A network of capillaries supply brain cells with nutrients. Tight seals in their walls keep blood toxins—and many beneficial drugs—out of the brain. 
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From: Bridging the Blood-Brain Barrier: New Methods Improve the Odds of Getting Drugs to the Brain Cells That Need Them Ferber D PLoS Biology Vol. 5, No. 6, e169 doi:10.1371/journal.pbio.0050169
Dan Ferber
CC BY 2.5
<a rel="nofollow" class="external free" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1892841/">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1892841/</a>