? : this dissertation is submitted for the degree of Doctor of Philosophy, School of Natural and Computational Sciences, Massey University, New Zealand
    (Massey University, 2019) Zaidi, Syed Muhammad Faheem
    One of the open questions in relation to the control of amplitude and frequency of breathing is why a particular pattern of breathing is observed. This thesis explores the hypothesis that the particular combination of breathing frequency and amplitude realised, is optimal with respect to some objective function. Several objective functions have been suggested in the literature, such as the rate of work during inhalation, the average force exerted by the respiratory muscles, and the weighted sum of volumetric acceleration and work during inhalation; all of these objective functions were studied using 1D models and all provided physiologically acceptable minima under normal conditions. The thesis investigates optimal solutions of mathematical models that range from 2D to 6D and reflect more accurately the coupling between lung mechanics and gas exchange. It shows how published 6D and 5D models can be reduced to new 3D and 2D models. At its simplest, the 2D model consists of two piecewise linear differential equations. The use of higher dimension models require a new definition of the optimization problem as minimizing a given objective function subject to several constraints, such as satisfying the differential equations and maintaining one of the variables at a given average value. The optimal problem can be solved analytically in the case of the simplest 2D model, using concepts from optimal control theory. The analytical solution is used to verify a numerical algorithm that is then used to solve the more complex models. Solutions of the optimization problem for the different objective functions, previously suggested in the literature have been calculated. In all the optimal solutions found in this thesis, the duration of inhalation is equal to the duration of exhalation. However, under normal conditions, the time duration of inhalation is expected to be shorter than that of exhalation. This might be resolved by imposing additional constraints or by proposing a different hypothesis to explain why a particular pattern of breathing is observed.
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    Vagal influences on respiratory reflexes : interaction of P.S.R. and R.A.R. on the inflation and deflation reflex, their role in linking respiratory cycles : and postvagotomy effect of P.D.G. : a thesis in partial fulfilment of the requirements for the degree of Master of Science, Massey University
    (Massey University, 1985) Jones, Heather
    There is evidence that changes in one respiratory cycle may influence subsequent cycles by a central mechanism. Thus the influence of P.S.R. and R.A.R. activity from within one respiratory cycle on subsequent cycles, which we have called "memory", needed to be examined in the determination of duration of expiration (tE) and inspiration (tI). this study was designed to investigate the relative roles of P.S.R. and R.A.R. stimulation in expiration influencing tI arnd tE over several subsequent breaths. In particular to investigate their role in linking respiratory cycles. In 14 anaesthetized spontaneously breathing rabbits we studied the response of tI and tE to +ve and -ve pressure pulses of 20KPa applied to the lung at various stages in expiration before and during P.S.R. block with S02. Before P.S.R. block, +ve pressure pulses early in expiration generally shortened tE containing the pulse, applied later +ve pressure pulses lengthened tE. Positive pressure pulses after P.S.R. block, and -ve pressure pulses before and after block always shortened tE. Regardless of sign of pulse tE was shortened in subsequent breaths before and after block. The inspiration after negative pulse application was usually lengthened. After effective block -ve pulses rarely lengthened tl. Large shortening of tE containing the pulse was usually followed bv a shortened tl. Positive pulses did not significantly effect the duration of tl. Regardless of sign of pulse tl was not usually changed but occasional large shortening occured in subsequent breaths before and after P.S.R. block. This indicates that the tE containing the stimulation is governed by a balance between P.S.R. and R.A.R. activity. The tI following t.he stimulus is governed by a balance between memory 11 of P.S.R. and R.A.R. activity. In the breaths following both tE and tI were influenced by memory of R.A.R. activity only. However "memory'' of strong R.A.R. activity is required to affect tI. During this study it was intended to use phenyldiguanide (P.D.G.) to test J receptor patency. Intravenous injections of P.D.G. have been used to provoke respiratory reflexes, these have been considered to be due mainly to stimulation of type J receptors. However although most workers demonstrated that vagotomy abolished or reduced these reflexes, some still had significant response to P.D.G. after vagotomy. A study was conducted to resolve this difference and demonstrate the sites at which P.D.G. acts in rabbits. We measured tE and tI in 10 anaesthetized spontaneously breathing rabbits. 50 g/kg P.D.G. was given intravenously (via a catheter with its tip close to the right atrium) to the intact rabbit; after blocking epicardial receptors; immediately after bilateral cervical vagotomy; 15 minutes after vagotomy; and after the glossopharyngeal nerves were cut near the base of the skull. The respiratory reflex after injection of xylocaine, 15 minutes after vagotomy, and after cutting the glossopharyngeal nerves was as pronounced as in the intact state, and consisted of an increase in frequency almost totally due to a reduction in tE. With injections given up to 3 minutes after bilateral vagotomy the respiratory response was greatly attenuated and variable. We suggest this question of timing may contribute to the differences seen by different groups of workers. It is clear that intravenous injection of P.D.G. is not an adequate test of J receptor presence in the rabbit.