SALIVA
Saliva has several roles, including:
- Oral hygiene (saliva cleanses the mouth)
- Dilution and buffering of ingested materials
- Dissolution of foods, which is necessary for digestion and enhances taste
- Moistening of foods, which is necessary for bolus formation
- Initiation of carbohydrate digestion via salivary amylase
The importance of healthy salivary glands and ducts can be appreciated in conditions like Sjogren's Syndrome, an autoimmune disorder in which the mucosal tissues are damaged. Among other issues, patients experience excessive dry mouth, and fissures (cracks) can form in their tongues and cheeks.
Anatomy of Salivary Glands
The oral cavity, houses the tongue, and the pharynx, which we commonly refer to as the throat.
Minor, aka, intrinsic salivary glands are embedded within the oral submucosa, such as in the cheeks.
Most of our saliva is produced and delivered by the extrinsic salivary glands, which lie outside the oral cavity; these are paired mixed glands that connect to the mouth via ducts.
The parotid gland is a large gland anterior to the ear between the masseter muscle and the skin; this gland comprises mostly serous cells, so its saliva is a watery secretion with enzymes.
The submandibular gland, which lies along the medial aspect of the mandibular body (sub = under, mandible).
The sublingual gland, which lies anterior to the submandibular gland under the tongue (lingual = tongue). These glands comprise serous and mucus cells.
Combined, the intrinsic and extrinsic glands produce an average of 1500 mL of saliva per day.
We show a single salivary acinus and duct; in the tissues, the acini and ducts resemble bunches of grapes, and the small, individual ducts join together to drain into larger collective ducts.
Water, bicarbonate, potassium, sodium, and chloride are secreted by the acinar cells; because this is the primary site of secretion, we call the acinus the "secretory region" of the gland.
When saliva is initially formed in the acinus, it is isotonic – that is, its electrolyte profile is similar to plasma's.
However, as saliva travels through the duct, selective electrolyte secretion and absorption modify its composition – thus, we call the duct the modifying region.
Chloride and sodium are reabsorbed from the saliva, and bicarbonate and potassium are secreted into it. Thus, the final saliva product is hypotonic compared to plasma.
Saliva is primarily water; its osmolarity depends on the stimulus for salivation and which glands are most active. It is also generally acidic.
Compared to plasma, saliva has higher potassium and bicarbonate concentrations, and lower sodium and chloride concentrations.
Saliva also contains the digestive enzymes salivary amylase and lingual lipase; salivary amylase initiates carbohydrate digestion and lingual lipase initiates triglyceride digestion.
Salivary mucin helps with lubrication and bolus formation, and lysozymes and IgA antibodies protect the mouth from microorganisms.
As mentioned, the composition of saliva varies; a key determent of its osmolarity is salivary flow rate.
When flow rate is low, that is, saliva is moving relatively slowly through the ducts, there is more time for ductal modification of electrolyte concentrations – thus, the final saliva is more hypotonic than when it was produced in the acinus.
However, when flow rate is high, and saliva is moving relatively quickly through the ducts, there is less time for ductal modification of electrolyte concentrations – thus, the final saliva remains more isotonic.
Regulation of Salivary Flow
First, be aware that the minor, intrinsic glands secrete saliva nearly constantly to keep the mouth moist and protected.
The extrinsic glands are under autonomic control; both the parasympathetic and sympathetic arms of the ANS trigger saliva release, but parasympathetic stimulation is dominant.
Food ingestion (or even its sight or smell) triggers parasympathetic activation, which, via the facial and glossopharyngeal nerves, leads to output of watery, enzyme-rich saliva. Note that nausea is also a powerful stimulator of salivary release.
Parasympathetic stimulation is reduced by fear, sleep, and dehydration; this is why anticholinergic drugs, which block the activity of acetylcholine in parasympathetic pathways, often cause "dry mouth."
Sympathetic stimulation produces a thick, mucin-rich saliva.
Minor, aka, intrinsic salivary glands are embedded within the oral submucosa, such as in the cheeks.
Most of our saliva is produced and delivered by the extrinsic salivary glands, which lie outside the oral cavity; these are paired mixed glands that connect to the mouth via ducts.
The parotid gland is a large gland anterior to the ear between the masseter muscle and the skin; this gland comprises mostly serous cells, so its saliva is a watery secretion with enzymes.
The submandibular gland, which lies along the medial aspect of the mandibular body (sub = under, mandible).
The sublingual gland, which lies anterior to the submandibular gland under the tongue (lingual = tongue). These glands comprise serous and mucus cells.
Combined, the intrinsic and extrinsic glands produce an average of 1500 mL of saliva per day.