Saliva

Related Topics:
Salivary_Gland

For the band, see Saliva (band).

Saliva, often informally known as spit, is the moist, clear, and usually somewhat frothy substance produced in the mouths of some animals, including humans. To spit or the action of spitting (also called expectoration) is to expel saliva or other substances from the mouth.

Contents of Saliva

In animals, saliva is produced in and secreted from the salivary glands. It is a fluid containing:

Electrolytes: (2-21 mmol/L sodium, 10-36 mmol/L potassium, 1.2-2.8 mmol/L calcium, 0.08-0.5 mmol/L magnesium, 5-40 mmol/L chloride, 2-13 mmol/L bicarbonate, 1.4-39 mmol/L phosphate)
Mucus. Mucus in saliva mainly consists of mucopolysaccharides and glycoproteins;
Antibacterial compounds (thiocyanate, hydrogen peroxide, and secretory immunoglobulin A)
Various enzymes. The major enzymes found in human saliva are alpha-amylase (EC3.2.1.1), lysozyme (EC3.2.1.17), and lingual lipase (EC3.1.1.3). Amylase starts the digestion of starch and lipase fat before the food is even swallowed. It has a pH optima of 7.4. Lingual lipase has a pH optimum ~4.0 so it is not activated till entering an acidic environment. Lysozyme acts to lyse bacteria. Human saliva contains also salivary acid phosphatases A+B (EC3.1.3.2), N-acetylmuramyl-L-alanine amidase (EC3.5.1.28), NAD(P)H dehydrogenase-quinone (EC1.6.99.2), salivary lactoperoxidase (EC1.11.1.7), superoxide dismutase(EC1.15.1.1), glutathione transferase (EC2.5.1.18), class 3 aldehyde dehydrogenase (EC1.2.1.3), glucose-6-phosphate isomerase (EC5.3.1.9), and tissue kallikrein (EC3.4.21.35). The presence of these products causes saliva to sometimes have a foul odor.

There has been some disagreement regarding the daily salivary output in a healthy individual. Today, it is believed that the average person produces about 700mL of saliva per day - much less than originally thought.

Functions

The digestive functions of saliva include moistening food, and helping to create a food bolus, so it can be swallowed easily. Saliva contains the enzyme amylase that breaks some starches down into maltose and dextrin. Thus, digestion of food occurs within the mouth, even before food reaches the stomach.

Saliva also has an important protective function, in that it helps prevent against tooth decay and tooth erosion by way of its buffering properties. Tooth decay (or erosion) progresses rapidly when the oral environment becomes acidic (eg. after consuming a soft drink or from bacterial fermentation). Bicarbonate ions contained within saliva will act to neutralize this change in acidity, thus helping maintain a constant pH. By maintaining optimal pH levels, the rate of demineralisation (loss of tooth structure) is minimized.

Saliva also carries many important ions (Calcium, phosphate, fluoride) that play a role in tooth remineralisation (rebuilding lost tooth structure). The result of this is that, when acid attack occurs on a tooth, there is an equilibrium between demineralization and remineralisation. It is ideal for these two processes to occur at equal rates, however, if demineralization exceeds remineralisation, then loss of tooth structure occurs. For this reason, it is important for salivary funtion to be normal, otherwise an individual carries a higher risk of dental decay.

The importance of the salivary protective function can be demonstrated by considering a scenario where an individual is about to vomit. Vomit contains gastric substances which are extremely acidic and will erode teeth. A protective reflex occurs before the individual prepares to vomit. Signals are sent from the brain to the salivary glands (via the involuntary nervous system) to cause increased saliva secretion, even before vomiting occurs. Thus, when vomitting does occur, there is already saliva present in the mouth acting to minimize the acidity and thus prevent destruction of tooth structure.

In addition to this, saliva is responsible for depositing salivary pellicle that covers the entirety of the tooth surfaces. This pellicle is believed to play a role in plaque formation, though there is evidence that it may also act as a protective barrier between acids and the tooth surface (1).

Beliefs

A common belief is that saliva contained in the mouth has natural disinfectants, which leads people to believe it is beneficial to "lick their wounds". Researchers at the University of Florida at Gainesville have discovered a protein called nerve growth factor (NGF) in the saliva of mice. Wounds doused with NGF healed twice as fast as untreated and unlicked wounds; therefore, saliva does have some curative powers in some species. NGF has not been found in human saliva; however, researchers find human saliva contains such antibacterial agents as secretory IgA, lactoferrin, and lactoperoxidase. It has not been shown that licking your wounds actually disinfects them, but the licking action is likely to help clean the wound by removing larger contaminants such as dirt and may help to directly remove infective bodies by brushing them away.

References

(1) http://www.dentistry.nus.edu.sg/cdearchive/cdeclasen.htm

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Contents of Saliva

Functions

Beliefs

References

See also