The concept of a blood-brain barrier includes overlapping control mechanisms which work together to produce a constant microenvironment. Most important is the barrier to macromolecule passage located in cerebral capillary endothelium. Many drugs bound to proteins cannot pass this obstruction. Also important are membrane properties shared by all living cells. Lipid soluble molecules pass cell membranes easily; water soluble and ionized molecules do not. Other components include selective ion regulation, facilitated sugar transport, and resorption by the choroid plexus. The bulk flow of cerebrospinal fluid washes all solutes, and even particulate debris, from the system.