Each day, about 9 liters of fluid enters the duodenum. Most chemical digestion takes place in the duodenum by chemicals secreted by the liver, pancreas and small intestine. The other two sections of the small intestine, the jejunum and the ileum, absorb food molecules by way of the villi directly into the blood stream.
Histamine is synthesised in the brain in the hypothalamus, in a specific area known as the tuberomamillary nucleus (TMN), where it acts as a signalling messenger (or neurotransmitter). The hypothalamus is the area of the brain that overseas the metabolic processes of the body found in the central nervous system.
Both the brain and the gut produce neurotransmitters. The brain produces around 100 different neurotransmitters, and the gut produces 40 of the same ones. Serotonin, the feel good neurotransmitter, is found in both, but 90-95% of its production is in the gut, not the brain. Dopamine, another major neurotransmitter, has 50% of its production in the gut and the other 50% in the brain.