saliva

Saliva is the watery and usually somewhat frothy substance produced in the mouths of some animals, including humans. Produced in salivary glands, saliva is 98% water, but it contains many important substances, including electrolytes, mucus, antibacterial compounds and various enzymes. 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.

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 vomiting 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. [2] It has not been shown that human licking of wounds disinfects them, but licking 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.

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, which is much less than originally thought.

Contents

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 α-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.

References

• The acquired enamel pellicles in adults and children
• Discover Magazine, "The Biology of ...Saliva" October 2005

See also

• Tongue