Serotonin is also a growth factor for some types of cells, which may give it a role in wound healing. In normal physiologic states, vasodilation occurs through the serotonin mediated release of nitric oxide from endothelial cells, and the inhibition of release of norepinephrine from adrenergic nerves. The vasoconstrictive property is mostly seen in pathologic states affecting the endothelium – such as atherosclerosis or chronic hypertension. In high concentrations, serotonin acts as a vasoconstrictor by contracting endothelial smooth muscle directly or by potentiating the effects of other vasoconstrictors (e.g. When the platelets bind to a clot, they release serotonin, where it can serve as a vasoconstrictor or a vasodilator while regulating hemostasis and blood clotting. There, it is actively taken up by blood platelets, which store it. Serotonin secreted from the enterochromaffin cells eventually finds its way out of tissues into the blood. The remainder is synthesized in serotonergic neurons of the CNS, where it has various functions, including the regulation of mood, appetite, and sleep. The serotonin is secreted luminally and basolaterally, which leads to increased serotonin uptake by circulating platelets and activation after stimulation, which gives increased stimulation of myenteric neurons and gastrointestinal motility.

About 8% is found in platelets and 1–2% in the CNS. Additionally, it is stored in blood platelets and is released during agitation and vasoconstriction, where it then acts as an agonist to other platelets. Serotonin is also produced in the central nervous system (CNS), specifically in the brainstem's raphe nuclei, the skin's Merkel cells, pulmonary neuroendocrine cells and the tongue's taste receptor cells. Approximately 90% of the serotonin the human body produces is in the gastrointestinal tract's enterochromaffin cells, where it regulates intestinal movements. Its biological function is complex and multifaceted, modulating mood, cognition, reward, learning, memory, and numerous physiological processes such as vomiting and vasoconstriction. Serotonin ( / ˌ s ɛr ə ˈ t oʊ n ɪ n, ˌ s ɪər ə-/) or 5-hydroxytryptamine (5-HT) is a monoamine neurotransmitter.

How many neurons in the human brain