William Wells et al.

Outside the body, NO is an unstable, potentially toxic gas that forms in lightning strikes and car exhaust. But as a messenger molecule inside the body it plays a crucial regulatory role. Every cell type and tissue sends and receives messages—telling muscle cells when to contract, for instance, or fat cells when to release their stores. Several message systems regulate our web of blood vessels so that they deliver oxygen-carrying blood to the tissues and organs that need it most while also keeping our blood pressure at an appropriate level. The various messengers selectively dilate or constrict blood vessels, diverting blood flow as the body requires—to the gastrointestinal tract after a meal, for example, or to the muscles of movement in an emergency.

Nitric oxide is at the center of the most important relaxation system, thus explaining why nitroglycerin helps angina patients. But NO’s importance does not stop with angina. Inhaled NO can help premature babies when the blood vessels in their lungs are not absorbing oxygen adequately. Local application of NO-related drugs may prevent cells from growing and blocking repaired arteries. Drugs that release NO at the site of an infection also may help immune cells kill invading pathogens and tumor cells.