BackgroundTransport 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.MethodsThis article evaluates the plausibility of steady, pressure-driven flow in these channels with one-dimensional branching models.ResultsAccording 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.ConclusionsPeriarterial flow and glymphatic circulation driven by steady pressure are both found to be implausible, given current estimates of anatomical and fluid dynamic parameters.

The unfolding COVID-19 pandemic has exposed the vulnerability of the Pacific food system to externalities and has had farreaching impacts, despite the small number of COVID-19 cases recorded thus far. Measures adopted to mitigate risk from the pandemic have had severe impacts on tourism, remittances, and international trade, among other aspects of the political economy of the region, and are thus impacting on food systems, food security and livelihoods. Of particular concern will be the interplay between loss of incomes and the availability and affordability of local and imported foods. In this paper, we examine some of the key pathways of impact on food systems, and identify opportunities to strengthen Pacific food systems during these challenging times. The great diversity among Pacific Island Countries and Territories in their economies, societies, and agricultural potential will be an important guide to planning interventions and developing scenarios of alternative futures. Bolstering regional production and intraregional trade in a currently import-dependent region could strengthen the regional economy, and provide the health benefits of consuming locally produced and harvested fresh foodsas well as decreasing reliance on global supply chains. However, significant production, processing, and storage challenges remain and would need to be consistently overcome to influence a move away from shelf-stable foods, particularly during periods when human movement is restricted and during post-disaster recovery.

No abstract

The prediction of liquefaction and resulting displacements is a major concern for earth structures located in regions of moderate to high seismicity. Conventional procedures used to assess liquefaction commonly predict the triggering of liquefaction to depths of 50 m or more. Remediation to prevent or curtail liquefaction at these depths can be very expensive. Field experience during past earthquakes indicates that liquefaction has mainly occurred at depths less than about 15 m, and some recent dynamic centrifuge model testing initially appeared to confirm a depth or confining-stress limitation on the occurrence of liquefaction. Such a limitation on liquefaction could greatly reduce remediation costs. In this paper an effective stress numerical modeling procedure is used to assess these centrifuge tests. The results indicate that a lack of complete saturation and densification at depth arising from the application of the high-acceleration field are largely responsible for the apparent limitation on liquefaction at depth observed in some centrifuge tests.Key words: liquefaction, dynamic centrifuge modeling, numerical modeling, depth limitation.