A molecule called serotonin carries signals between nerve cells. Tryptophan is an amino acid that is converted into the neurotransmitter serotonin. The neurological system, mostly the brain, but also specific cells in the bronchial tubes (lungs) and gastrointestinal tract, produces it as needed. The platelets contain more than 90% of the serotonin in the blood. Serotonin participates in the wake-sleep cycle, regulates mood    and helps transfer nerve impulses and constrict blood vessels. The liver converts serotonin into its many metabolites, the majority of which are excreted in the urine as 5-HIAA (5-hydroxyindoleacetic acid).