Figure 2 : The entero-salivary circulation of nitrate in humans.

From: The nitrate–nitrite–nitric oxide pathway in physiology and therapeutics

Figure 2

Ingested inorganic nitrate from dietary sources is rapidly absorbed in the small intestine. Although much of the circulating nitrate is eventually excreted in the urine, up to 25% is actively extracted by the salivary glands and concentrated in saliva. In the mouth, commensal facultative anaerobic bacteria effectively reduce nitrate to nitrite by the action of nitrate reductase enzymes. Nitrate reduction to nitrite requires the presence of these bacteria, as mammalian cells cannot effectively metabolize this anion. In the acidic stomach, nitrite is spontaneously decomposed to form nitric oxide (NO) and other bioactive nitrogen oxides, which regulate important physiological functions. Nitrate and remaining nitrite is absorbed from the intestine into the circulation and can convert to bioactive NO in blood and tissues under physiological hypoxia.