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Biomechanics of Microcirculatory Blood Perfusion

Biomechanics of Microcirculatory Blood Perfusion -...

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Unformatted text preview: Biomechanics of Microcirculatory Blood Perfusion By: Shin Chang Outline Microvasculature Growth Viscoelasticity Myogenic Response Endothelium Effects Microvascular Blood Flow Properties Pressure Flow Relationship Capillaries Blood Properties Blood Contents Non-Newtonian Properties of Blood NonCell Distribution in Small Vessels Leukocyte Activation and Adhesion Microvasculature Arteriloes Capillaries Venules Figure 1. Arterioles, capillaries, and venules. en.wikipedia.org/wiki/areterioles Microvasculature - Rat Muscle Microvasculature - Growth Growth influenced by: Angiogenic Factors Mechanical Stresses Example: Capillary Connections between two terminal arterioles (Price et al) Elevated wall tension New arterioles originate from capillary network Microvasculature - Viscoelastic Properties Microvessels are distensible to stress: Pressure Venules ~103 dyn/cm2 Arterioles ~105 dyn/cm2 Shear Stress ~50-100 dyn/cm2 Pulmonary arterioles and venules Pulmonary capillary thickness Microvasculature Pressure relationship with Strain Viscoelastic Standard Model Strain Component Microvasculature -Myogenic Response Active stimulation of arterial smooth muscle cells by: Circumferential stress generated by the pressure Shear Stress Metabolic Stimuli (oxygen, ph, etc) Nitric Oxide produced by Endothelial Cells Microvasculature - Myogenic Response cont. Pressure Function Creep Function Elastic Function Microvasculature - Effects of Endothelium Endothelium Branch Vessel Figure: Dr. Eberhart's BE5361 Lecture 13a. Microvasculature - Effects of Endothelium Endothelial Cell Nuclei causes non-circular cross sections Pressure <20 mmHg Nuclei presence is most significant in capillaries Can increase resistance twofold Endothelial Cell stimulation producing cytoplasmic projections via actin polymerization Histamine Platelet Activating Factor Microvasculature Endothelium Glycocalyx Layer Layer of glycoproteins and glycolipids on the surface of the endothelium 0.5-1.0um thick Prevents RBCs from approaching the endothelium when traveling 20um/s Resistance reduced by enzyme breakdown (neuraminidase or heparinase) Microvascular Flow Reynolds Number - 0.1-0.001 Viscous vs. Inertial Forces Peclet Number <1 Convective Forces Neglected Womersely Number - <1 Velocity Profile in Phase with Waveform Parabolic velocity profile at all times Blood Flow Pressure Flow Relationship Pulmonary arterioles/venules Skeletal Muscle Microvessels Pulmonary Capillaries Blood Flow - Capillaries Capillaries RBC ~10um Goes in single file High pressure differences Deforms to `parachute' or `umbrella' shape Assists in nutrient/gas exchange Blood Blood Contents Aqueous Phase Plasma, contains salts, sugars, and proteins Cellular Phase RBC 30-40% WBC <1% Platelets Blood Properties - Non-Newtonian Properties Increases in shear rate decrease viscosity Newtonian Above 100s-1 Fibrinogen and Globulin bind to RBC causing aggregation Deformation of RBC leads to Lower Viscosity Blood Properties NonNewtonian Properties Cont' Casson's Equation Equation of Fluid Motion Distensible Microvessel Blood Properties - Cell Distribution in Small Vessels Fahreus Effect RBCs like to travel through the center of a tube. Flow is influenced by blood's two phase nature. Blood Properties - Effects of Leukocytes Leukocytes causes increased resistance in microvessels Larger and stiffer compared to RBCs Increased resistance is due to build up of RBCs, depends on Hematocrit Higher apparent viscosity Larger resistance in organs with larger capillary length Smaller resistance in pulmonary circulation with shorter capillaries Blood Properties Leukocyte Activation Essential Membrane Molecules in the Multistep Leukocyte Adhesion Cascade von Andrian et al., "T-Cell Function and Migration -- Two Sides of the Same Coin". New England Journal of Medicine 343:1020-34 (2000) Leukocyte membrane Endothelial cell membrane Conclusion Biomechanics of Microcirculatory Blood Perfusion is dependent on many variables Microvasculature Composition - Distensible Walls, Endothelium, Myogenic Response Flow Nature Re, Pe, Wm, compensation for Microvasculature Fluid Properties RBCs, WBCs Questions? References Schmid-Schonbein G. Biomechanics of Microcirculatory SchmidBlood Perfusion. Annual Review Biomedical Engineering. 01:73-102. 1999. 01:73Lipwsky H. Microvascular Rheology and Hemodynamics. Microcirculation. 12:5-15. 2005. 12:5Truskey G, Yuan F, Katz D. Transport Phenomena in Biological Systems. Pearson Prentice Hall. Upper Saddle River, NJ. 2004. ...
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