scholarly journals A Constrained Mixture Theory Model to Study Autoregulation in the Coronary Circulation

2020 ◽  
Author(s):  
Hamidreza Gharahi ◽  
Daniel A. Beard ◽  
C. Alberto Figueroa ◽  
Seungik Baek
Keyword(s):  
Coronary Circulation ◽  
Mixture Theory ◽  
Extensive Literature ◽  
Optimization Approach ◽  
List Type ◽  
Growth And Remodeling ◽  
Constrained Mixture Theory ◽  
Constrained Mixture ◽  
Wide Range

AbstractCoronary autoregulation is a short-term response manifested by a relatively constant flow over a wide range of perfusion pressures for a given metabolic state. This phenomenon is thought to be facilitated through a combination of mechanisms, including myogenic, shear dependent, and metabolic controls. The study of coronary autoregulation is challenging due to the coupled nature of the mechanisms and their differential effects through the coronary tree. In this paper, we developed a novel framework to study coronary autoregulation based on the constrained mixture theory. This structurally-motivated autoregulation model required calibration of anatomical and structural parameters of coronary trees via a homeostatic optimization approach using extensive literature data. Autoregulation was then simulated for two different coronary trees: subepicardial and subendocardial. The structurally calibrated model reproduced available baseline hemodynamics and autoregulation data for each coronary tree. The autoregulation analysis showed that the diameter of the intermediate and small arterioles varies the most in response to changes in perfusion pressure. Finally, we demonstrated the utility of the model in two application examples: 1) response to drops in epicardial pressure, and 2) response to drug infusion in the coronary arteries. The proposed structurally-motivated model could be extended to study long-term growth and remodeling in the coronary circulation in response to hypertension, atherosclerosis, etc.Key pointsCoronary autoregulation is defined as the capability of the coronary circulation to maintain the blood supply to the heart over a range of perfusion pressures. This phenomenon is facilitated through intrinsic mechanisms that control the vascular resistance by regulating the mechanical function of smooth muscle cells. Understanding the mechanisms involved in coronary autoregulation is one of the most fundamental questions in coronary physiology.This paper presents a structurally-motivated coronary autoregulation model that uses a nonlinear continuum mechanics approach to account for the morphometry and vessel wall composition in two coronary trees in the subepicardial and subendocardial layers.The model is calibrated against diverse experimental data from literature and is used to study heterogeneous autoregulatory response in the coronary trees. This model drastically differs from previous models, which relied on lumped parameter model formulations, and is suited to the study of long-term pathophysiological growth and remodeling phenomena in coronary vessels.

scholarly journals VASCULAR MECHANICS, MECHANOBIOLOGY, AND REMODELING

2009 ◽  
Vol 09 (02) ◽  
pp. 243-257 ◽  
Author(s):  
J. D. HUMPHREY
Keyword(s):  
Constitutive Relations ◽  
Mixture Theory ◽  
Clinical Treatment ◽  
Vascular Mechanics ◽  
Growth And Remodeling ◽  
Theoretical Frameworks ◽  
Future Directions ◽  
Constrained Mixture Theory ◽  
Constrained Mixture ◽  
Recent Developments

Arteries exhibit a remarkable ability to adapt in response to sustained alterations in hemodynamic loading as well as to disease, injury, and clinical treatment. A better understanding of such adaptations will be aided greatly by formulating, testing, and refining appropriate theoretical frameworks for modeling the biomechanics and associated mechanobiology. The goal of this brief review is to highlight some recent developments in the use of a constrained mixture theory of arterial growth and remodeling, with particular attention to the requisite constitutive relations, and to highlight future directions of needed research.

scholarly journals A Two-Dimensional Model of Hypertension-Induced Arterial Remodeling With Account for Stress Interaction Between Elastin and Collagen

2019 ◽  
Vol 142 (4) ◽  
Author(s):  
Alexander Rachev ◽  
Tarek Shazly
Keyword(s):  
Mixture Theory ◽  
Natural State ◽  
Arterial Remodeling ◽  
Two Dimensional ◽  
Structural Reorganization ◽  
Constrained Mixture Theory ◽  
Novel Structure ◽  
Constrained Mixture ◽  
Arterial Tissue ◽  
The Individual

Abstract We propose a novel structure-based two-dimensional (2D) mathematical model of hypertension-induced arterial remodeling. The model is built in the framework of the constrained mixture theory and global growth approach, utilizing a recently proposed structure-based constitutive model of arterial tissue that accounts for the individual natural configurations of and stress interaction between elastin and collagen. The basic novel predictive result is that provided remodeling causes a change in the elastin/collagen mass fraction ratio, it leads to a structural reorganization of collagen that manifests as an altered fiber undulation and a change in direction of the helically oriented fibers in the tissue natural state. Results obtained from the illustrative simulations for a porcine renal artery show that when remodeling is complete the collagen reorganization might have significant effects on the initial arterial geometry and mechanical properties of the arterial tissue. The proposed model has potential to describe and advance mechanistic understanding of adaptive arterial remodeling, promote the continual refinement of mathematical models of arterial remodeling, and provide motivation for new avenues of experimental investigation.

scholarly journals A constituent-based model of age-related changes in conduit arteries

2011 ◽  
Vol 301 (4) ◽  
pp. H1286-H1301 ◽  
Author(s):  
Alkiviadis Tsamis ◽  
Alexander Rachev ◽  
Nikos Stergiopulos
Keyword(s):  
Mechanical Properties ◽  
Time Course ◽  
Present Report ◽  
Finite Elasticity ◽  
Mixture Theory ◽  
Constrained Mixture Theory ◽  
Age Related ◽  
Constrained Mixture ◽  
Arterial Tissue ◽  
Age Related Changes

In the present report, a constituent-based theoretical model of age-related changes in geometry and mechanical properties of conduit arteries is proposed. The model was based on the premise that given the time course of the load on an artery and the accumulation of advanced glycation end-products in the arterial tissue, the initial geometric dimensions and properties of the arterial tissue can be predicted by a solution of a boundary value problem for the governing equations that follow from finite elasticity, structure-based constitutive modeling within the constrained mixture theory, continuum damage theory, and global growth approach for stress-induced structure-based remodeling. An illustrative example of the age-related changes in geometry, structure, composition, and mechanical properties of a human thoracic aorta is considered. Model predictions were in good qualitative agreement with available experimental data in the literature. Limitations and perspectives for refining the model are discussed.

A Constitutive Model for Soft Tissue and its Application to a Boundary Value Problem

2013 ◽  
Author(s):  
P. Mythravaruni ◽  
Parag Ravindran
Keyword(s):  
Soft Tissue ◽  
Mixture Theory ◽  
Axial Loading ◽  
Tissue Growth ◽  
Theory Approach ◽  
Growth And Remodeling ◽  
Constrained Mixture ◽  
Set Up ◽  
And Function ◽  
Turn Over

Mechanical loading induces changes in the structure and function of soft tissue. Growth and remodeling results from the production and removal of constituents. We consider a tissue constituted of elastin and collagen. The collagen turns over at a much higher rate than elastin. In this work we propose a two-constituent, constrained mixture model for this soft tissue. One constituent is modeled as a viscoelastic material and the other as an elastic material. It is assumed that the collagen turns over depending on the stress applied and the elastin does not turn over. The standard mixture theory approach is followed and the balance equations are set-up. The model is studied in simple uni-axial loading to test its efficacy.

scholarly journals Standard Astronomical Sources for the Space Telescope

1985 ◽  
Vol 111 ◽  
pp. 357-360
Author(s):  
Ralph C. Bohlin
Keyword(s):  
List Type ◽  
Scientific Instruments ◽  
Diverse Range ◽  
Space Telescope ◽  
Wide Range ◽  
Flat Field ◽  
Imaging Polarimetry ◽  
Working Groups ◽  
Measurement Capabilities

The Space Telescope (ST) will require many types of standard sources for a diverse range of calibrations to be performed after launch. The scientific instruments are sensitive to a wide range of wavelengths from 1050 to 11,000Å and encompass a broad range of measurement capabilities including astrometry, photometry, imaging, polarimetry, and spectroscopy. To verify proper operations of each instrument and to provide quantitative calibrations, a diverse range of standard sources and fields are required. In order to select targets that satisfy the requirements of the Instrument Definition Teams and the long term responsibilities of the Science Institute, six groups containing a total of 25 astronomers are defining the calibration targets to be observed after launch. The six categories of ST standard sources are: 1)Ultraviolet Spectrophotometric2)Ground Based Spectrophotometric and Photometric3)Wavelength4)Astrometric5)Polarimetric6)Spatially Flat FieldThe data in these categories will be collected from the literature or through new observing programs as appropriate. These six reports of the working groups outline the calibrations and proposed targets for all of the scientific instruments on ST. The collected data on each set of standard sources should be published in the refereed literature.

A CONSTRAINED MIXTURE MODEL FOR GROWTH AND REMODELING OF SOFT TISSUES

2002 ◽  
Vol 12 (03) ◽  
pp. 407-430 ◽  
Author(s):  
J. D. HUMPHREY ◽  
K. R. RAJAGOPAL
Keyword(s):  
Soft Tissues ◽  
Constitutive Relations ◽  
Viscoelastic Behavior ◽  
Mixture Theory ◽  
Theory Model ◽  
Structure And Function ◽  
Material Point ◽  
Growth And Remodeling ◽  
Constrained Mixture ◽  
And Function

Not long ago it was thought that the most important characteristics of the mechanics of soft tissues were their complex mechanical properties: they often exhibit nonlinear, anisotropic, nearly incompressible, viscoelastic behavior over finite strains. Indeed, these properties endow soft tissues with unique structural capabilities that continue to be extremely challenging to quantify via constitutive relations. More recently, however, we have come to appreciate an even more important characteristic of soft tissues, their homeostatic tendency to adapt in response to changes in their mechanical environment. Thus, to understand well the biomechanical properties of a soft tissue, we must not only quantify their structure and function at a given time, we must also quantify how their structure and function change in response to altered stimuli. In this paper, we introduce a new constrained mixture theory model for studying growth and remodeling of soft tissues. The model melds ideas from classical mixture and homogenization theories so as to exploit advantages of each while avoiding particular difficulties. Salient features include the kinetics of the production and removal of individual constituents and recognition that the neighborhood of a material point of each constituent can have a different, evolving natural (i.e. stress-free) configuration.

scholarly journals Dynamic visualisation of million-tip trees: the OneZoom project

2020 ◽  
Author(s):  
Yan Wong ◽  
James Rosindell
Keyword(s):  
Science Communication ◽  
Tree Of Life ◽  
Individual Species ◽  
List Type ◽  
Data Synthesis ◽  
Wide Range ◽  
Complete Tree ◽  
Client Side ◽  
Guided Tours

AbstractThe complete tree of life is now available, but methods to visualise it are still needed to meet needs in research, teaching and science communication. Dynamic visualisation of million-tip trees requires many challenges in data synthesis, data handling and computer graphics to be overcome.Our approach is to automate data processing, synthesise data from a wide range of available sources, then to feed these data to a client-side visualisation engine in parts. We develop a way to store the whole tree topology locally in a highly compressed form, then dynamically populate metadata such as text and images as the user explores.The result is a seamless and smooth way to explore the complete tree of life, including images and metadata, even on a relatively old mobile device.The underlying methods developed have applications that transcend tree of life visualisation. For the whole complete tree, we describe automated ID mappings between well known resources without resorting to taxonomic name resolution, automated methods to collate sets of public domain representative images for higher taxa, and an index to measure public interest of individual species.The visualisation layout and the client user interface are both abstracted components of the codebase enabling other zoomable tree layouts to be swapped in and supporting multiple applications including exhibition kiosks and digital art.After eight years of work, our tree of life explorer is now broadly complete, it has attracted over 1.3 million users, and is backed by a novel long-term sustainability plan. We conclude our description of the OneZoom project by suggesting the next challenges that need to be solved in this field: extinct species and guided tours around the tree.

Comparison of Deterministic and Linear Cosine Spatial Filtering of Transmission Electron Micrographs

1972 ◽  
Vol 30 ◽  
pp. 596-597
Author(s):  
David A. Ansley
Keyword(s):  
Spherical Aberration ◽  
Focal Length ◽  
Chromatic Aberration ◽  
Spatial Filter ◽  
Operating Conditions ◽  
Objective Lens ◽  
List Type ◽  
Spatial Filters ◽  
Transmission Electron ◽  
Wide Range

The coherence of the electron flux of a transmission electron microscope (TEM) limits the direct application of deconvolution techniques which have been used successfully on unmanned spacecraft programs. The theory assumes noncoherent illumination. Deconvolution of a TEM micrograph will, therefore, in general produce spurious detail rather than improved resolution.A primary goal of our research is to study the performance of several types of linear spatial filters as a function of specimen contrast, phase, and coherence. We have, therefore, developed a one-dimensional analysis and plotting program to simulate a wide 'range of operating conditions of the TEM, including adjustment of the:(1) Specimen amplitude, phase, and separation(2) Illumination wavelength, half-angle, and tilt(3) Objective lens focal length and aperture width(4) Spherical aberration, defocus, and chromatic aberration focus shift(5) Detector gamma, additive, and multiplicative noise constants(6) Type of spatial filter: linear cosine, linear sine, or deterministic

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