Time filter

Source Type

Évora, Portugal

Silva C.,Polytechnic Institute of Tomar | Silva C.,Evora Geophysics Center | Reis A.H.,Evora Geophysics Center | Reis A.H.,University of Evora
Journal of Biomechanics | Year: 2014

In this study we explore the ability of a previously developed model of pulsatile flow for explaining the observed reduction of arterial distensibility with heart rate. The parameters relevant for the analysis are arterial wall distensibility together with permeability and reflection coefficients of the end capillaries. A non-specific artery and the ensemble of tissues supplied by that artery were considered in the model. The blood current within that artery was equalized to the sum of all micro currents in the tissues supplied by that artery. A formula emerged that relates changes in arterial distensibility with heart rate, and also with some particular aspects of microcirculation. Then, that formula was tested with data of distensibilities of the radial and carotid arteries observed at the heart rates of 63, 90, and 110. b.p.m. The formula correctly predicted the trend of decreased distensibility with heart rate for both arteries. Moreover, due to the fact that the carotid artery supplies the brain, and because the Blood-Brain barrier is highly restrictive to colloids in the blood, for the carotid artery the formula predicted a less marked decrease in distensibility than in the case of the radial artery feeding muscle tissue, which has a greater permeability to colloids, a trend that was confirmed by data. It was found that reduction of arterial distensibility with heart rate was greater in arteries that supply end capillaries with high permeability and low reflection coefficients. © 2014 Elsevier Ltd. Source

Pereira S.N.,Evora Geophysics Center | Wagner F.,Evora Geophysics Center | Silva A.M.,Evora Geophysics Center | Silva A.M.,University of Evora
Atmospheric Environment | Year: 2012

Black carbon mass concentration (BCPM10) measurements carried out at évora, Portugal, between 2007 and 2009, were analyzed and interpreted at different timescales. Additional measurements of aerosol mass concentration (MPM10) were included and the black carbon to total mass fraction (BC mass fraction) was derived when measurements of both quantities were coincident. BCPM10 values were found to vary between 0.3 and 5 μg m -3, mainly in the range of 0.5-2 μg m -3. A clear and consistent seasonal behavior was found; an increase by a factor of two in the average BCPM10 values was observed from summer (0.9 μg m -3) to winter (1.8 μg m -3) which is reflected in the BC mass fraction, amplified from about 4 to 10%. Comparison of the average BCPM10 mass concentrations on week days and week ends indicate that local traffic emissions strongly influence the observed average diurnal patterns. Other factors, such as wood burning for heating, lower boundary layer height and more frequent winter-time temperature inversions, also likely influence the observed BCPM10 but were not directly studied here. When different air mass types are considered then black carbon levels show a much lower variation than PM 10 mass concentrations, stressing the relevance of local emissions in the BCPM10 levels. BCPM10 values under continental or maritime influence only differ by a factor of approximately 1.4 only, much lower than the factor of 10 reported for a coastal rural site in Portugal. © 2012 Elsevier Ltd. Source

Silva C.,Polytechnic Institute of Tomar | Silva C.,Evora Geophysics Center | Reis A.H.,Evora Geophysics Center | Reis A.H.,University of Evora
International Journal of Thermal Sciences | Year: 2014

Apparently complex flow structures obey to scaling relations that enable to make it viable the study of their configuration and flow dynamics. This is the case of flow structures that exhibit several branching levels and are thought to perform optimally. Here we present scaling relations of diameters and lengths of branching cylindrical channels with pulsatile flows, and compare them with other relations published in the literature. It is shown that, under constant global volume of the flow tree, and for zero pulse frequency these scaling relations reduce to Murrays's law of consecutive diameters. Optimal scaling depends on pulse frequency, distensibility of the channel walls, and asymmetry of the daughter vessels. In case that in addition to global volume of the flow tree, the pressure head is also kept constant, a similar scaling law of channel lengths emerges that holds together with the law of diameter scaling. The effect of channel distensibility is shown to be somehow important, such that for achieving optimal performance (lowest impedance) channels with lower relative distensibility must have their diameter increased. Results are compared with those of other models for the case of some arteries. © 2014 Elsevier Masson SAS. All rights reserved. Source

Silva C.,Polytechnic Institute of Tomar | Silva C.,Evora Geophysics Center | Heitor Reis A.,Evora Geophysics Center | Heitor Reis A.,University of Evora
Medical Physics | Year: 2014

Purpose: The objectives are: (i) assess the development of the impedance of some arteries during the first decades of life; (ii) determine the influence of pulse rate in arterial impedance; (iii) compare the structure of some arterial segments with optimized structures with respect to blood flow; and (iv) explain the elongation of the ascending aorta throughout life in healthy subjects. Methods: A model of the arterial network previously developed by the authors, together with data of lengths, diameters, and distensibilities of arterial segments reported in the literature were used. The impedances of the aorta and carotid artery were calculated based on that model. Similarly, the impedances of various arteries corresponding to heart rates of 65 bpm and 120 bpm were calculated. Values observed in arterial segments were compared with the respective optimal values from the viewpoint of hemodynamic performance. This allowed drawing conclusions on the arterial segments that might be critical with regard to hemodynamics. Results: It was found that in healthy people impedances of the aorta and the carotid artery decrease markedly with age especially during body growth. It was also found that impedances of the main arteries do not significantly change with heart rate, even if sharp changes in arterial distensibility are observed. With respect to optimal flow performance, it was found that scaling between diameters of branching arteries is generally close to optimality, while the corresponding length scaling is far from optimality. It was also found that the ascending aorta and aortic arch are among those arterial segments whose lengths are much smaller than the optimum values. An explanation is offered for the age associated elongation of the aorta in healthy people. Conclusions: In healthy subjects, the human arterial system continues to optimize its performance at least until the age of 60. © 2014 American Association of Physicists in Medicine. Source

Pereira S.N.,Evora Geophysics Center | Wagner F.,Evora Geophysics Center | Silva A.M.,Evora Geophysics Center | Silva A.M.,University of Evora
Atmospheric Chemistry and Physics | Year: 2011

Aerosol scattering properties, near the surface (at about 10m height), were measured during a period of seven years (2002-2008) at ́ Evora, Portugal. The average (and median) scattering and backscattering coefficients, at the wavelength of 550 nm, were found to be 42.5Mm-1 (29.9Mm-1) and 5.9Mm-1 (4.4Mm-1), respectively. Also, the average and median scattering Ångstr̈om exponent (1.4 and 1.5) indicate that scattering was, in general, dominated by submicrometer particles. Both seasonal and daily cycles are shown, which were related to local production and transport of particles from elsewhere. Summer and winter average values of the scattering coefficient, at the wavelength of 550 nm (47 and 54Mm-1, respectively), correspond to a significant increase in the aerosol particle concentration when compared with spring and fall (35 and 37Mm-1, respectively). Also, the average increase in the Ångstr̈om exponent for summer and winter seasons is consistent with the input of sub-micrometer particles from anthropogenic origin in winter and forest fires in summer. Back-trajectory analysis indicated that the site was regularly under the influence of air masses from the Atlantic area, with low particle loads (low scattering coefficients), but as the influence of transport from the continent (Iberia Peninsula) increased, the aerosol particle load was observed to increase as well as the relative importance of fine particles over coarse ones, approaching the features observed at the site during European air masses influence. © 2011 Author(s). Source

Discover hidden collaborations