Serotonin is found all over the body: Blood, digestive system, skin, bones, lungs.

It doesn't migrate all over, though. Serotonin in the gut doesn't go into your brain like many people assume.

There has been research suggesting that the vagus nerve might carry serotonin from the gut to the brain:

https://www.psychologytoday.com/us/blog/the-athletes-way/201...

I'd note that this is a five year old article and I haven't heard of any followups. And the article itself may be misinterpreting the underlying actual research, which suggests that it's more about SSRIs:

https://www.nature.com/articles/s41598-019-50807-8

Still, I thought it was worth mentioning.