nitric oxide
a compound present in numerous body tissues, where it has a variety of functions. It is synthesized in the body by the enzyme nitric oxide synthase from arginine, nicotinamide adenine dinucleotide phosphate (NADPH), and oxygen. In the brain and other parts of the central nervous system, nitric oxide functions as a neurotransmitter, or as an agent that influences neurotransmitters. In peripheral tissues, it is involved in the relaxation of smooth muscle and thus acts as a vasodilator, as a bronchodilator, and as a relaxant of smooth muscle in the penis and clitoris, being involved in erection and other components of the sexual response.