more than 95 per cent of the serotonin in the body is actually used by the gastrointestinal tract. Serotonin released in the gut controls peristalsis, the involuntary muscle movement that pushes along the contents of the digestive system.
https://psyche.co/ideas/to-grasp-how-serotonin-works-on-the-brain-look-to-the-gut
@johnverdon I wonder if that's why so many people who take psilocybin mushrooms report that they can "feel" where the mushrooms are in their gut. Maybe it is not entirely a hallucination.