The blood-brain barrier (BBB) is a collection of specific cells and proteins that control the movement of molecules from the blood to the central nervous system (CNS). The blood vessel endothelial cells of the BBB are cemented together by protein structures called tight junctions, preventing diffusion of most molecules between cells. These endothelial cells display transporters, receptors, and that facilitate selective transport of vital nutrients into the CNS. They also possess efflux pumps that expel most small, amphiphilic molecules that are soluble in the blood and in cell lipid membranes. Pericytes and astrocyte pseudopods serve as an additional physical barrier between the blood vessel and brain tissue.