Nitrate metabolism in the human body is primarily a multiorgan cycle, in which salivary glands are considered to be important organs. In normal people, salivary nitrate level is much higher than blood nitrate level, suggesting that the salivary glands do not simply excrete nitrates, but actively take up nitrates from the blood. So, why do the salivary glands actively take up nitrates and secrete them into the saliva? Of the four groups of salivary glands in the mouth, which one actively absorbs nitrates from blood and then secretes nitrates into the saliva? How are nitrates distributed in saliva, blood, and urine? How will the levels of nitrates change in saliva, blood, and urine when salivary gland function is impaired?