Image caption : Nitric Oxide Molecule: Medical visualization of a nitric oxide molecule. Nitric oxide is an important signaling molecule, and is so small, that unlike other neurotransmitters, it can diffuse quite far and act on several nearby neurons, not just those within the synapse connection. Nitric oxide causes relaxation of smooth muscle and increases blood flow, thereby playing an important role in digestion as well as the development and maintenance of erection. Nitric oxide doesn't affect blood flow directly, but instead initiates a series of chemical reactions that allow the blood vessels to dilate. The most important milestones along this chemical pathway are two proteins known as cyclic GMP (cGMP) and PDE 5. cGMP is the protein that signals the muscles in the blood vessel walls to relax and fill with blood. PDE 5 breaks down cGMP to prevent overstimulation of the blood vessels and excessive relaxation. If not enough cGMP is made or if cGMP is broken down too quickly, the blood vessels won't remain relaxed long enough to fill with sufficient blood to allow the penis to become erect. Current erectile dysfunction drugs work by blocking the effect of PDE 5 and allowing cGMP to remain active longer. These drugs can have beneficial sexual effects for men-and, some studies have found, for women. They make it easier for men to both achieve and maintain an erection, and for women to experience the vaginal lubrication and the enhanced clitoral sensitivity associated with normal sexual arousal.
Nitric oxide is a toxic pollutant that we most often smell in car exhaust fumes, but it is critical to our cardiovascular health, brain function and immune system. It transmits signals in most living creatures to help dilate arteries, activate nerve cells and increase the number of white blood cells to kill invading bacteria and parasites. Interestingly, Tibetans living at high elevations—where there's less oxygen in the air—have much more nitric oxide in their blood than do people living closer to sea level. Scientists believe that this extra nitric oxide dilates the Tibetans' blood vessels, increasing blood flow to ensure that sufficient oxygen gets to their tissues. The discovery of nitric oxide's role in the body, particularly the cardiovascular system, netted a Nobel Prize in 1998.
National Institute of General Medical Sciences / NIH
Nitric oxide (nitrogen oxide, nitrogen monoxide) is a molecular, chemical compound with chemical formula of ·NO. One of several oxides of nitrogen, it is a colorless gas under standard conditions. Nitric oxide is a free radical—i.e., its bonding structure includes an unpaired electron, represented by the dot (·) on the nitrogen atom— and it is in the class of heteronuclear diatomic molecules that are of historic theoretical interest (for the insights they gave in formulating early modern theories of bonding). It is a practically important intermediate in the chemical industry. In addition, some is unavoidably produced during combustion of fossil fuels in power plants and automobile engines, with excess being created when there is present more air, or higher temperatures, than needed for efficient and complete combustion of the fuel. It is also produced naturally by the extremely high air temperatures produced along the path of lightning in thunderstorms.
In mammals including humans, ·NO is an important cellular signaling molecule involved in many physiological and pathological processes. It is a powerful vasodilator with a short half-life of a few seconds in the blood. Long-known pharmaceuticals such as nitroglycerine and amyl nitrite were found to be precursors to nitric oxide more than a century after their first use in medicine. Low levels of nitric oxide production are important in protecting organs such as the liver from ischemic damage. Nitric oxide production is associated with nonalcoholic fatty liver disease (NAFLD) and is essential for hepatic lipid metabolism under starvation.
As a consequence of its importance in neuroscience, physiology, and immunology, ·NO was proclaimed "Molecule of the Year" in 1992. Research into its function led to the 1998 Nobel Prize for discovering the role of nitric oxide as a cardiovascular signalling molecule. Nitric oxide should not be confused with nitrous oxide (N2O), an anaesthetic, or with nitrogen dioxide (NO2), a brown toxic gas and a major air pollutant, the latter being a product to which nitric oxide is rapidly oxidised in air.
It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.
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