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. 2018 Jun 15;15(1):17.
doi: 10.1186/s12987-018-0103-8.

Is bulk flow plausible in perivascular, paravascular and paravenous channels?

Mohammad M Faghih  1 , M Keith Sharp  2
Affiliations

Is bulk flow plausible in perivascular, paravascular and paravenous channels?

Mohammad M Faghih et al. Fluids Barriers CNS. .

Abstract

Background: Transport of solutes has been observed in the spaces surrounding cerebral arteries and veins. Indeed, transport has been found in opposite directions in two different spaces around arteries. These findings have motivated hypotheses of bulk flow within these spaces. The glymphatic circulation hypothesis involves flow of cerebrospinal fluid from the cortical subarachnoid space to the parenchyma along the paraarterial (extramural, Virchow-Robin) space around arteries, and return flow to the cerebrospinal fluid (CSF) space via paravenous channels. The second hypothesis involves flow of interstitial fluid from the parenchyma to lymphatic vessels along basement membranes between arterial smooth muscle cells.

Methods: This article evaluates the plausibility of steady, pressure-driven flow in these channels with one-dimensional branching models.

Results: According to the models, the hydraulic resistance of arterial basement membranes is too large to accommodate estimated interstitial perfusion of the brain, unless the flow empties to lymphatic ducts after only several generations (still within the parenchyma). The estimated pressure drops required to drive paraarterial and paravenous flows of the same magnitude are not large, but paravenous flow back to the CSF space means that the total pressure difference driving both flows is limited to local pressure differences among the different CSF compartments, which are estimated to be small.

Conclusions: Periarterial flow and glymphatic circulation driven by steady pressure are both found to be implausible, given current estimates of anatomical and fluid dynamic parameters.

Keywords: Brain clearance system; Bulk flow; Glymphatic system; Paravascular flow; Paravenous flow; Perivascular flow.

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Figures

Fig. 1
Fig. 1
Hypothetical perivascular and paravascular flow pathways in an artery. Paravascular flow moves inward to the brain tissue between astrocyte end feet and pia mater. Perivascular flow moves outward from the brain tissue in basement membranes between SMCs
Fig. 2
Fig. 2
Schematic of the arterial tree
Fig. 3
Fig. 3
Cumulative periarterial resistance
Fig. 4
Fig. 4
Cumulative paraarterial resistance
Fig. 5
Fig. 5
Cumulative paravenous resistance
See this image and copyright information in PMC

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