Adjustment of the human respiratory system to increased upper airway resistance during sleep

A cardiorespiratory model incorporating control of the human upper airway during sleep is described. Most previous models have not considered the possibility that the upper airway could be a limiting factor for gas exchange. Our model was developed to also predict certain pathophysiological phenomena in the cardiorespiratory system that characterize heavy snoring or sleep apnea. We started by adapting our collapsible upper airway model to include the impact of nasal passage and larynx, and extended the model with equations for gas exchange in the lungs. A feedback loop both to the respiratory pump and the upper airway dilator muscles was included. The model enabled successful breath-by-breath simulations of obstructive events of the upper airway. Although the model incorporates several physiologically relevant components of the system, the simulation results suggest that only few parameters suffice to predict the key adjustments that the cardiorespiratory system is known to make in patients with heavy snoring.     

Soil heterogeneity in tree mixtures depends on spatial clustering of tree species

Heterogeneity in soil characteristics promotes and maintains coexistence between a diverse set of species. In forests, trees have species-specific impacts on soil abiotic characteristics and mixing of tree species is being promoted as a tool to ensure high levels of diversity and functioning. Yet, limited knowledge is available on the effect of tree species composition and spatial clustering on heterogeneity in soil characteristics. In this paper we derived heterogeneity of key characteristics of the leaf litterfall, the forest floor and the mineral topsoil (C, N and base cation concentration, C:N ratio and mass) in 53 plots of 7 different tree species compositions. We found that heterogeneity increased from the leaf litterfall, through the forest floor down to the mineral topsoil. Mixing tree species did not lead to an increased heterogeneity in the forest floor and topsoil compared to monocultures. However, we did find that mixed plots where conspecific trees stand in groups are more heterogeneous than plots where species are intimately mixed. Our results imply that heterogeneity in soil characteristics does not necessarily increase with tree diversity, but that within mixed stands the spatial organization of tree species should be considered in relation to the scale at which heterogeneity is desired.     

Small-scale spatial heterogeneity and seasonal variation in a population of a cave-dwelling Mediterranean mysid

Seasonal changes in small-scale spatial distribution patternsof abundance and composition of Hemimysis speluncola Ledoyer(1963) (Crustacea, Mysidacea) were studied in a population ofcave-dwelling mysids off the Medes Islands (NW MediterraneanSea). The distribution pattern was characterized all year roundby marked spatial segregation of juveniles, which occupied areascloser to the cave entrance, while adults were concentratedmainly in the innermost part of the cave. The small-scale spatialheterogeneity observed appears to be regulated by biologicalfactors, particularly social and reproductive behaviour. Nevertheless,a certain adaptation of the distribution of the swarms to hydrodynamicfactors suggests that physical factors may also play a role.The heterogeneous aggregation pattern recorded would clearlyappear to be an adaptive strategy by the population to enableit to thrive within its habitat. That adaptation is designedto reduce predation mortality, enhance mating efficiency andregulate the population generally. Marked seasonal fluctuationsin the density and composition of the population were recorded,with high density levels in winter and throughout the spring.Two methods were employed to quantify the population: haulswith plankton nets towed by divers and collection of faecalpellets. The patterns observed using both these methods weresimilar, although the latter method yielded total density valuesthat were an order of magnitude greater.     

Modeling the bifurcating flow in a human lung airway

The inspiratory flow characteristics in a three-generation lung airway have been numerically investigated using a control volume method to solve the fully three-dimensional laminar Navier-Stokes equations. The three-generation airway is extracted from the fifth to seventh branches of the model of Weibel (Morphometry of the Human Lung, Academic Press, New York, Springer, Berlin, 1963) with in-plane and 90 degrees off-plane configurations. Computations are carried out in the Reynolds number range of 200-1600, corresponding to mouth-air breathing rates ranging from 0.27 to 2.16l/s, or an averaged height of a man breathing from quiet to vigorous state. Particular attention is paid to establishing relations between the Reynolds number and the overall flow characteristics, including flow patterns and pressure drop. The ratio of airflow rate through the medial branch to that of the lateral branch for an in-plane airway increases as Re(0.227). However, the total pressure drop coefficient varies as Re(-0.497) for an in-plane airway and as Re(-0.464) for an off-plane airway. These pressure drop results are in good agreement with the experimentally measured behavior of Re(-0.5) and are more accurate than the numerically determined behavior of Re(-0.61) assuming the airways to be approximated by two-dimensional channels.     

Individual size variation reduces spatial variation in abundance of tree community assemblage,not of tree populations

Research on individual trait variation has gained much attention because of its implication for ecosystem functions and community ecology. The effect of individual variation on population and community abundance (number of individuals) variation remains scarcely tested. Using two established ecological scaling laws (Taylor's law and abundance–size relationship), we derived a new scaling relationship between the individual size variation and spatial variation of abundance. Tested against multi‐plot tree data from Diaoluo Mountain tropical forest in Hainan, China, the new scaling relationship showed that individual size variation reduced the spatial variation of community assemblage abundance, but not of taxon‐specific population abundance. The different responses of community and population to individual variation were reflected by the validity of the abundance–size relationship. We tested and confirmed this scaling framework using two measures of individual tree size: aboveground biomass and diameter at breast height. Using delta method and height‐diameter allometry, we derived the analytic relation of scaling exponents estimated under different individual size measures. In addition, we used multiple regression models to analyze the effect of taxon richness on the relationship between individual size variation and spatial variation of population or community abundance, for taxon‐specific and taxon‐mixed data, respectively. This work offers empirical evidence and a scaling framework for the negative effect of individual trait variation on spatial variation of plant community. It has implications for forest ecosystem and management where the role of individual variation in regulating population or community spatial variation is important but understudied.     

Impact of spatial heterogeneity on a predator-prey system dynamics

This paper deals with the study of a predator-prey model in a patchy environment. Prey individuals moves on two patches, one is a refuge and the second one contains predator individuals. The movements are assumed to be faster than growth and predator-prey interaction processes. Each patch is assumed to be homogeneous. The spatial heterogeneity is obtained by assuming that the demographic parameters (growth rates, predation rates and mortality rates) depend on the patches. On the predation patch, we use a Lotka-Volterra model. Since the movements are faster that the other processes, we may assume that the frequency of prey and predators become constant and we would get a global predator-prey model, which is shown to be a Lotka-Volterra one. However, this simplified model at the population level does not match the dynamics obtained with the complete initial model. We explain this phenomenom and we continue the analysis in order to give a two-dimensional predator-prey model that gives the same dynamics as that provided by the complete initial one. We use this simplified model to study the impact of spatial heterogeneity and movements on the system stability. This analysis shows that there is a globally asymptotically stable equilibrium in the positive quadrant, i.e. the spatial heterogeneity stabilizes the equilibrium.     

Interspecific variation in resistance of two host tree species to spruce budworm

Woody plants regularly sustain biomass losses to herbivorous insects. Consequently, they have developed various resistance mechanisms to cope with insect attack. However, these mechanisms of defense and how they are affected by resource availability are not well understood. The present study aimed at evaluating and comparing the natural resistance (antibiosis and tolerance) of balsam fir (Abies balsamea [L.] Mill.) and white spruce (Picea glauca [Moench) Voss] to spruce budworm, Choristoneura fumiferana (Clem.), and how drainage site quality as a component of resource availability affects the expression of resistance over time (6 years). Our results showed that there are differences in natural resistance between the two tree species to spruce budworm, but it was not significantly affected by drainage quality. Balsam fir exhibited higher foliar toxic secondary compounds concentrations than white spruce in all drainage classes, resulting in lower male pupal mass, survival and longer male developmental time. This, however, did not prevent spruce budworm from consuming more foliage in balsam fir than in white spruce. This response suggests that either natural levels of measured secondary compounds do not provide sufficient toxicity to reduce defoliation, or spruce budworm has developed compensatory mechanisms, which allow it to utilize food resources more efficiently or minimize the toxic effects that are produced by its host's defensive compounds. Larvae exhibited lower pupal mass and higher mortality in rapidly drained and subhygric sites. Drainage class also affected the amount of foliage destroyed but its impact varied over the years and was probably influenced by climatic variables. These results demonstrate the complexity of predicting the effect of resource availability on tree defenses, especially when other confounding environmental factors can affect tree resource allocation and utilization.     

Modeling the bifurcating flow in a CT-scanned human lung airway

The inspiratory flow characteristics in a CT-scanned human lung model were numerically investigated using low Reynolds number (LRN) kappa-omega turbulent model. The five-generation airway is extracted from the trachea to segmental bronchi of a 60-year-old Chinese male patient. Computations were carried out in the Reynolds number range of 900-2100, corresponding to mouth-air breathing rates of 190-440ml/s. Flow patterns on the Re=2100 and flow rate distribution were presented. In this model, the flow pattern is very complex. To count the effect of laryngeal jet on trachea inlet, the trachea was extended and modified to simulate the larynx, consequently the inlet velocity profile is biased towards the rear wall. In the inferior lobar bronchi, there are two stems in which the axial velocity is stronger but secondary velocity is weaker. Secondary flow in the lateral bronchi is stronger than the medial ones. With increasing Re, the air flow increases in the middle, inferior lobes and left main bronchus, i.e., flow biases to left and downward.     

Phenotypic variation and spatial structure of secondary chemistry in a natural population of a tropical tree species

To understand herbivore selection in natural plant populations, it is important to understand the landscape of plant chemical phenotypes that herbivores face and the sources of variation that will define this landscape. We studied the spatial patterns of variation in leaf secondary chemistry of the tropical tree Quararibea asterolepis , Pitt. (Bombacaceae) in a natural population on Barro Colorado Island, Panama, and used this background to discuss hypotheses of natural selection by herbivores. Quararibea plants collected from different sites had consistent differences in their chemical phenotypes. Some of these differences were explained by developmental and environmental sources of variation. Canopy trees had 13% lower yield of leaf extracts than gap seedlings, explained by 41% lower concentrations of the more abundant metabolites in the secondary compound profile. Also, plants growing in gaps had 25% higher yield than those in the understory, explained by two-fold increases in the concentration of some of the less polar secondary compounds in the profile. Differences in soil type did not affect the secondary chemistry of leaves, but sites with different topography had differences in the secondary compound profile that were not explained by any of the measured environmental sources of variation. Neighboring parent-offspring pairs and sibling/half sibling clusters displayed equal or higher variance among themselves than unrelated individuals at farther distances. Assuming that related plants should be more similar in their phenotypes, this pattern is consistent with local selection by herbivores overriding the similarity of related plants in a frequency- or distance-dependent manner.     

Interrupter airway and tissue resistance: errors caused by valve properties and respiratory system compliance.

The interrupter technique is used to determine airway and tissue resistance. Their accuracy is influenced by the technical properties of the interrupter device and the compliance of the respiratory system. We investigated the influence of valve characteristics and respiratory system compliance on the accuracy of determining airway and tissue resistance by means of a computer simulation. With decreasing compliance we found increasing errors in both airway and tissue resistance determination of up to 34 and 71%, respectively. On this basis we developed a new occlusion valve, with special emphasis on rapid closing time and tightness in the closed state to improve the accuracy of resistance determination. The newly developed occlusion device greatly improves the accuracy of airway and tissue resistance determination. We conclude that respiratory system compliance is a limiting factor for the accuracy of the interrupter technique. To apply the interrupter technique in patients with extremely low respiratory system compliances, we need sophisticated technical devices.     

Body size and tree species composition determine variation in prey consumption in a forest‐inhabiting generalist predator

Trophic interactions may strongly depend on body size and environmental variation, but this prediction has been seldom tested in nature. Many spiders are generalist predators that use webs to intercept flying prey. The size and mesh of orb webs increases with spider size, allowing a more efficient predation on larger prey. We studied to this extent the orb‐weaving spider Araneus diadematus inhabiting forest fragments differing in edge distance, tree diversity, and tree species. These environmental variables are known to correlate with insect composition, richness, and abundance. We anticipated these forest characteristics to be a principle driver of prey consumption. We additionally hypothesized them to impact spider size at maturity and expect shifts toward larger prey size distributions in larger individuals independently from the environmental context. We quantified spider diet by means of metabarcoding of nearly 1,000 A. diadematus from a total of 53 forest plots. This approach allowed a massive screening of consumption dynamics in nature, though at the cost of identifying the exact prey identity, as well as their abundance and putative intraspecific variation. Our study confirmed A. diadematus as a generalist predator, with more than 300 prey ZOTUs detected in total. At the individual level, we found large spiders to consume fewer different species, but adding larger species to their diet. Tree species composition affected both prey species richness and size in the spider''s diet, although tree diversity per se had no influence on the consumed prey. Edges had an indirect effect on the spider diet as spiders closer to the forest edge were larger and therefore consumed larger prey. We conclude that both intraspecific size variation and tree species composition shape the consumed prey of this generalist predator.     

Modeling airway resistance dynamics after tidal and deep inspirations.

Using the forced oscillation technique, we tracked airway resistance continuously during quiet breathing (QB) and deep inspiration (DI), thus observing fluctuations in resistance that may reflect mechanisms of airway stretch and renarrowing. After DI, however, the resistance may be depressed for a period not related to volume changes. We hypothesized that this gradual increase in resistance after DI-induced dilation was determined by a simple time constant. Furthermore, to the extent that this effect reflects dynamic characteristics of airway renarrowing, the resistance change after each tidal inspiration should also be constrained by this temporal limit. A model relating resistance fluctuations to the breathing pattern, including both instantaneous and delayed effects, was developed and applied to data from 14 nonasthmatic and 17 asthmatic subjects (forced expiratory volume in 1 s = 103 +/- 13 and 83 +/- 12%, respectively, means +/- SD) after methacholine challenge (dose 145 +/- 80 and 3.0 +/- 3.4 micromol, respectively) that resulted in respective forced expiratory volume in 1 s reductions of 16 +/- 7 and 24 +/- 6% from baseline. Resistance was measured continuously for 1 min of QB, a DI, followed by a further minute of QB. Resistance values at end expiration (Ree) and end inspiration were calculated. We found that the sequence of Ree after DI was best modeled by a power-law function of time rather than an exponential decay (r2 = 0.82 +/- 0.18 compared with 0.63 +/- 0.16; P < 0.01). Furthermore, the coefficient characterizing this "renarrowing function" was close to equal to the coefficient characterizing the equivalent function of resistance change between each resistance value at end inpiration and subsequent Ree during QB, particularly in the nonasthmatic subjects for whom the intraclass correlation was 0.66. This suggests that the same time-dependent factors determine renarrowing after both large and small breaths.     

A quantitative approach to spatial variation of human cerebral sulci

The surfaces and the sulci of the lateral hemispheres of 30 postmortem brain specimens were three-dimensionally measured. The coordinates of all the structures registered constitute a digital brain model. The models were scaled linearly until they fitted a defined cuboid. Then all the models within the reference volume, each represented by a set of points, were considered together. The reference volume was divided into small cubic volume elements (voxels). Two parameters of the voxels were studied: the point density of each structure per voxel and the contribution index which designates how many of the hemispheres contribute points to a certain voxel. The latter parameter may serve as an index for the presence probability of a sulcus, which means the probability with which a sulcus will be contained in a defined volume element. The presence probability of a sulcus corresponds well with its first appearance in embryological development. Spatial variation in the early developing principal sulci is far less than in those which develop in later stages. As regards some defined regions of the brain, statements about the probable location of cortical areas in relation to the commissural system can be made.     

Modeling of respiratory system impedances in dogs

Mechanical impedances between 4 and 64 Hz of the respiratory system in dogs have been reported (A.C. Jackson et al. J. Appl. Physiol. 57: 34-39, 1984) previously by this laboratory. It was observed that resistance (the real part of impedance) decreased slightly with frequency between 4 and 22 Hz then increased considerably with frequency above 22 Hz. In the current study, these impedance data were analyzed using nonlinear regression analysis incorporating several different lumped linear element models. The five-element model of Eyles and Pimmel (IEEE Trans. Biomed. Eng. 28: 313-317, 1981) could only fit data where resistance decreased with frequency. However, when the model was applied to these data the returned parameter estimates were not physiologically realistic. Over the entire frequency range, a significantly improved fit was obtained with the six-element model of DuBois et al. (J. Appl. Physiol. 8: 587-594, 1956), since it could follow the predominate frequency-dependent characteristic that was the increase in resistance. The resulting parameter estimates suggested that the shunt compliance represents alveolar gas compressibility, the central branch represents airways, and the peripheral branch represents lung and chest wall tissues. This six-element model could not fit, with the same set of parameter values, both the frequency-dependent decrease in Rrs and the frequency-dependent increase in resistance. A nine-element model recently proposed by Peslin et al. (J. Appl. Physiol. 39: 523-534, 1975) was capable of fitting both the frequency-dependent decrease and the frequency-dependent increase in resistance. However, the data only between 4 and 64 Hz was not sufficient to consistently determine unique values for all nine parameters.     

Influence of respiratory drive on upper airway resistance in normal men

The variations in nasal and pharyngeal resistance induced by changes in the central inspiratory drive were studied in 10 normal men. To calculate resistances we measured upper airway pressures with two low-bias flow catheters; one was placed at the tip of the epiglottis and the other in the posterior nasopharynx, and we measured flow with a Fleisch no. 3 pneumotachograph connected to a tightly fitting mask. Both resistances were obtained continuously during CO2 rebreathing (Read's method) and during the 2 min after a 1-min voluntary maximal hyperventilation. The inspiratory drive was estimated by measurements of inspiratory pressure generated at 0.1 s after the onset of inspiration (P0.1) and by the mean inspiratory flow (VT/TI). In each subject both resistances decreased during CO2 rebreathing; these decreases were correlated with the increase in P0.1. During the posthyperventilation period, ventilation fell below base line in seven subjects; this was accompanied by an increase in both nasal and pharyngeal resistances. These resistances increased exponentially as VT/TI decreased. Parallel changes in nasal and pharyngeal resistances were seen during CO2 stimulus and during the period after the hyperventilation. We conclude that 1) the indexes quantifying the inspiratory drive reflect the activation of nasopharyngeal dilator muscles (as assessed by the changes in upper airway resistance) and 2) both nasal and pharyngeal resistances are similarly influenced by changes in the respiratory drive.