Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session M03: Biological Fluid Dynamics: Physiological Microcirculation |
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Chair: Marcus Roper, UCLA Room: North 121 A |
Monday, November 22, 2021 1:10PM - 1:23PM |
M03.00001: The effects of the endothelial surface layer's (ESL's) hydraulic resistivity and resistance to compression on red blood cell partitioning, deformation, and penetration of the ESL Jared Barber, Carlson Triebold The heterogeneous red blood cell (RBC) distribution seen in the microvasculature has important consequences for transport of materials such as oxygen, nutrients, and drugs. Such heterogeneity is due, in part, to nonuniform partitioning of RBCs at microvascular bifurcations where higher flow downstream branches tend to claim a disproportionately high number of RBCs compared to the low flow branch. RBC distribution heterogeneity is also influenced by the presence of an endothelial surface layer (ESL), a layer on the order of 1 micron thick that coats the vessel wall. While many studies have considered RBC dynamics at bifurcations and many others have considered ESL effects on flow, none have considered both. To better understand how the ESL may affect partitioning, we constructed a computational model of RBCs passing one at a time through a bifurcation lined with an ESL. RBCs are modeled as viscoelastic networks immersed in Stokes flow while the ESL is modeled as a porous media that resists compression. We found that decreasing the ESL's hydraulic resistivity and/or its resistance to compression generated more nonuniform partitioning. In addition, because experiments have shown evidence that RBC deformation and adhesion may play important roles in various pathologies (e.g. cardiovascular disease and thrombosis), we also considered how ESL properties affected RBC deformation and penetration of the ESL. We found that decreasing the ESL's hydraulic resistivity and/or its resistance to compression increased RBC penetration of the ESL and, usually, decreased RBC deformation. We will also share preliminary results on how varying ESL properties affect RBC interactions with each other as they pass through a bifurcation. While all results are limited to very low hematocrits and a somewhat specific setting, they provide a strong start towards more fully characterizing ESL-RBC interactions. |
Monday, November 22, 2021 1:23PM - 1:36PM |
M03.00002: Computational Modeling of Flow-Mediated Fibrin Degradation in Arterial Blood Clots During Thrombolysis. Lindsey S Nast, Debanjan Mukherjee Thrombolysis is the process by which a blood clot is broken down by a drug (tissue Plasminogen Activator or tPA) through degradation of fibrin, the structural component of the clot. Comprehensive understanding of thrombolysis of real human arterial clots is important for improving treatment efficacy. In real arterial clots with heterogeneous microstructure, the lysis process is often not well-understood and is difficult to investigate experimentally or in vivo. In-silico computational models coupling fluid flow and biochemical processes can be a viable alternative to inform underlying thrombolytic processes and resulting treatment efficacy. Here, we use a compartmental model to simulate tPA infusion into the body and biochemical reactions in the fibrinolysis cascade. We couple this with a stabilized finite element formulation for fluid flow within and around an arterial blood clot, and species transport and local species reaction with surface fibrin in the clot geometry. We will present simulations using different clot microstructural properties, to illustrate how microstructural variations can lead to differences in flow mediated transport of tPA drug and subsequent lysis of the clot. |
Monday, November 22, 2021 1:36PM - 1:49PM |
M03.00003: Control of low flow regions in the cerebral vasculature sets an optimal arteriole-venule number ratio. Yujia Qi, Marcus Roper Lacking any ability to store glucose, the mammalian brain relies on a constant glucose and oxygen supply via the cerebral vasculature. In the cortex, this supply is maintained by parallel arterioles and venules. Yet, analysis of both real cortical microvasculature modeled as networks and idealized vasculature modeled by a slender-body theory approach shows that far from being perfused uniformly, the cortex is strewn with regions of very low flow. Increasing the number of perfusing vessels increases the number of low-flow spots. Minimizing the influence of low flow spots sets an optimal arteriole-venule ratio that we find to be closely recapitulated in data from real mammalian cortices. Further, low flow regions complicate the regulation of metabolite delivery with neuronal activity, leading to unintuitive changes in perfusion when penetrating vessels dilate. |
Monday, November 22, 2021 1:49PM - 2:02PM |
M03.00004: Application of Echo-PTV and Deep Learning for the Visualization and Quantification of the Cerebral Microcirculation Zeng Zhang, Misun Hwang, Todd J Kilbaugh, Anush Sridharan, Joseph Katz Ultrasound Localization Microscopy involves tracking of microbubbles (<5 μm) in contrast enhanced ultrasound (CEUS) images for reconstructing the microvasculature and microcirculation in organs. The current work introduces several methods for enhancing the localization and tracking of microbubbles in clinical CEUS images for measuring the cerebral microcirculation in piglets. A U-net based deep learning method is used for enhancing noisy raw CEUS images, improving the precision of microbubble detection. Bubble tracking uses Kalman filtering along with a series of criteria to insure the spatio-temporal consistency in flow direction, velocity magnitude, and bubble image morphology. Trajectory assignments are then globally optimized. Based on synthetic data, the U-net enhancement significantly improves the processing speed and localization accuracy over conventional methods. Heatmaps of the bubble trajectories depict the complex cerebral micro-vessel network, where neighboring vessels separated by 40 μm can be distinguished. Based on the current framerate (44 fps), blood flow speeds in the 0.1 to 12 cm/s range can be well captured, but non-clinical systems with much higher framerates can capture higher speeds. |
Monday, November 22, 2021 2:02PM - 2:15PM |
M03.00005: Multi-stability in blood vessel network motifs George W Atkinson, Helen Byrne, Philip Maini, Joe Pitt-Francis The presence of low oxygen, or hypoxia, within tumours can significantly impact patient responses to therapy. Experimental results have shown that transient periods of cycling hypoxia can select for cancers that are difficult to treat and highly invasive. A useful metric for quantifying hypoxia is the haematocrit (ratio of red blood cells to plasma) distribution in the tumour’s micro-circulation. We model blood flow through a network of blood vessels by assuming ‘Poiseuille’ flow through individual vessels, conserving blood flow and red blood cells, and using a previously proposed phenomenological rules for haematocrit splitting at vessel bifurcations. The blood flow dictates the distribution of haematocrit within the network. We hypothesise that cycling hypoxia can be generated by fluctuations between distinct stable equilibria. We use continuation techniques to find multiple equilibria, and determine their stability as physiologically relevant model parameters are varied. Hence, we showcase our approach by applying it to several vessel network motifs to uncover a link between the existence of equilibria and network geometry that supports our hypothesis. |
Monday, November 22, 2021 2:15PM - 2:28PM |
M03.00006: Red blood cell shape transitions and dynamics in time-dependent capillary flows Christian Wagner, Stephan Gekle, Steffen M Recktenwald, Katharina Graessel, Felix Maurer We present experimental and numerical data on the dynamics and shape transitions of RBCs in unsteady flow conditions using a combination of microfluidic experiments and numerical simulations. Tracking RBCs in a comoving frame in time-dependent flows showed that the mean transition time from the symmetric croissant to the off-centered, non-symmetric slipper shape is significantly faster than the opposite shape transition, which exhibits pronounced cell rotations. Complementary simulations indicate that these dynamics depend on the orientation of the RBC membrane in the channel during the time-dependent flow. Moreover, we show how the tank-treading movement of slipper-shaped RBCs in combination with the narrow channel leads to oscillations of the cell's center of mass. The frequency of these oscillations depends on the cell velocity, the viscosity of the surrounding fluid, and the cytosol viscosity. These results provide a potential framework to identify and study pathological changes of RBC properties. |
Monday, November 22, 2021 2:28PM - 2:41PM |
M03.00007: Relations between network topology, hemodynamic and oxygen supply investigates by a 1D numerical model of microcirculation Laureline JULIEN, Michel PAQUES, José-Maria FULLANA Pathologies like diabetes or arterial hypertension affect vessels of the microcirculation and lead to the emergence of vascular anomalies as occlusions. Non-oxygen perfused areas (ischemic) then appear, altering surrounding tissues and organ function. In the retina, this leads to local or global vision disorders. In-vivo imaging of the retina allows establishing correlations between ischemic area, troubles of blood circulation, and morphology. We wonder to what extent the topological characteristics of the network, unique to each person, are involved in the development of vascular anomalies. We want then to investigate the relations between network topology, hemodynamics, and oxygen supply. The oxygen distribution in a microvascular network is heterogeneous and complex because of the variety of physical phenomena that exists at this scale close to that of a red blood cell (RBC). We develop a 1D numerical model of circulation with the specificities of the micro-scale blood rheology (Fahraeus-Lindqvist, Zweifach-Fung effects). We simulate blood flow and transport of RBCs and oxygen through networks with several spatial symmetries. These simulations allow an exhaustive study on global quantities as energy, blood resistance, or amount of delivered oxygen. |
Monday, November 22, 2021 2:41PM - 2:54PM |
M03.00008: A Fully Second Order Constitutive Theory of Fluids Samuel Paolucci A fully second order continuum theory of fluids is developed. The conventional balance equations of mass, linear momentum, energy and entropy are used. Constitutive equations are assumed to depend on density, temperature and velocity, and their derivatives up to second order. The principle of equipresence is used along with the Coleman-Noll procedure to derive restrictions on the constitutive equations by utilizing the second law. The entropy flux is not assumed to be equal to the heat flux over the temperature. We obtain explicit results for all constitutive quantities up to quadratic nonlinearity so as to satisfy the Clausius-Duhem inequality. Our results are shown to be consistent but much more general than other published results. |
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