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Microvasculature is defined as vessels (venules or capillaries) with a maximum average diameter of 0.3 millimeters.〔("Microvascular" ) Merriam-Webster. N.p., n.d. Web. 21 May 2012.〕 As the vessels decrease in size, they increase their surface-area-to-volume ratio. This allows surface properties to play a significant role in the function of the vessel. Diffusion occurs through the walls of the vessels due to a concentration gradient, allowing the necessary exchange of ions, molecules, or blood cells. The permeability of a capillary wall is determined by the type of capillary and the surface of the endothelial cells. A continuous, tightly spaced endothelial cell lining only permits the diffusion of small molecules. Larger molecules and blood cells require adequate space between cells or holes in the lining. The high resistivity of a cellular membrane prevents the diffusion of ions without a membrane transport protein. The hydrophobicity of an endothelial cell surface determines whether water or lipophilic molecules will diffuse through the capillary lining. The blood brain barrier restricts diffusion to small hydrophobic molecules, making drug diffusion difficult to achieve. Blood flow is directly influenced by the thermodynamics of the body. Changes in temperature affect the viscosity and surface tension of the blood, altering the minimum blood flow rate. At high temperatures the minimum flow rate will decrease and the capillary will expand. This allows heat transfer through the increased surface area of the inner capillary lining and through increased blood flow. At low temperatures the minimum flow rate will increase and the capillary will constrict. This restricts blood flow and decreases the surface area of the capillary, reducing heat transfer. Fluid mechanics are primarily affected by pressure, temperature, heat transfer, and electrokinetics. An increase in pressure increases the flow rate given by the Starling equation. An increase in temperature increases the wettability of the surface, promoting fluid flow. Heat also decreases the viscosity of lumen. Heat transfer is monitored by thermoreceptors which regulate the amount of capillary beds open for heat dissipation. The surface chemistry of the endothelial cell lining also dictates fluid flow. A charged surface will acquire a layer of stagnant diffuse ions that hinder the flow of ions in the lumen. This decreases the lumen velocity and promotes the exchange of molecules through the capillary lining. == Diffusion == Diffusion is the movement of molecules due to a concentration gradient. The molecules move in a random walk pattern in order to achieve a uniform solution. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Surface Chemistry of Microvasculature」の詳細全文を読む スポンサード リンク
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