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Results for “Models, Theoretical”

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At least 325 records · Page 18Linked to original sources

PubMed2002

[Mechanisms of organic cementing soil aggregate formation and its theoretical models].

Soil aggregate is the basic unit of soil structure, and its formation and stabilization has direct effect on soil fertility and crop growth. For black soil with high content of organic matter, organic cementing substance is an important factor in forming soil aggregates. Based on the recent researches on organic cementing agents, this paper was focused on the nature of organic cementing substances, the effect of soil organisms and plant roots in forming soil aggregates, and the theoretical models and stable mechanisms of soil aggregate formation.

Cementation↗

PubMed1979

A theoretical model for adhesion between cells mediated by multivalent ligands.

A theoretical model is developed for cell-to-cell binding by bivalent ligands that can bind to mobile receptors on the cell surfaces. Monovalent inhibitors that can bind either to receptors or ligands are also included. For symmetrical ligands, that is, ligands in which both binding sites are the same, it is shown that crosslinking of receptors on each cell will interfere with intercellular bridge formation. At equilibrium, such interference is not drastic, but if the crosslinks can form before the cells are brought into contact, crosslinking may greatly impede the rate of intercellular binding. Comparison is made with experiments, and the importance of receptor mobility is discussed. It is noted that ligands can also bind a cell to itself or to a surface.

Agglutination↗

PubMed2000

Theoretical models for circadian rhythms in Neurospora and Drosophila.

We examine theoretical models proposed for the molecular mechanism of circadian rhythms in Drosophila. The models are based on the negative feedback exerted by a complex between the PER and TIM proteins on the expression of the per and tim genes. We show that a similar model can account for circadian oscillations in Neurospora, where the protein FRQ negatively regulates the expression of the frq gene. The effect of light on the circadian rhythms is included by considering that it elicits a rise in the rate of TIM degradation in Drosophila, whereas in Neurospora it enhances the rate of frq transcription. The models account for the occurrence of sustained circadian oscillations in continuous darkness in Drosophila and Neurospora. Numerical simulations further indicate that the periodic forcing of circadian oscillations by light-dark cycles can result either in the entrainment to the external periodicity or in aperiodic oscillations (i.e. chaos), depending on the magnitude of the periodic changes in the light-controlled parameter.

Animals↗

PubMed1994

A theoretical model for magneto-acoustic imaging of bioelectric currents.

A theoretical model of magneto-acoustic current imaging is derived, based on fundamental equations of continuum mechanics and electromagnetism. In electrically active tissue, the interaction between an applied magnetic field, B, and action currents, J, creates a pressure distribution. In the near field limit, this pressure obeys Poisson's equation, with a source term (delta x J).B. The displacement and pressure fields are calculated for a dipole (q), oriented either parallel or perpendicular to the applied magnetic field (B), at the center of an elastic, conducting sphere (radius a, shear modulus G). Surface displacements are on the order of qB/(4 pi Ga), which is about 1 nm for typical biological parameters. If the applied magnetic field is changing with time, eddy currents induced in the tissue may be larger than the action currents themselves. The frequency of the pressure and displacement arising from these eddy currents, however, is twice the frequency of the applied magnetic field, so it may be possible to eliminate this artifact by filtering or lock-in techniques. Magneto-acoustic and biomagnetic measurements both image delta x J in a similar way, although magneto-acoustic current imaging has the disadvantage that acoustic properties vary among tissues to a greater degree than do magnetic properties.

Acoustics↗

PubMed2001

A theoretical model for the elastic properties of very soft tissues.

A theoretical model is developed to predict the elastic properties of very soft tissues such as glands, tumors and brain. Tissues are represented as regular arrays of polyhedral (cubic or tetrakaidecahedral) cells, surrounded by extracellular spaces of uniform width. Cells are assumed to be incompressible, with very low resistance to shear deformation. Tissue shear rigidity is assumed to result mainly from the extracellular matrix, which is treated as a compressible elastic mesh of interconnected fibers. Small-strain elastic properties of tissue are predicted using a finite-element method and an analytical method. The model can be used to estimate the compressibility of a very soft tissue based on its Young's modulus and extracellular volume fraction.

Animals↗

PubMed1978

Dimensions of black suicide: a theoretical model.

This paper has two purposes: (1) to develop a theoretical model of suicide, based on the previous work of Henry and Short, and (2) to apply that model in the study of black suicides in Orleans Parish, Louisiana. In so doing, we are focusing on suicide among a traditionally low-status, low-suicide population group, one which Henry and Short cited in support of the positive relationship between status and suicide. The generality of the theory is thus examined in this and other ways. The theory of "external restraints" is only weakly sustained by this analysis. More adequate testing of the theory must await further data, with careful attention to the subjective aspects of hypothesized structural relationships.

Black or African American↗

PubMed1999

Integrating response shift into health-related quality of life research: a theoretical model.

Patients confronted with a life-threatening or chronic disease are faced with the necessity to accommodate to their illness. An important mediator of this adaptation process is 'response shift' which involves changing internal standards, values and the conceptualization of quality of life (QOL). Integrating response shift into QOL research would allow a better understanding of how QOL is affected by changes in health status and would direct the development of reliable and valid measures for assessing changes in QOL. A theoretical model is proposed to clarify and predict changes in QOL as a result of the interaction of: (a) a catalyst, referring to changes in the respondent's health status; (b) antecedents, pertaining to stable or dispositional characteristics of the individual (e.g. personality); (c) mechanisms, encompassing behavioral, cognitive, or affective processes to accommodate the changes in health status (e.g. initiating social comparisons, reordering goals); and (d) response shift, defined as changes in the meaning of one's self-evaluation of QOL resulting from changes in internal standards, values, or conceptualization. A dynamic feedback loop aimed at maintaining or improving the perception of QOL is also postulated. This model is illustrated and the underlying assumptions are discussed. Future research directions are outlined that may further the investigation of response shift, by testing specific hypotheses and predictions about the QOL domains and the clinical and psychosocial conditions that would potentiate or prevent response shift effects.

Adaptation, Psychological↗

PubMed2005

Chronic inflammation and breast pathology: a theoretical model.

Breast cell pathology results from biochemical and molecular changes that culminate in the cell's loss of functional responsiveness. The epithelial cell compartment in the breast ductal system is the site of approximately 98% of malignant proliferations, and it is from within these cells that the first biochemical signal of change may be expressed as an inflammatory response. Inflammation may be represented by biomarkers of early pathologic changes in breast cells and be associated with risk for the development of breast cancer. A theoretical model of the inflammatory process is proposed showing predictive linkages among stimuli in the breast microenvironment and the development of breast pathology, in particular, breast cancer. This model fuels intervention concepts that may prevent malignant breast health outcomes.

Biomarkers, Tumor↗

PubMed2006

Induced airflow in flying insects I. A theoretical model of the induced flow.

A strong induced flow structure envelops the body of insects and birds during flight. This flow influences many physiological processes including delivery of odor and mechanical stimuli to the sensory organs, as well as mass flow processes including heat loss and gas exchange in flying animals. With recent advances in near-field aerodynamics of insect and bird flight, it is now possible to determine how wing kinematics affects induced flow over their body. In this paper, I develop a theoretical model based in rotor theory to estimate the mean induced flow over the body of flapping insects. This model is able to capture some key characteristics of mean induced flow over the body of a flying insect. Specifically, it predicts that induced flow is directly proportional to wing beat frequency and stroke amplitude and is also affected by a wing shape dependent parameter. The derivation of induced flow includes the determination of spanwise variation of circulation on flapping wings. These predictions are tested against the available data on the spanwise distribution of aerodynamic circulation along finite Drosophila melanogaster wings and mean flows over the body of Manduca sexta. To explicitly account for tip losses in finite wings, a formula previously proposed by Prandtl for a finite blade propeller system is tentatively included. Thus, the model described in this paper allows us to estimate how far-field flows are influenced by near-field events in flapping flight.

Air Movements↗

PubMed2001

Postmenarchal Down's syndrome: a theoretical model of growth hormone action on ovarian function.

Much evidence indicates that blunted ovarian sensitivity to follicle-stimulating hormone (FSH) and lower growth hormone (GH) plasma concentrations, as often occur in women with Down's syndrome (DS), may contribute to the gonadal disfunction frequently present in such subjects. In this review, we analyze the more recent advances in this field, and then discuss from a clinical point of view the potential role of GH on ovarian function, since DS patients may also constitute a theoretical model for investigating this particular aspect of reproductive physiology.

Down Syndrome↗

PubMed1984

Application of gas adsorption kinetics. I. A theoretical model for respirator cartridge service life.

Using the principles of gas adsorption kinetics, we have developed a new theoretical model addressing the adsorption and breakthrough of contaminant vapors or gases with respect to solid sorbents. Specifically, we have applied the theory to predict respirator cartridge service life in connection with individual contaminant exposure to toluene, vinyl chloride, ethyl acetate, and each of several different trichlorinated hydrocarbons at several levels of concentration. Theoretical expressions and contaminant breakthrough curves derived from our new approach are compared with those published previously by Mecklenburg and by Wheeler. The breakthrough curves derived by Mecklenburg and by Wheeler have approximately the same shape as the corresponding experimental curves for 0% to 40% breakthrough. However, these curves deviate in varying degrees from the experimental data at breakthrough values exceeding 40%. By contrast, our new theory agrees with published experimental results over the entire range of 0% to 100% breakthrough. The new model not only agrees with experimental observation, it is also less complicated and easier to apply to practical industrial hygiene problems than theories developed previously.

Adsorption↗

PubMed1986

A theoretical model simulating the anomalous concentration dependence of the equilibrium thermal unfolding curve of noncrosslinked tropomyosin.

Thermal unfolding curves of tropomyosin have so far been fit only semi-quantitatively by the statistical-mechanical theory of the helix-coil transition. The calculated values of helix content are a bit too small for the most dilute solutions and a bit too large for the most concentrated ones. The theory, as hitherto used, assumes a uniform helix-helix interaction, whereas evidence from studies on molecular segments suggests otherwise. A theoretical model incorporating such non-uniformity in helix-helix interaction is used to produce simulated thermal unfolding curves. These simulated curves, when fit to the theory using the assumption of uniformity, reveal precisely the same discrepancies seen with the experimental data. We conclude that non-uniformity in helix-helix interaction along the tropomyosin molecule is responsible for the small discrepancy between experimental data and the uniform-model theory previously employed.

Models, Theoretical↗

PubMed2004

On the mechanism of stomatocyte-echinocyte transformations of red blood cells: experiment and theoretical model.

This study represents an attempt to achieve a better understanding of the stomatocyte-echinocyte transition in the shape of red blood cells. We determined experimentally the index of cell shape at various ionic strengths and osmolarities for native and trypsin-treated human erythrocytes. For every given composition of the outer phase, we calculated the ionic strength in the cells and the transmembrane electric potential using a known theoretical model. Next, we described theoretically the electric double layers formed on both sides of the cell membrane, and derived expressions for the tensions of the two membrane leaflets. Taking into account that the cell-shape index depends on the tension difference between the two leaflets, we fitted the experimental data with the constructed physicochemical model. The model, which agrees well with the experiment, indicates that the tension difference between the two leaflets is governed by the different adsorptions of counterions at the two membrane surfaces, rather than by the direct contribution of the electric double layers to the membrane tension. Thus, with the rise of the ionic strength, the counterion adsorption increases stronger at the outer leaflet, whose stretching surface pressure becomes greater, and whose area expands relative to that of the inner leaflet. Hence, there is no contradiction between the bilayer-couple hypothesis and the electric double layer theory, if the latter is upgraded to account for the effect of counterion-adsorption on the membrane tension. The developed quantitative model can be applied to predict the shape index of cells upon a stomatocyte-discocyte-echinocyte transformation at varying composition of the outer medium.

Adsorption↗

PubMed1976

[Theoretical models of chromatid exchange established after endoreplication analysi].

In endoreduplicating cells, two thirds of the chromatid exchanges observed after BrdU treatment are symetrical, like on the two elements of the diplochromosome. The other third seems to occur at random on one or the other element. Each double symetrical exchange would result from a single event occurring at the next to last cell cycle before the mitosis observed. The other exchanges would result from last events of the cycle. Thus the frequency of events would be the same at each cell generation. A theoretical model is proposed which explains the mechanism responsible for chromatid exchanges at the DNA level.

Acridines↗

PubMedDate not supplied

A theoretical model for coordinating and documenting patient education.

The Joint Commission on Accreditation of Healthcare Organizations' patient and family education standards pose a challenge to large medical centers. Prior to 1994, Joint Commission standards required all healthcare disciplines to instruct their patients about issues relevant to their health. There was no requirement that the information that was given to patients by providers in various disciplines, units, or clinics be coordinated. Now that education is a functional chapter in the Joint Commission's Accreditation Manual for Hospitals, an integrated approach to providing such education is necessary. This article proposes a theoretical model intended to help medical centers meet the new standards, improve patient education, and improve communication among healthcare providers.

Family↗

PubMed2006

Theoretical modeling for radiofrequency ablation: state-of-the-art and challenges for the future.

Radiofrequency ablation is an interventional technique that in recent years has come to be employed in very different medical fields, such as the elimination of cardiac arrhythmias or the destruction of tumors in different locations. In order to investigate and develop new techniques, and also to improve those currently employed, theoretical models and computer simulations are a powerful tool since they provide vital information on the electrical and thermal behavior of ablation rapidly and at low cost. In the future they could even help to plan individual treatment for each patient. This review analyzes the state-of-the-art in theoretical modeling as applied to the study of radiofrequency ablation techniques. Firstly, it describes the most important issues involved in this methodology, including the experimental validation. Secondly, it points out the present limitations, especially those related to the lack of an accurate characterization of the biological tissues. After analyzing the current and future benefits of this technique it finally suggests future lines and trends in the research of this area.

Animals↗

PubMed2006

A theoretical model for the production of Ac-225 for cancer therapy by photon-induced transmutation of Ra-226.

Radium needles that were once implanted into tumours as a cancer treatment are now obsolete and constitute a radioactive waste problem, as their half-life is 1600 years. We are investigating the reduction of radium by transmutation on a small scale by bombarding Ra-226 with high-energy photons from a medical linear accelerator (linac) to produce Ra-225, which subsequently decays to Ac-225, which can be used as a generator to produce Bi-213 for use in 'targeted alpha therapy' for cancer. This paper examines the possibility of producing Ac-225 with a linac using an accurate theoretical model in which the bremsstrahlung photon spectrum at 18 MV linac electron energy is convoluted with the corresponding photonuclear cross sections of Ra-226. The total integrated yield can then be obtained and is compared with a computer simulation. This study shows that at 18 MV, the photonuclear reaction on Ra-226 can produce low activities of Ac-225 with a linac. However, a high power linac with high current, pulse length and frequency is needed to produce practical amounts of Ac-225 and a useful reduction of Ra-226.

Actinium↗

PubMed1995

A theoretical model for 222Rn adsorption on activated charcoal canisters in humid air based on Polanyi's potential theory.

Water vapor interferes with the adsorption of 222Rn gas by passive activated charcoal devices used to estimate indoor air concentrations. The 222Rn adsorption coefficient is the fundamental parameter characterizing charcoal's ability to adsorb 222Rn. The Dubinin-Radushkevich equation, based on Polanyi's potential theory, was modified to include two terms quantifying the effect of both water vapor and sampling time on the 222Rn adsorption coefficient of passive charcoal devices. A single equation was derived that quantifies the 222Rn adsorption coefficients at any temperature, humidity and exposure time using six experimentally determined physical constants that are unique for a particular passive charcoal device. The theoretical model was verified with published experimental data, and it showed a good correlation between theory and experiment. The model proved to be consistent with experimental data, provided that the amount of water vapor adsorbed by the charcoal device during sampling remains below a critical level, termed the breakpoint.

Adsorption↗