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Results for “diffusion theory”

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At least 19 recordsLinked to original sources
PubMedDate not supplied

A diffusion theory model of spatially resolved, steady-state diffuse reflectance for the noninvasive determination of tissue optical properties in vivo.

A model based upon steady-state diffusion theory which describes the radial dependence of diffuse reflectance of light from tissues is developed. This model incorporates a photon dipole source in order to satisfy the tissue boundary conditions and is suitable for either refractive index matched or mismatched surfaces. The predictions of the model were compared with Monte Carlo simulations as well as experimental measurements made with tissue simulating phantoms. The model describes the reflectance data accurately to radial distances as small as 0.5 mm when compared to Monte Carlo simulations and agrees with experimental measurements to distances as small as 1 mm. A nonlinear least-squares fitting procedure has been used to determine the tissue optical properties from the radial reflectance data in both phantoms and tissues in vivo. The optical properties derived for the phantoms are within 5%-10% of those determined by other established techniques. The in vivo values are also consistent with those reported by other investigators.

Diffusion↗
PubMed1987

What is the status of reaction-diffusion theory thirty-four years after turing?

Physicochemical explanations of phenomena are divisible into three classes: structure, equilibrium and kinetics. For the phenomena of biological development, many physical scientists have the preconception that the explanations must turn out to be principally kinetic. In this class of theory, reaction-diffusion is by far the most extensively developed, and is worthy of attention both for its own sake and because many of its features well exemplify the nature of the broader field of kinetic theory. Reaction-diffusion should be thought of as a class, rather than a species, of theory. This review addresses three aspects: first, the general nature of the two-morphogen interaction as first proposed by Turing and incorporated in many later models; second, the specifics of these later models and their probable relative scope; third, the current state of attempts to identify the chemical nature of morphogens. It is concluded that reaction-diffusion in particular, and kinetic theory in general, are now slowly emerging from the almost total neglect by biologists which reaction-diffusion suffered for its first 20 years.

Animals↗
PubMed1989

Monte Carlo modeling of light propagation in highly scattering tissue--I: Model predictions and comparison with diffusion theory.

Using optical interaction coefficients typical of mammalian soft tissues in the red and near infrared regions of the spectrum, calculations of fluence-depth distributions, effective penetration depths and diffuse reflectance from two models of radiative transfer, diffusion theory, and Monte Carlo simulation are compared for a semi-infinite medium. The predictions from diffusion theory are shown to be increasingly inaccurate as the albedo tends to zero and/or the average cosine of scatter tends to unity.

Light↗
PubMed1988

Analysis of the diffusion theory of negative capacitance: the role of K+ and the unstirred layer thickness.

The diffusion theory of negative capacitance is extended to take into account potassium transport as well as proton or hydroxyl transport. It is shown that both the capacitance spectrum and the frequency at which the capacitance is zero can be used to experimentally test the theory. The effects of the fraction of potassium current, membrane conductance, NaCl concentration, and unstirred layer thickness on these two characteristics is investigated. Maximum negative capacitance can be obtained when the current flowing through the membrane is mainly carried by protons, the membrane conductance is high, the solution conductivity is low, and the unstirred layer thickness is large. The effect of a dominant hydroxyl transport in place of a proton transport is also discussed. We suggest simple experiments to test the theory on Characeaen plant cells.

Cell Membrane↗
PubMedDate not supplied

Innovation-diffusion theory and the evolution of the nurse practitioner role: how a good thing has caught on.

Nurse practitioners are one of the most unique innovations in health care delivery in recent decades. In this article, Roger's innovation-diffusion theory is applied to an analysis of the evolution of the nurse practitioner role. The four elements of the diffusion process, the innovation, communication, time, and social system, are addressed. Consequences of the innovation-diffusion process, as well as research implications, are presented.

Communication↗
PubMed1989

Community programs to enhance in-school anti-tobacco efforts.

This article presents theoretical bases for the development of community-wide anti-tobacco programs to reinforce school-based programs, discusses the problems involved in translating these theoretical bases into practice, and suggests means for minimizing those problems. Spiral of silence theory, diffusion theory, and expectancy value theory are covered. Special attention is given to the importance of a community program based in and "belonging" to the community, the need for high visibility, maximum participation by community individuals and organizations, mechanisms for program self-perpetuation, and explicit and visible ties to school-based anti-tobacco programs.

Adolescent↗
PubMed1987

The association reaction of yeast alcohol dehydrogenase with coenzyme is partly diffusion-controlled in solvents of increased viscosity.

The steady-state kinetics of the yeast and liver alcohol dehydrogenase catalyzed reduction of aldehydes were examined in solvent mixtures of increased viscosity. This was done to investigate the effects of diffusion control on the fast association of NADH with the enzymes. Both glycerol and sucrose were unsatisfactory as viscosogens, as they inhibited the enzyme, but poly(ethylene glycol)/water mixtures were satisfactory. The 5-fold faster reaction of yeast alcohol dehydrogenase with NADH is partly diffusion controlled, whereas the slower liver alcohol dehydrogenase reaction showed no diffusion effects. These results are consistent with a yeast alcohol dehydrogenase active site that has relatively little steric hindrance to NADH binding. It is estimated that contributions to this association reaction from diffusion control and chemical activation control are equal at a solvent viscosity of 10 cP. Thus, under physiological conditions of increased viscosity the NADH association may be significantly affected by diffusion effects. In order to estimate accurately the maximum diffusion-controlled rate constant from diffusion theory, the diffusion coefficients of NADH were measured in poly(ethylene glycol)/water mixtures and were found to vary inversely as the solvent viscosity raised to the power of 0.5. The non-Stokesian behaviour of molecules as large as NADH in polymer/water mixtures may be a serious limitation to the routine use of poly(ethylene glycol) as a viscosogen for diffusion studies.

Alcohol Dehydrogenase↗
PubMed1992

Diffusion-perfusion inhomogeneity and alveolar-arterial O2 diffusion limitation: theory.

Unequal distribution of pulmonary O2 diffusing capacity (D) to pulmonary blood flow (Q) (D/Q heterogeneity) leads to decreased alveolar O2 exchange efficacy. It is shown on simple models that the effect increases with increasing amount of inequality and with increasing value of the equilibration index, D/(Q beta) (beta, increment in blood O2 content per partial pressure increment). This inhomogeneity effect, if not taken into account, leads to spurious increases of D in hypoxia and with elevated O2 uptake.

Animals↗