Secretion of Saliva

The body produces around 1.5 litres of saliva every day and it plays a vital role in lubricating food, digestion and protecting the oral environment.

This article shall consider the production of saliva, how it varies on consumption of food and the consequences of low secretion.

Function

Saliva has many important roles within the gastrointestinal system. Not only does it help to lubricate and wet food but it also begins the digestion of carbohydrates.

One of its most important roles is protection of the oral environment:

• Keeping mucosa moist
• Washing teeth
• Maintaining an alkaline environment
• Contains a high concentration of calcium

Constituents of Saliva

Saliva is made up from several components in varying concentrations:

• Water
• Electrolytes – sodium and chloride ions are usually at a lower concentration than the plasma (hypotonic). Potassium, calcium and iodide ions are usually at a higher concentration than plasma (hypertonic)
• Bicarbonate – present at a higher concentration than in plasma to maintain an alkaline environment
• Bacteriostats – chemicals that prevent the multiplication of bacteria. These include lysozymes, lactoferrin, immunoglobulin A as well as salivary lactoperoxidase.
• Mucus- aids to moisten food contents and eases swallowing.
• Enzymes – salivary amylase, lingual lipase, and kallikrein (helps to produce bradykinin-> a vasodilator)

Salivary Glands

There are three paired salivary glands that are responsible for the production of saliva, the submandibular glands, the sublingual glands, and the parotid glands. They are all ducted, exocrine glands.

Each of the glands produces a slightly different secretion. The parotid glands produce serous saliva: a watery secretion that is rich in enzymes. The secretion from the sub-lingual glands is known as mucous saliva; a viscous secretion containing no enzymes and large amounts of mucus. Finally, the sub-mandibular glands produce a mixed secretion, that is both serous and mucous.

Further information on the anatomy of these glands can be found here.

Secretion

General Secretion

Saliva is always a hypotonic solution but it needs to be produced from concentrated extra-cellular fluid and modified.

The fluid secreted from the acini is overall isotonic with the extracellular fluid:

• Sodium and potassium ions are equivalent
• Iodide ions are present at an increased concentration
• Chloride ions are present at a decreased concentration
• Bicarbonate is present at the same concentration

During ductal modification, there is little change in volume however concentrations of some of the ions change:

• Sodium concentration decreases
• Potassium concentration increases
• Bicarbonate concentration decreases at rest and increases when stimulated

The ductal cells have a maximum rate of modification and therefore the more rapidly saliva is produced, the less it is modified (excluding bicarbonate).

By Boumphreyfr (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

Fig 1 – Diagram showing the modification of saliva.

Resting Saliva

At rest, the acinar secretion is highly modified and has the following characteristics:

• Low volume
• Very hypotonic
• Neutral or slightly acidic
• Few enzymes

Stimulated Saliva

When the production of saliva is stimulated, flow exceeds the ductal cells’ maximum rate of modification and so the acinar secretion is modified less:

• High volume
• Less hypotonic than resting saliva
• Alkaline
• Many enzymes

Clinical Relevance – Xerostomia

Xerostomia is the feeling of a dry mouth and it occurs when there is either a change in the composition of saliva or reduced salivary flow.

It can be caused by many things, but most typically it is due to side effects from medication. Other causes include:

• Dehydration
• Radiotherapy of the salivary glands
• Old age

Generally, the treatment is often unsatisfactory. If there is an identifiable cause then it should be dealt with first, but often the xerostomia cannot be corrected in spite of this. Saliva substitutes are available, as well as stimulants designed to encourage the salivary glands to increase production – although these stimulants are generally ineffective unless there is still salivary function.

In addition, education plays an important role in terms of improving dental hygiene.

By Christopher from Salem, OR, USA (248/365: The burnt tongue) [CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons

Fig 2 – Image showing the mouth of a patient suffering from xerostomia.