Nitric oxide (NO) is a molecule that is embedded into almost every function of the body and has been discovered for the continued functions in the bioregulatory system. Although it is a highly reactive gas, it has a lifespan of maximum five seconds. It plays a role in many physiological processes that include blood clotting, blood pressure control, inhibition of blood platelet aggregation , nerve transmission, penile erection, uterine contraction, and also as part of the immune system as an agent against tumor cells and internal pathogens. It is formed biosynthetically in the tissues of mammals in the endothelial lining of the blood vessels, cells of the nervous system, and in the immune system.

NO is synthesized by an enzyme called the Nitric Oxide Synthase (NOS). There are three forms of this synthase: epithelial NOS (eNOS), neuronal NOS (nNOS), and inducible NOS (iNOS). Chemically, NOS catalyzes the two-step reaction of L-arginine to N-hyroxy-L-arginine to Citrulline and NO comes out as a by-product in that reaction.

In the endothelial cells of the blood vessels, the form of eNOS is embedded and is activated by an influx of Ca²⁺ that occurs after binding of the calmodulin. The enzyme produces NO gas and it diffuses out of the blood vessels into the smooth muscle cells. There, it binds with the heme group to form cyclic guanosine monophosphate that causes a decrease in the levels of Ca²⁺. Because Ca²⁺ is needed for muscle contraction, the lowering of the Ca²⁺ concentration results in muscle relaxation, dilation of the blood vessels, and ultimately, a drop in the blood pressure that increases blood flow to the heart. In order to sustain normal blood pressure, a constant supply of NO is needed to be synthesized or it could result in hypertension.

In the central nervous system, NO plays the role of messenger and neurotransmitter between the synapses of neurons. Once the pre-synaptic neuron releases the neurotransmitter, glutamate, it diffuses across the synaptic cleft and interacts with receptors at the postsynaptic neuron. They go to calcium channels which cause an influx of Ca²⁺ and after binding with calmodulin, it activates NO synthesis. The NO gas diffuses out of the postsynaptic neuron to interact with the next pre-synaptic neuron. It also stimulates glutamate formation in the nerve cells.

NO is also important in the immune system as a cytostatic agent that helps phagocytes engulf particles from their surroundings by endocytosis. Macrophages contain iNOS which is inducible and the only one that is not Ca2+ dependent. NO is produced constantly as long as the target cells is there. Because it always contains tightly bound calmodulin, it takes place of needing to have Ca²⁺ influx in order to activate it. The NO formed can also diffuse into tumor cells and disrupt DNA production in the nucleus, the TCA cycle, and the electron transport chain.