Most biologically relevant gases are produced within the CNS. NO is synthesized in regulated ways by three forms of NO synthase (NOS): two constitutive, neuronal and endothelial NOS (nNOS and eNOS); and one inducible NOS (iNOS) (Paakkari and Lindsberg, 1995). NH3 is produced as an end product of amino acid degradation. Gases that were classically considered toxic, such as CO and H2S, have been found to be produced by the brain at subtoxic concentrations and to act as signaling molecules (Kajimura et al., 2010). Unlike all of the above, the blood is the only source of O2 for the brain. Even if the blood compartment is not considered part of the CNS, the brain is a highly vascularized organ, and changes in substances carried by the blood and alterations in cerebral blood flow (CBF) can impact brain function. Inhaled O2 is transported in the blood, mostly bound to the iron of the carrier protein hemoglobin, and released in brain capillaries where it diffuses into the brain parenchyma. CO2, which also plays a role in the regulation of O2 exchange, is produced as a by‐product of respiration and other metabolic reactions in the brain and other organs.
Gases are relatively short‐lived in comparison with other types of molecules. This is partly due to the multiplicity of gas scavengers and gas targets, and the fact that “free” movement of gases in and across the cellular milieu facilitates their fast encounter with these targets. The site of gas production, the location of the gas target, and the path of preferred or faster gas diffusion are therefore important considerations in the context of gas biology (Kajimura et al., 2010).