Nitric oxide ( nitrogen oxide [3] or nitrogen monoxide), a chemical compound, is a colorless gas under standard conditions, and is one of several oxides of nitrogen. Nitric oxide is a free radical, i.e., its bonding structure includes an unpaired electron, [4] denoted by a dot in its chemical formula, · N O. Nitric oxide is also a heteronuclear diatomic molecule, a historic class that drew researches which spawned early modern theories of chemical bonding.
A particularly important intermediate in the chemical industry, nitric oxide forms in mixtures of nitrogen and oxygen at high temperatures, as found in combustion systems such as internal-combustion piston engines, gas turbines, furnaces, and boilers. Likewise, nitric oxide forms naturally along the pathway of lightning in thunderstorms.
In mammals, including humans, nitric oxide is a signaling molecule in many physiological and pathological processes. [5]Nitric oxide dilates blood vessels, raising blood supply and lowering blood pressure. [6] Conversely, it helps protect tissues from damage due to low blood supply. [6] Also a neurotransmitter, nitric oxide acts in the nitrergic neurons active on smooth muscle, abundant in the gastrointestinal tract and erectile tissue. [7] Nitric oxide is also an immune effector, mediating diverse activities of immune cells. [8]
In 1992, for its importance in neuroscience, physiology, and immunology, nitric oxide was proclaimed the " Molecule of the Year". [9] The 1998 Nobel Prize in Physiology or Medicine was awarded for discovering nitric oxide's role as a cardiovascular signalling molecule. Over a century after their medical introduction, the organic-nitrate pharmaceuticals, including nitroglycerine and amyl nitrite, were identified as precursors of nitric oxide.
Nitric oxide should not be confused with nitrous oxide (N2O), an anesthetic, or with nitrogen dioxide (NO2), a brown toxic gas and a major air pollutant, that nitric oxide is rapidly oxidized to within air. In the United States, nitric oxide is classified as an extremely hazardous substance, [10] and is subject to strict reporting requirements imposed on facilities that produce, store, or use it in significant quantities. [11]
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