The brain is permeated by a vast network of tiny blood vessels called capillaries- so tiny and thin that blood cells have to pass through in single file. In the brain alone there are enough capillaries that if you laid them all out end to end they would stretch from Tucson to Tijuana.
These capillaries are surrounded by a single layer of cells.
That layer of cells forms a barrier between the capillaries and the cells and fluid of the brain.
These barrier-forming cells are called "endothelial cells". You can think of "endothelial" as a synonym for "lining" or even just "barrier".
When we use the phrase Blood Brain Barrier, (which for obvious reasons we'll refer to as BBB from here on out!), we're talking about all of these endothelial ("barrier") cells collectively.
Function of the BBB
The cell membranes of the BBB contain transport proteins. If the brain is a nightclub, the transport proteins are bouncers. They decide who gets in, and who gets kicked out.
On this website we'll be introducing you to the most important transport proteins- OATP, MDR1, and MDR2.
Don't let all the acronyms intimidate you- read carefully and you'll be fine. If the nightclub/bouncer analogy doesn't work for you, you could also think of them as little vacuum pumps and blowers. An extremely detailed view of their actual mechanisms is beyond current knowledge.
Importance of the BBB
Without the BBB, undesirable molecules could freely diffuse from the capillaries to the fluid that surrounds the brain cells.
These undesirable molecules include:
TOXINS- poisons taken in from the environment.
IONS- that might upset the delicate electrochemical gradients of the cerebral fluid.
ACIDS and BASES- that might upset the cerebral fluid's delicate pH balance.
These substance often diffuse into the cerebral fluid anyway, in spite of the BBB- but the BBB is not just a static barrier- it incorporates mechanisms to pump such substances back out once they've gotten in.
The BBB also incorporates mechanisms to shepherd or transport certain substances *in*:
GLUCOSE- the universal energy source, which is needed by every cell in your body...
OXYGEN- electron acceptor par excellence, do without it and die...
CERTAIN IONS- cations, anions, Chlorine, Calcium, etc.. involved in nervous system transmission...Â
Finally, the BBB also lets various substances in because it either can't pump them out or its mechanisms can't tell the difference between them and one of the substances that it naturally tranports across:
ETHANOL- enters by the same pathways that let glucose in, which is why drinking alcohol affects the brain...
THC, NICOTINE, CAFFEINE...
Obstacles For Scientists
The BBB evolved as an extremely tight barrier to protect the brain from potentially toxic compounds. Though it is an indispensable part of the Central Nervous System, its tightness makes delivering therapeutic drugs to the brain very difficult.