Saliva

Saliva is the watery and usually frothy substance produced in the mouths of humans and most other animals. Saliva is produced in and secreted from the salivary glands. Human saliva is 98% water, which carries electrolytes, mucus, antibacterial compounds and various enzymes. ^[1] Enzymes begin the digestion process, breaking down some starch and fat at the molecular level. Saliva breaks down food caught in the teeth, protecting them from bacteria that cause decay. Saliva also lubricates and protects the teeth, the tongue, and the tender tissues inside the mouth.

Saliva is inhabited by bacteria. Human bite wounds have a high risk of infection unless treated with antibiotics.

Various species have evolved special uses for saliva that go beyond predigestion. Some swifts use their gummy saliva to build their nests. Some Aerodramus swiftlet nests are made only from saliva and used to make bird's nest soup. Cobras, vipers, and certain other members of the venom clade hunt with venomous saliva injected by fangs. Some arthropods, such as spiders and caterpillars, create thread from salivary glands.

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Functions

Digestion

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. Salivary glands also secrete enzyme to start fat digestion.

Pellicle deposits

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.^[2]

Disinfectants

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.^[3] 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. Therefore, licking would be a way of washing, useful if purer water isn't available to the animal or person.

The mouth of animals is the habitat of many bacteria, some of which may be pathogenic. Animal (including human) bites are routinely treated with systemic antibiotics because of the risk of septicemia.

Recent research suggests that the saliva of birds is a better indicator of avian influenza than are faecal samples. ^[4]

Stimulation

The production of saliva is stimulated both by the sympathetic nervous system and the parasympathetic.^[5]

The saliva stimulated by sympathetic innervation is thicker, and saliva stimulated parasympathetically is more watery.

Daily salivary output

There has been some disagreement regarding the daily salivary output in a healthy individual. Today, it is believed that the average person produces approximately 0.75 l of saliva per day, less than half of the output originally thought produced.

It is produced at a rate of 1-1.5 l/day. 20ml/hr at rest, 250 ml/hr under stimulated conditions. While sleeping, salivary flow drops to almost zero.

Contents

Produced in salivary glands, human saliva is 98% water, but it contains many important substances, including electrolytes, mucus, antibacterial compounds and various enzymes. ^[6]

It is a fluid containing:

• Water
• Electrolytes:
  • 2-21 mmol/L sodium (lower than blood plasma)
  • 10-36 mmol/L potassium (higher than plasma)
  • 1.2-2.8 mmol/L calcium
  • 0.08-0.5 mmol/L magnesium
  • 5-40 mmol/L chloride (lower than plasma)
  • 25 mmol/L bicarbonate (higher than plasma)
  • 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. There are three major enzymes found in saliva.
  • α-amylase (EC3.2.1.1). Amylase starts the digestion of starch and lipase fat before the food is even swallowed. It has a pH optima of 7.4.
  • lysozyme (EC3.2.1.17). Lysozyme acts to lyse bacteria.
  • lingual lipase (EC3.1.1.3). Lingual lipase has a pH optimum ~4.0 so it is not activated till entering an acidic environment.
  • Minor enzymes include 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).
• Cells: Possibly as much as 8 million human and 500 million bacterial cells per mL. The presence of bacterial products (small organic acids, amines, and thiols) causes saliva to sometimes exhibit foul odor.
• Opiorphin, a newly researched pain-killing substance found in human saliva.

References