To better understand xerostomia or dry mouth, you should know what salivary glands are and what their function is.
Saliva is a watery substance secreted by the salivary glands which are part of the upper digestive system. The salivary glands are exocrine glands, that is, they secrete substances outside the body or within a body cavity, in this case the oral cavity.
The function of the salivary glands is simply to produce and discharge saliva in the oral cavity. The salivary glands secrete between 1 to 1.5 liters of saliva in a 24 hour period.
As explained in this article, over 99% of saliva is water and the remaining 1% is made of proteins, electrolytes, enzymes, amino acids and other substances essential for good oral health, chewing, and food digestion.
Saliva keeps the mouth moist and clean, moistens food to facilitate chewing and swallowing, and contributes to the digestive process.
The salivary glands are located around the mouth. There are two types of salivary glands according to their size and functional importance.
Minor salivary glands
The mucosa of the mouth, lips, tongue, and palate is filled with many small salivary glands that moisten these surfaces. Between 800 and 1000 minor salivary glands are located throughout the mouth and they secrete saliva into the mouth directly.
Major salivary glands
Most of the saliva in the mouth is secreted by three pairs of glands symmetrically located on both sides of the jaw. These major salivary glands include the parotid, submandibular, and sublingual glands.
These glands are the largest salivary glands and the main producers of saliva. They are located outside the mouth and secrete saliva through ducts.
Parotid glands are the largest of the major salivary glands. Each of these glands measures 6 cm long and 3-4 cm wide and can weigh up to 30 grams.
They are located on both sides of the face below the zygomatic arch and behind the jaw.
Approximately 20% of saliva is produced by parotid glands. This saliva is serous, that is, more liquid and fluid.
Certain diseases or the intake of substances like alcohol or certain drugs can affect the sympathetic nervous system (SNS), producing vasoconstriction of the parotid glands and reducing the secretion of saliva. This is the main reason why alcohol abuse is directly related to the lack of saliva.
These little glands are the size of a walnut and are located beneath the lower jaw, on the back of the floor of the mouth (outside of the oral cavity).
Despite their small size, approximately 65-70% of saliva entering the oral cavity comes from these glands. Saliva secreted by the submandibular glands is more viscous than that secreted by the parotid, but it is still quite fluid.
Sublingual glands are the smallest of the major salivary glands. They are located on the floor of the mouth under the tongue. Approximately 5% of the saliva comes from these glands. This saliva is predominantly mucous, has a viscous texture, and flows into the mouth through the sublingual ducts.
Sometimes, sialoliths (stones) form in the salivary gland ducts, blocking the flow of saliva, and causing pain and swelling in the affected gland. Salivary stones most often affect the submandibular glands but may also affect the parotid.