Salivary glands are organs in the human head, which secrete a chemical substance called saliva. This substance is a fluid that contains water, electrolytes, enzymes and mucus. Saliva flows out from the glands through a system of ducts. The produced saliva keeps the mouth moist, and its enzymes initiate digestion of food.
The saliva that comes out from the glands can either be serous (watery fluid), mucous (thick and slippery fluid), or seromucous (a mix of both). In serous saliva, the main type of protein secreted is alpha-amylase, which is an enzyme that breaks down starch from food into smaller particles. In mucous saliva the main protein secreted is mucin, which acts as lubricant.
There are three major salivary glands in humans, each present in pairs. Parotid glands produce a serous secretion. Submandibular glands produce a mix of serous and mucous secretion. Sublingual glands produce mainly mucous saliva.
Humans have also hundreds of minor salivary glands located all over the mouth, and their secretion is mainly mucous.
Each day, we produce between 0.5 and 1.5 litres of saliva. This is important in the initial steps of digestion, because saliva moistens and breaks down foods. This initial process enables mastication and swallowing. The cell unit of salivary gland is called acinus (plural acini).
Salivary gland functions
The saliva secreted by salivary glands has many benefits for the mouth and a person’s health in general:
- Digestion, as the enzyme amylase contained in saliva breaks down carbohydrates of foods into maltose and dextrin (chains of glucose), which initiates the process of digestion. This creates the food bolus, which is the finely packed ball of food that ends up inside the mouth, and which simplifies its safe passage down the oesophagus to the stomach.
- Protection, the mucus lubricates and protects teeth and all other parts of the mouth.
- Antimicrobial, as saliva has lactoferrin, histatins, and Immunoglobulin A that break down bacteria and microorganisms of dental plaque.
- Taste, pieces of foods can be dissolved by saliva before they enter the taste buds.
- Cleaning, as saliva washes the mouth after a meal and dissolves the remaining foods.
- Maintenance of tooth integrity, following demineralization of enamel from acids contained in food breakdown, saliva helps the remineralisation of teeth with calcium and phosphate minerals.
- Tissue repair, as saliva decreases clotting time and increases wound contraction when there are injuries of parts of the mouth other than teeth.
Salivary gland disorders
Many diseases can affect salivary glands, on various levels whether they are infective, dysfunctional, autoimmune or cancerous. Here are just a few of them:
- Xerostomia – or dry mouth, occurs when salivary glands produce less saliva; this can be secondary to many other diseases, or a side effect of several medications. This condition increases the risk of having tooth decay.
- Hypersalivation – or the excessive production of saliva, can follow some conditions and diseases. It can also be a side effect of several medications, or the result of exposure to a few substances such as mercury, copper, arsenic.
- Mumps – a highly contagious viral infection of the parotid glands.
- Mucocele – is a harmless cyst-like lesion that is expressed by swelling or a bump, and is caused by a ruptured salivary gland duct, mostly from local trauma. Mucoceles are mostly located on the cheeks, the tongue, or the inner surface of the lower lip.
- Ranula – is a mucocele found on the floor of the mouth (below the tongue).
- Nicotinic stomatitis – irritation of the palate caused by the heat of smoking, or consuming hot foods or liquids. The continuous heat causes inflammation of the ducts of minor salivary glands.
- Sjögren’s syndrome – an autoimmune disease that causes dry mouth and dry eyes. Scientists do still not understand its detailed cause.
- Frey’s syndrome – a neurological disorder that produces damage to the parotid gland.
- Sialolithiasis – is the formation of salivary stones, within a salivary gland or in its duct, and that can have many causes.
- Salivary gland tumours can affect both major and minor glands, and have different levels of aggressiveness.
Parotid glands are the largest of the major salivary glands. They are located behind the last molar teeth, more precisely around the mandibular ramus (the back perpendicular extremity of the mandible on either side). They produce 20 % of the total saliva in the mouth, and the secretion is made through the parotid duct (also called Stensen duct), of which the exit is around the second upper molar.
The saliva that is produced by parotid glands is of the serous type, which means it contains the enzyme alpha-amylase that breaks down carbohydrates into glucose and maltose.
This pair of major salivary glands, also sometimes called the submaxillary glands, is located beneath and to the back of the lower jaw. Even though they are smaller than the parotid glands, the submandibular glands produce about 65 to 70 % of the total saliva in the mouth.
The secretion produced is a mixture of serous and mucous fluid. Saliva comes out via the submandibular duct (or Wharton duct), of which the exit is below the tongue, on the lateral front side.
These major salivary glands are smaller and are located in front of the submandibular glands, under the tongue. They produce about 5% of the saliva of the mouth, and the secretion is mostly mucous, but seldom serous.
Each sublingual gland doesn’t have only one excretory duct, but a series of 8 to 20 ducts called the Rivinus ducts, that come out on the front side of the floor of the mouth, under the tongue.
Minor salivary glands
Those much smaller salivary glands are 1 to 2 mm in diameter and their secretion is mainly mucous. Most of those glands are too small to be seen with the naked eye. There are almost 1000 of them in and around the mouth, and their main function is coating the mouth with saliva for lubrication.
Minor salivary glands are located on the mucosa tissue of most parts of the mouth, including the cheeks, the lips, the palate, the floor of the mouth (under the tongue), and also the nose, the sinuses, the pharynx, the larynx and the trachea.
Von Ebner’s glands
Those salivary glands are located on the backside of the tongue in the circumvallate papillae area. The saliva secreted is purely serous that initiates breakdown of the fats in ingested foods. They also secrete digestive enzymes that help the perception of taste.
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