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Mimicking microcapillaries

Experiments reveal why the flow rate of red blood cells in the narrowest blood vessels is lower than expected.
Thanks to microscopes and high-speed video cameras, it's possible to follow the flow of red blood cells (RBCs) through the 10-μm-diameter capillaries that service mammalian cells. It's also possible to follow RBCs through 10-μm-diameter glass tubes—which is how researchers discovered that under outwardly similar conditions, the flow of RBCs is significantly slower in vivo than in vitro. The forest of molecules—mainly protein–sugar hybrids—that sprouts from the inner surface of capillaries and known collectively as the endothelial glycocalyx is suspected as the discrepancy’s principal source. That attribution is now on firmer experimental ground. Giovanna Tomaiuolo of the University of Naples in Italy and her colleagues have mimicked the effect of the endothelial glycocalyx by using methacrylate polymer chains. When RBCs are sent through polymer-lined microcapillaries, they conceivably encounter two sources of resistance. The first is the restriction of the channel's diameter by the polymer layer. The second source is the increased dissipation that results when RBCs pass by the chains and cause them to jiggle. It turned out that the second source is the more important: For the same surface density of chains, the reduction in RBC flow was independent of layer thickness. Tomaiuolo's results could help elucidate the pathology of diabetes, atherosclerosis, and other vascular diseases that entail alterations to the endothelial glycocalyx. (L. Lanotte et al., Biomicrofluidics 8, 014104, 2014.)—Charles Day

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