*1.1.1 The blood-brain barrier (BBB)*

It is a tight physical junction present at the interface of CNS and blood circulation. It consists of endothelial cells that do not have fenestrations and thus restrict the influx of ions and other solutes into the brain from surrounding blood capillaries. Astrocytes and pericytes surround endothelial cells and thus make it almost an impermeable barrier. BBB allows paracellular transport of small lipophilic compounds (<400 Da) via passive diffusion. This barrier also offers active transport of some hydrophilic compounds by the means of transport proteins (e.g., P-glycoprotein) present at the junction. The transcellular pathway that is used by some compounds to enter the brain includes different mechanisms such as passive diffusion, specific transporters, and transcytosis [4].
