Nitric oxide (NO), also called nitrogen monoxide, is an important natural component of the biochemistry of mammals, including humans. In normal, healthy mammalian biochemistry, nitric oxide's primary function is as a chemical messenger. Healthy levels of nitric oxide are important to the nervous, circulatory, and immune system. Although the effects of nitric oxide are essential for human life, excessive levels, whether due to endogenous production or environmental exposure, can be toxic.
When the body needs to increase the flow of blood through the circulatory system, the inner lining of the blood vessels releases nitric oxide into the surrounding smooth muscle tissue in the blood vessel walls. This causes the smooth muscle to relax and induce vasodilation, or widening of the blood vessels. This helps to regulate blood flow and blood pressure and allows people to adapt to low-oxygen conditions such as high altitudes. It also causes erection of the penis during sexual arousal by increasing blood flow to the genitals in response to sexual stimulation. In addition to affecting smooth muscle, it can also affect cardiac muscle tissue of the heart to reduce heart rate.
In the nervous system, nitric oxide functions as a neurotransmitter to carry messages between neurons and is part of the process of memory formation and learning. It is also produced in the stomach, through reactions between stomach acid and nitrate in swallowed saliva, to maintain blood flow to the stomach and sterilize food as it is digested. In the immune system, a type of white blood cell called macrophages produce nitric oxide to poison and kill bacteria.
The effects of nitric oxide can be damaging to health at excessive levels. It can aggravate sepsis, as the release of large amounts of the chemical by the immune system in an attempt to kill infectious bacteria can cause the amount of it to rise to cytotoxic levels, damaging the body's own cells. As one of the effects of nitric oxide is vasodilation, excess levels can cause blood pressure to drop to dangerously low levels, causing the organism to go into shock.
Nitric oxide can also be harmful when it is absorbed into the body from the outside environment, and it is a common industrial pollutant. Contact causes irritation of the eyes, skin, and mucous membranes. Inhalation can interfere with breathing by causing coughing and shortness of breath as well as nausea and painful burning in the chest and throat. In addition to initial breathing difficulties, the effects of nitric oxide when inhaled can also include potentially dangerous conditions such as pulmonary edema, an accumulation of excess fluid in the lungs, and the blood disorder methemoglobinemia.