Nitric oxide

Nitric oxide, known chemically as NO, is a dissolved gas that can be generated and used in the body. In the 1980s NO was discovered to have many vital roles in the human body, including acting as a unique kind of transmitter for the nervous system. The enzyme NO Synthase (NOS) synthesizes NO from the amino acid L-arginine.

NO has a number of roles in the respiratory system, including promoting vascular and bronchial dilation, and mucus secretion, and it is an important neurotransmitter for neurons in the bronchial wall. NO can also influence immune cells, and is a mediator of inflammatory phenomena within lung cells.

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Nitric oxide - Encyclopedia of Neuroscience

Nitric oxide and it's possible role in ME/CFS - Andrew Gladman