The blood-brain barrier is pretty much what its name sounds like; a barrier that separates your blood from your brain. But what's the point? Doesn't our brain want the sweet sweet nutrients in our blood?
I like to think of your brain as a nightclub, let's call it Club Head. Inside it's popping, there's lights and electronic music and the mood is good. The circulatory system is like the street outside the nightclub, and just anybody can walk up to the club's door via your bloodstream.
Conditions in your blood change dramatically all the time, like after you eat or exercise, but things need to be stable in your brain to keep the party going. So Club Head has a bouncer to keep the riffraff out -- the blood-brain barrier.
The bouncer, in this case, are endothelial cells. These cells line the walls throughout your whole vascular system, but in your brain the gaps between them are extremely small, forming what are called tight junctions. These small gaps, along with a lack of pathways through the endothelial cells, keeps your blood and your brain separate.
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