Saliva
For the band, see Saliva (band).Saliva, 98% water 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.
In
animals,
saliva is produced in and secreted from the
salivary glands. It is a
fluid containing:
*
Water*
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.
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).
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.
(1) http://www.dentistry.nus.edu.sg/cdearchive/cdeclasen.htm
*
Tongue
