Why can’t you just inject serotonin to feel happy?
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Your brain is actually somewhat separated from your bloodstream. We call the border between the two the blood-brain barrier. Think of it like a fine mesh strainer.
Some molecules are small enough to fit through this barrier, but others are not. Serotonin is too big to fit through, so injecting it into your blood won’t do anything for your brain. So the only way to increase serotonin in the brain is to either give the brain more of the ingredients it needs to make more, or to use drugs that can cross the blood-brain barrier to trick the brain into making more of it, or keeping it around for longer (that’s how SSRIs work).
ELI15: the blood brain barrier is a lot more complicated than just a fine strainer. It’s actually selective (allowing some ways for certain molecules that would otherwise be too large to get in) and its permeability (how much, and what kind of stuff it lets in) varies across different regions of the brain.
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