Salivary glands (overview)
The salivary glands are a group of exocrine glands located within the soft tissue of the head and neck near or within the oral cavity. They produce and excrete a substance called saliva, which is a mixture of water, electrolytes and enzymes.
Saliva contributes to digestion by moistening and lubricating food in the oral cavity and beginning the digestion process. All salivary glands excrete saliva in the oral cavity via their ducts.
The salivary glands belong to the accessory organs of the digestive system. These glands are classified regarding their size and duct types, and they can be subdivided into the following two main groups:
- Minor salivary glands
- Major salivary glands
Overall, the salivary glands are essential structures for digestion, maintaining oral hygiene, and speech.
Minor salivary glands
Each person has numerous minor salivary glands scattered throughout the oral cavity and oropharynx. They are embedded in the mucosa of mentioned structures and are only 0.04 - 0.2 inches (1 to 5 mm) large. Overall, the oral cavity contains about 600 - 1000 dispersed minor salivary glands. They are classified depending on their location, and the most common minor salivary gland types include the following glands:
- Labial glands - located in the upper and lower lips;
- Buccal glands - found within the cheeks;
- Palatine glands - situated in the hard and soft palates; the anterior aspect of the hard palate usually does not contain any of these glands;
- Lingual glands - found in the tongue.
Besides the mentioned glands, small amounts of minor salivary glands can also be seen within the anterior aspect of the floor of the oral cavity, tonsils, tonsillar pillars, uvula, supraglottis and paranasal sinuses. The minor salivary glands produce a relatively small amount of saliva (5 - 10%) compared to the major salivary glands. Their secreted saliva mainly helps to lubricate, moisten and protect the oral mucosa.
Most of the glands produce mucous saliva (palatine and some lingual). However, some are mixed (labial, buccal and some palatine) or even serous glands. Minor serous salivary glands are found only within the tongue and are called the lingual glands of von Ebner. They are positioned close to the foliate and circumvallate papillae. The lingual glands of von Ebner are essential glands, as they secrete saliva that modulates taste perception by providing an environment for the taste buds.
Recent studies have shown that there are also some minor salivary glands in the nasopharynx. These glands are called the tubarial glands.
Major salivary glands
The major salivary glands are a collection of exocrine tissue that secrete saliva through one central duct. Each person has three pairs of major salivary glands:
- Parotid gland (Read more!) - the largest salivary gland; located in front of the external ear; it secretes saliva through the parotid duct, which opens into the oral cavity opposite the maxillary second molar tooth;
- Submandibular gland (Read more!) - the second largest gland; positioned behind the mandible and under the lower jaw; it secretes saliva through the submandibular duct, which opens into the floor of the oral cavity near the frenulum of the tongue;
- Sublingual gland (Read more!) - the smallest gland; situated below the tongue; it mainly secretes saliva through the Bartholin's duct that also opens into the floor of the oral cavity.
Overall, the major salivary glands are the primary glands responsible for producing about 90 - 95% of saliva.
As mentioned above, the major and minor salivary glands produce a clear, colorless and a bit stretchy liquid called saliva. Its normal pH varies from 6.2 to 7.6. Saliva starts the chemical food procession and helps in digestion. Also, it lubricates food during swallowing, helps in bolus creation, and maintains oral health.
Saliva is composed of water, mucus, electrolytes, antibacterial substances and enzymes. However, it mainly contains water - around 99.5%. Three types of saliva are secreted from salivary glands - serous, mucous and seromucous (mixed).
All salivary glands produce about 0.5 - 1.5 liters of saliva per day. Some of the main functions of saliva include the following:
- Moistening the oral cavity - saliva helps to keep the oral cavity moist;
- Food lubrication - saliva lubricates the food bolus and makes it easy to swallow it;
- Regulation of food temperature - it helps to lower the temperature of ingested food;
- Initiation of the digestion process and food breakdown - saliva contains enzymes (such as amylase) that begin the chemical food procession and start to break down carbohydrates in the oral cavity;
- Buffering function - it regulates the balance between acids and alkalines and maintains the pH of the oral cavity in the neutral range;
- Teeth protection from bacterial decay - saliva helps to neutralize the acid produced by bacteria;
- Antimicrobial properties - it contains various antimicrobial compounds, such as lactoferrin and lysozymes, which help to protect the oral cavity against pathological agents;
- Immune properties - saliva contains immunoglobulins that also help to protect the oral cavity against infections;
- Taste perception - it helps dissolve the food molecules to allow the taste receptors to perceive the flavor of food.
The amount and composition of saliva can vary depending on several factors, such as time of day, diet, and medications.