What exactly is the blood-brain barrier?

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What exactly is the blood-brain barrier?

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Think of your brain as the city. Powerful and complicated. Activity everywhere doing everything imaginable.

And think of your body as the country side, where all of the resources and factories are.

How does what needs to move flow between the city and the countyside? The network of roads. The vascular system.

Except right at the entrance to the city. There’s a customs office. Passport control. The blood-brain barrier.

It blocks what shouldn’t go onto the brain from entering. It allows what should to pass.

As a concept, it’s the idea that your body seriously restricts what can get from your bloodstream to your brain and spinal cord (which are bathed in cerebrospinal fluid.) The brain is delicate and needs to be protected from stuff that will mess with its function, and in particular from infection. Infection in the brain is very bad news, as both the infection and your immune response can damage things.

Practically, the blood-brain barrier is mostly made up of tight junctions between cells that limit what gets through, plus pumps that get rid of many things that do get in.

(As you may have gathered from the existence of medications and street drugs that do affect the brain, it’s still not 100%.)

The brain is full of tiny blood vessels (capillaries) which provide oxygen and nutrients. But the capillary walls are composed of tightly connected cells, which prevent most large molecules from getting out of the blood and into the neural space. This is normally a good thing.

However, some medical therapies involve placing large molecules (antibodies, for example) next to neurons. Glioblastoma, a type of cancer, is treated this way.

Technologies are being developed, like focused ultrasound, to open up the capillary wall cell gaps, to allow drugs to pass through.

It is a wall of tiny blood vessels that defends the brain and protects central nervous system homeostasis. Homeostasis basically means balance. I like to think of it as home-state.

Blood circulates through different vessels in the body. Along the way it picks up different passengers. These passengers are checked at the border and either admitted or denied entry to the brain.

In the simplest terms, it is just a barrier. We use the word barrier not blockade because a barrier can be selective as is the case with BBB. Why do certain drugs reach our brains and some don’t? Up to BBB. Why some infections spread to the brain and some don’t? BBB.