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Biomedical subjects

A S Popel

Publications and source records attributed to A S Popel.

At least 19 recordsLinked to original sources
PubMed2000

A computational study of the effect of capillary network anastomoses and tortuosity on oxygen transport.

The objective of this study was to investigate the effects of capillary network anastomoses and tortuosity on oxygen transport in skeletal muscle, as well as the importance of muscle fibers in determining the arrangement of parallel capillaries. Countercurrent flow and random capillary blockage (e.g. by white blood cells) were also studied. A general computational model was constructed to simulate oxygen transport from a network of blood vessels within a rectangular volume of tissue. A geometric model of the capillary network structure, based on hexagonally packed muscle fibers, was constructed to produce networks of straight unbranched capillaries, capillaries with anastomoses, and capillaries with tortuosity, in order to examine the effects of these geometric properties. Quantities examined included the tissue oxygen tension and the capillary oxyhemoglobin saturation. The computational model included a two-phase simulation of blood flow. Appropriate parameters were chosen for working hamster cheek-pouch retractor muscle. Our calculations showed that the muscle-fiber geometry was important in reducing oxygen transport heterogeneity, as was countercurrent flow. Tortuosity was found to increase tissue oxygenation, especially when combined with anastomoses. In the absence of tortuosity, anastomoses had little effect on oxygen transport under normal conditions, but significantly improved transport when vessel blockages were present.

Animals↗
PubMed2000

An analysis of the hydraulic conductivity of the extracisternal space of the cochlear outer hair cell.

The cylindrically shaped cochlear outer hair cell (OHC) plays an important role in the transduction of acoustic energy into electrical energy in the cochlea. The extracisternal space (ECiS) of the lateral wall of the OHC is the fluid-filled space between the plasma membrane (PM) and the intracellular subsurface cisterna (SSC). In the ECiS, an array of cylindrical micropillars extends from the SSC to the PM. We obtain equations for the pressure, osmotic concentration and fluid velocity in the ECiS from the Brinkman-Stokes equations for steady incompressible flow in a plane channel that encloses an array of cylinders and whose upper wall, i.e. the plasma membrane, has a hydraulic conductivity of P(PM). From these equations we obtain an estimate for the hydraulic conductivity of the ECiS, P(ECiS). We show that the ECiS geometry accounts for P(ECiS) being several orders of magnitude larger than P(PM) and that P(ECiS) increases with the width of the ECiS and decreases with the length of the ECiS.

Cochlea↗
PubMed2000

A membrane bending model of outer hair cell electromotility.

We propose a new mechanism for outer hair cell electromotility based on electrically induced localized changes in the curvature of the plasma membrane (flexoelectricity). Electromechanical coupling in the cell's lateral wall is modeled in terms of linear constitutive equations for a flexoelectric membrane and then extended to nonlinear coupling based on the Langevin function. The Langevin function, which describes the fraction of dipoles aligned with an applied electric field, is shown to be capable of predicting the electromotility voltage displacement function. We calculate the electrical and mechanical contributions to the force balance and show that the model is consistent with experimentally measured values for electromechanical properties. The model rationalizes several experimental observations associated with outer hair cell electromotility and provides for constant surface area of the plasma membrane. The model accounts for the isometric force generated by the cell and explains the observation that the disruption of spectrin by diamide reduces force generation in the cell. We discuss the relation of this mechanism to other proposed models of outer hair cell electromotility. Our analysis suggests that rotation of membrane dipoles and the accompanying mechanical deformation may be the molecular mechanism of electromotility.

Animals↗
PubMed2000

Calculations of intracapillary oxygen tension distributions in muscle.

Characterizing the resistances to O(2) transport from the erythrocyte to the mitochondrion is important to understanding potential transport limitations. A mathematical model is developed to accurately determine the effects of erythrocyte spacing (hematocrit), velocity, and capillary radius on the mass transfer coefficient. Parameters of the hamster cheek pouch retractor muscle are used in the calculations, since significant amounts of experimental physiological data and mathematical modeling are available for this muscle. Capillary hematocrit was found to have a large effect on the PO(2) distribution and the intracapillary mass transfer coefficient per unit capillary area, k(cap), increased by a factor of 3.7 from the lowest (H=0.25) to the highest (H=0.55) capillary hematocrits considered. Erythrocyte velocity had a relatively minor effect, with only a 2.7% increase in the mass transfer coefficient as the velocity was increased from 5 to 25 times the observed velocity in resting muscle. The capillary radius is varied by up to two standard deviations of the experimental measurements, resulting in variations in k(cap) that are <15% at the reference case. The magnitude of these changes increases with hematocrit. An equation to approximate the dependence of the mass transfer coefficient on hematocrit is developed for use in simulations of O(2) transport from a capillary network.

Animals↗
PubMed2000

Diameter changes in skeletal muscle venules during arterial pressure reduction.

Previous studies in skeletal muscle have shown a substantial (>100%) increase in venous vascular resistance with arterial pressure reduction to 40 mmHg, but a microcirculatory study showed no significant venular diameter changes in the horizontal direction during this procedure. To examine the possibility of venular collapse in the vertical direction, a microscope was placed horizontally to view a vertically mounted rat spinotrapezius muscle preparation. We monitored the diameters of venules (mean diameter 73. 8 +/- 37.0 microm, range 13-185 microm) oriented horizontally and vertically with a video system during acute arterial pressure reduction by hemorrhage. Our analysis showed small but significant (P < 0.0001) diameter reductions of 1.0 +/- 2.5 microm and 1.8 +/- 3. 1 microm in horizontally and vertically oriented venules, respectively, upon reduction of arterial pressure from 115.0 +/- 26. 3 to 39.8 +/- 12.3 mmHg. The venular responses were not different after red blood cell aggregation was induced by Dextran 500 infusion. We conclude that diameter changes in venules over this range of arterial pressure reduction are isotropic and would likely increase venous resistance by <10%.

Animals↗
PubMed2000

Estimating oxygen transport resistance of the microvascular wall.

The problem of diffusion of O(2) across the endothelial surface in precapillary vessels and its utilization in the vascular wall remains unresolved. To establish a relationship between precapillary release of O(2) and vascular wall consumption, we estimated the intravascular flux of O(2) on the basis of published in vivo measurements. To interpret the data, we utilized a diffusion model of the vascular wall and computed possible physiological ranges for O(2) consumption. We found that many flux values were not consistent with the diffusion model. We estimated the mitochondrial-based maximum O(2) consumption of the vascular wall (M(mt)) and a possible contribution to O(2) consumption of nitric oxide production by endothelial cells (M(NO)). Many values of O(2) consumption predicted from the diffusion model exceeded M(mt) + M(NO). In contrast, reported values of O(2) consumption for endothelial and smooth muscle cell suspensions and vascular strips in vitro do not exceed M(mt). We conjecture that most of the reported values of intravascular O(2) flux are overestimated, and the likely source is in the experimental estimates of convective O(2) transport at upstream and downstream points of unbranched vascular segments.

Animals↗
PubMed2000

Computational fluid dynamic studies of leukocyte adhesion effects on non-Newtonian blood flow through microvessels.

The study of the effect of leukocyte adhesion on blood flow in small vessels is of primary interest to understand the resistance changes in venular microcirculation. Available computational fluid dynamic studies provide information on the effect of leukocyte adhesion when blood is considered as a homogeneous Newtonian fluid. In the present work we aim to understand the effect of leukocyte adhesion on the non-Newtonian Casson fluid flow of blood in small venules; the Casson model represents the effect of red blood cell aggregation. In our model the blood vessel is considered as a circular cylinder and the leukocyte is considered as a truncated spherical protrusion in the inner side of the blood vessel. The cases of single leukocyte adhesion and leukocyte pairs in positions aligned along the same side, and opposite sides of the vessel wall are considered. The Casson fluid parameters are chosen for cat blood and human blood and comparisons are made for the effects of leukocyte adhesion in both species. Numerical simulations demonstrated that for a Casson fluid with hematocrit of 0.4 and flow rate Q = 0.072 nl/s, a single leukocyte increases flow resistance by 5% in a 32 microns diameter and 100 microns long vessel. For a smaller vessel of 18 microns, the flow resistance increases by 15%.

Animals↗
PubMed1999

Mechanical and electromotile characteristics of auditory outer hair cells.

The passive and active properties of the cochlear outer hair cell are studied. The outer hair cell is currently considered the major candidate for the active component of mammalian hearing. Understanding of its properties may explain the amplification and sharp frequency selectivity of the ear. To analyse the cell behaviour, a model of a nonlinear anisotropic electro-elastic shell is used. Using the data from three independent experiments, where the mechanical strains of the cell are measured, estimates of the cell wall in-plane Young's moduli and Poisson's ratios are given, as well as estimates of three modes of bending stiffness. Based on these estimates and data from the microchamber experiment, where the cell is under the action of transmembrane potential changes, the characteristics of the outer hair cell active behaviour are found. These characteristics include the coefficients of the active force production per unit of the transmembrane potential change and limiting parameters of the electromotile response for extreme hyperpolarisation and depolarisation of the cell. The obtained estimates provide important information for the modelling of organ-level cochlear mechanics.

Biomechanical Phenomena↗
PubMed1999

Canine sternal force-displacement relationship during cardiopulmonary resuscitation.

A viscoelastic model developed to model human sternal response to the cyclic loading of manual cardiopulmonary resuscitation (CPR) [8] was used to evaluate the properties of canine chests during CPR. Sternal compressions with ventilations after every fifth compression were applied to supine canines (n = 7) with a mechanical resuscitation device. The compressions were applied at a nominal rate of 90/min with a peak force near 400 N. From measurements of sternal force, sternal displacement, and tracheal airflow, model parameters were estimated and their dependence on time and lung volume evaluated. The position to which the chest recoiled between compressions changed with time at a mean rate of 1.0 mm/min. Within each ventilation cycle (five compressions) the sternal recoil position decreased by 2.0 cm for each liter of decrease in lung volume. The elastic force and damping decreased with time and decreasing lung volume. Canine and human [8] model parameters were found to be similar despite the notable differences in thoracic anatomy between the species, supporting the continued use of canines as models for human CPR. These parameters may be useful in the development of a model relating sternal compression forces to blood flow during CPR.

Animals↗
PubMed1999

Nonlinear active force generation by cochlear outer hair cell.

We analyze the nonlinear behavior of the longitudinal and circumferential components of the active force generated by the outer hair cell wall in response to changes of its transmembrane potential. We treat the material of the wall as electroelastic, linear orthotropic in terms of strains and as nonlinear in terms of the transmembrane potential. To describe the nonlinear behavior of the active force versus the transmembrane potential, we use two (Boltzmann and simple exponential) types of approximation. We estimate free parameters of these approximations by combining the previously reported passive stiffnesses with the active strains measured in the microchamber experiment. We analyze the sensitivity of the estimated parameters corresponding to changes of the cell axial stiffness, a characteristic independently measured by several groups. We also study the effect of combining the active strains measured in the microchamber experiment with those measured in the whole cell recording experiment. We show agreement between our prediction of the active force and measurements in the whole cochlea and in isolated cells.

Cell Movement↗
PubMed1999

Analysis of phosphorescence decay in heterogeneous systems: consequences of finite excitation flash duration.

Analysis of phosphorescence lifetimes using the Stern-Volmer equation is a reliable means of determining quencher concentration for a uniform sample. Methods of analysis for heterogeneous systems are based on the assumption that the excitation is produced by a momentary flash. This condition is an idealization because a real flash has a finite duration and a complex time profile. In the case of a heterogeneous quencher concentration, an excitation flash produces different initial intensities and different times of peak intensity from compartments having different concentrations of quencher. We formulated a model to explore the effects of flash duration on the shape of the emission curve obtained from systems in which the heterogeneity is continuous. We developed mathematical models that can be used to recover fitting parameters of continuous distributions of reciprocal lifetimes approximated as rectangular or Gaussian distributions, or an arbitrary histogram. We also formulated a procedure to convert the distribution of reciprocal lifetimes into a volume distribution of quencher concentration. We found that (1) the Stern-Volmer ratio of phosphorescence intensities cannot be employed for interpretation of pulse phosphorometric data in terms of a volume distribution of quencher; (2) shortening the flash duration decreases the difference of initial intensities between compartments having high and low quencher concentration; (3) the parameters of the volume distribution of quencher concentration can be recovered correctly only after taking account of the difference in initial intensities; and (4) calibration of the initial intensities for a given fitting delay and flash function is necessary.

Kinetics↗
PubMed1998

Parametric analysis of the relationship between end-capillary and mean tissue PO2 as predicted by a mathematical model.

An increase of the partial pressure of oxygen in venules towards larger vessels has been observed experimentally, but the mechanism of this phenomenon has not been established. The present study considers a simple mathematical model of oxygen transport from a capillary to the surrounding tissue cylinder and analyses the conditions under which the end-capillary partial pressure of oxygen is lower than the mean tissue pressure. Under these conditions oxygen would diffuse into the venules since they are surrounded by the tissue with a higher partial pressure of oxygen. Cerebral circulation is chosen for these calculations and conditions of normoxia, hypoxic hypoxia, carbon monoxide hypoxia, anemia, and polycythemia are simulated. The tissue metabolic rate is also varied. It is found that under most conditions the relationship between the end-capillary and mean tissue partial pressures of oxygen can be reversed when one of the parameters is varied within its physiological range, i.e. the difference between these variables could be either positive or negative depending on the value of the parameters. Therefore, under many realistic conditions this mechanism would contribute to an increase of the partial pressure of oxygen in the venules. This conclusion should hold for a more realistic geometrical model of capillary network, but the relationships between the end-capillary and mean tissue partial pressures of oxygen, in addition to their dependence on the parameters considered in this study, would likely be dependent on the spatial location within the network.

Animals↗
PubMed1998

Analysis of the micropipet experiment with the anisotropic outer hair cell wall.

The in-plane and bending stiffnesses of the outer hair cell wall are characteristics crucial to the understanding of force and energy transmission between cochlear elements. A mathematical interpretation is given of the micropipet experiment directed at determining the outer hair cell wall stiffness. On the basis of the microstructural observation of the wall showing that it comprises protein networks with different elastic properties, the model of an orthotropic cylindrical shell is used. The boundary-value problem is analyzed corresponding to the stress-strain of the wall (shell) caused by the action of the micropipet. The solution is given in terms of Fourier series with respect to the circumferential coordinate. An asymptotic analysis of the solution is developed and an approximate formula for the length of the tongue aspirated within the pipet is derived. This leads to an analytical expression for the stiffness parameter measured in the micropipet experiment in terms of Young's moduli and Poisson's ratios of the wall. This expression is an important part of the estimation of the elastic constants of the wall.

Anisotropy↗
PubMed1998

Estimation of elastic moduli and bending stiffness of the anisotropic outer hair cell wall.

The outer hair cell makes both passive and active contributions to basilar membrane mechanics. The outer hair cell mechanics is strongly coupled to the elastic properties of the cell lateral wall. The lateral wall experiences both in-plane deformations and bending under physiological and experimental conditions. To characterize the outer hair cell wall, the model of an orthotropic cylindrical shell is used. The elastic constants of the wall are estimated by solving a set of three equations based on the analyses of three independent experiments. The first equation is derived from a new interpretation of the micropipet experiment; the other two are obtained from the axial loading and the osmotic challenge experiments. The two Young's moduli corresponding to the longitudinal and circumferential directions and two Poisson's ratios are estimated. The longitudinal, circumferential, and mixed modes of the bending stiffness are also estimated. The sensitivity of the derived constants to the variation of the cell axial stiffness, which has been measured by several independent groups, is examined. The new estimates are also compared with results obtained by using the assumption of the wall isotropy.

Anisotropy↗
PubMed1998

Predictions of capillary oxygen transport in the presence of fluorocarbon additives.

A mathematical model of capillary oxygen transport was formulated to determine the effect of increasing plasma solubility, e.g., by the addition of an intravascular fluorocarbon emulsion. The effect of increased plasma solubility is studied for two distributions of fluorocarbon, when the fluorocarbon droplets are uniformly distributed throughout the plasma and when the fluorocarbon droplets are concentrated in a layer adjacent to the endothelium. The model was applied to working hamster retractor muscle at normal and lowered hematocrit. The intracapillary mass transfer coefficient was found to increase by 18% as the solubility was increased by a factor of 1.7 at a hematocrit of 43%. An additional increase of 6% was predicted when the solubility increase was concentrated in the layer adjacent to the endothelium. At a hematocrit of 25%, the intracapillary mass transfer coefficient increased 14% when the solubility was increased by a factor of 1.7.

Animals↗