The term refers to the fact that very few substances have the ability to get from the bloodstream and into the brain tissue as easily as they can get into most of the body's other tissue areas. An important exception is glucose (glucose), which passes freely into the brain tissue and is responsible for the energy supply.
Some kind of wall or barrier obstructs the free passage.
The barrier function is considered to be due to special properties of:
1/ Brain capillary wall cells (endodothel cells)
2/ astrocyte protrusions forming a more or less coherent layer on the outside of the cerebral capillaries, and
3/ pericytes outside the endothel cells (Nature, vol.468, Nov.2010).