Various transport mechanism allow molecules to be moved across the BBB. (A) Receptor-mediated endocytosis involves the binding of a molecule to a receptor, triggering endocytosis and transport across the endothelium. (B) Adsorptive endocytosis is the transport across the endothelium by membrane encapsulation of a molecule. (C,D) There are multiple transporters expressed on the vascular and parenchymal membranes of the endothelial cells that facilitate the influx and efflux of molecules. (E) Small lipophilic compounds and molecules can passively diffuse across the membranes of the endothelial cell to access the CNS, although efflux transporters shown in mechanism (D) can remove these compounds back into the systemic circulation. (F) Paracellular transport between cells is limited due to the presence of junctional complexes between neighbouring cells. Scaffolding proteins consisting of complexes of ZO-1 and 2, along with catenin, link the ECM to the intracellular actin cytoskeleton through junctional complexes: adherens junctions, gap junctions and tight junctions.