Blood brain barrier – this refers to the separation of circulating blood and its contents from the central nervous system’s cells and extracellular space. It is the result of tight junctions between the cells making up the walls of the blood system’s capillaries so that the only route available for substances seeking to move from the circulatory system to the central nervous system is through the endothelial cells that make up the capillary walls of the brain and spinal cord’s blood vessels. These cells of the barrier actively transport materials across this barrier into the central nervous system in a selective fashion. The net effect is that only certain substances, drugs, etc. can move from the blood stream int the brain and spinal cord.