Solutes can enter and leave gray matter in the brain by perivascular routes. The glymphatic hypothesis supposes that these movements are a consequence of inward flow along periarterial spaces and an equal outward flow along perivenous spaces. The flow through the parenchyma between periarterial and perivenous spaces is the same as the inflow and the outflow. Ray et al. (Fluids Barriers CNS 16:6, 2019) have investigated how this flow could interact with diffusion using numerical simulations of real-time iontophoresis experiments that monitor the concentrations of tetramethylammonium ions (TMA+) injected into the parenchyma via iontophoresis. For this purpose they have devised a description of the parenchyma incorporating perivascular spaces. Their simulations show that superficial flow velocities of about 50 µm min-1 are needed to produce changes in TMA+ fluxes comparable to those accounted for by diffusion. In the glymphatic hypothesis the proposed flow through the parenchyma can be estimated from the clearance of solutes that are present in the perivenous outflow at the same concentration as in the interstitial fluid of the parenchyma. Reported clearances are approximately 1 µL min-1 g-1. This flow can be converted to a superficial flow velocity using the area available for the flow, which can be estimated using Ray et al.'s description of the tissue as 40 cm2 g-1. The best available estimate of the flow velocity is thus 0.25 µm min-1 which is 200 times smaller than the flow that produces effects comparable to diffusion for TMA+. Thus it follows in Ray et al.'s description of the parenchyma that diffusion rather than flow accounts for TMA+ movements. Because the diffusion constant depends only weakly on molecular weight the same is expected to apply even for solutes somewhat larger than serum albumin.

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脑灰质细胞外空间内的溶质运动主要是由扩散还是流动引起的？L. Ray、JJ Iliff 和 JJ Heys 对“脑间质中的对流和扩散运输的分析”中枢神经系统的流体和屏障 (2019) 16:6 的评论。

溶质可以通过血管周围途径进入和离开大脑中的灰质。淋巴假说假设这些运动是沿动脉周围空间向内流动和沿静脉周围空间等量向外流动的结果。通过动脉周围和静脉周围空间之间的实质的流动与流入和流出相同。雷等人。(Fluids Barriers CNS 16:6, 2019) 使用实时离子电渗实验的数值模拟研究了这种流动如何与扩散相互作用，该实验监测通过离子电渗注入薄壁组织的四甲基铵离子 (TMA+) 的浓度。为此，他们设计了对包含血管周围空间的实质的描述。他们的模拟表明，需要大约 50 µm min-1 的表观流速才能产生与扩散所引起的变化相当的 TMA+ 通量变化。在 glymphatic 假设中，可以根据静脉周围流出物中与实质间质液中浓度相同的溶质清除率来估计通过实质的拟议流量。报告的清除率约为 1 µL min-1 g-1。可以使用可用于流动的面积将该流动转换为表面流速，可以使用 Ray 等人对组织的描述为 40 cm2 g-1 来估计该面积。因此，流速的最佳可用估计值为 0.25 µm min-1，它比产生与 TMA+ 扩散相当的效果的流量小 200 倍。因此，它遵循 Ray 等人。s 描述了 TMA+ 运动的扩散而不是流动的实质。由于扩散常数仅微弱地依赖于分子量，因此即使对于比血清白蛋白稍大的溶质，预期同样适用。