There are two ways to produce nitric oxide (NO) in our bodies.
In one way, NO is by the reduction of dietary nitrates to nitrite by bacteria and gastric acid to then be converted into NO. Another way to naturally create NO is by enzymatic reactions (nitric oxide synthase (NOS) enzyme) which converts L-arginine to NO.
nitrate-nitrite-nitric oxide pathway
The main source of nitrate is diet (especially leafy green vegetables).
Once a drink or a meal rich in nitrate is in contact with your oral cavity, commensal anaerobic bacteria reduce salivary nitrate to nitrite by the action of nitrate reductases enzyme. This reaction does not require oxygen).
In the gut, the gastric acidic milieu convert salivary nitrite to NO.
Nitrate and remaining nitrite is absorbed from the intestine into the circulation and can convert to bioactive NO in blood.
An active uptake of nitrate from the blood occurs in the salivary glands to start the process all over again.
L-arginine-nitric oxide pathway
After supplementation or dietary intake, the amino acid citrulline is converted into arginine. During a cascade of enzymatic reactions, arginine is converted into nitric oxide, urea and ornithine.
Then ornithine can be converted into citrulline (to start the process all over again) and proline, a key amino acid needed for the biosynthesis of collagen.
Interestingly, the L‑arginine–NO pathway is oxygen dependent, whereas the nitrate–nitrite–NO pathway is gradually activated as oxygen tensions falls.