In the environment, nitric oxide is a precursor of smog and acid rain. Nitric oxide in minute amounts serves as a source of energy in certain bacteria. In the body, it serves as a chemical messenger with a wide range of functions. It acts as a neurotransmitter and is necessary for penile erection. It affects blood pressure and is produced by macrophages in the immune system to help defend against infection and cancer. Despite its usefulness, nitric oxide can have a toxic effect on body cells and has been implicated in Huntington's disease and Alzheimer's disease.
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