Saliva

Taking a saliva sample for testing

Saliva is complex fluid, or watery or viscous consistency, produced in and secreted from the major and minor salivary glands in the mouths of humans and most other animals. Depending on the species, it provides digestive, protective, and lubricative functions. Some swifts use their gummy saliva to build their nests, and some Aerodramus swiftlet nests are made only from saliva (and used to make bird's nest soup)(Marcone 2005). 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.

Saliva, which has increased flow in humans when they are eating ****

Human saliva

Human saliva is produced by the three pairs of major salivary glands (parotid, submandibular, and sublingual glands) and the many minor salivary glands. The parotid glands produce saliva with a watery (serous) consistency, the sublingual and minor salivary glands produce a more viscous (mucous) fluid, and the submandibular glands produce a mixture of watery and viscous fluid (Orchardson 2001). Human saliva is composed mostly of water, about 98 to 99 percent, but also includes dissolved inorganic ions and numerous organic substances, including proteins, such as mucins (heavily glycosylated proteins) (Orchardson 2001).

As part of the initial process of food digestion, the enzymes in saliva break down some of the starch and fat in the food at the molecular level. Saliva also breaks down food caught in the teeth. Furthermore, saliva lubricates and protects the teeth, the tongue, and the tender tissues inside the mouth. Saliva also assists in swallowing by moistening ingested food, has proteins that provide a barrier against bacteria, fungi, and viruses, and has anti-micobrial substances that likewise protect the body. Saliva even contributes to taste by dissolving sapid substances in food and making them more accessible to taste buds (Orchardson 2001).

The average person produces a total of about 600 milliliters of saliva per day from all the salivary glands in total (Orchardson 2001). While eating, flow rates may reach five milliliters per minute, while the resting flow rates are about 0.3 milliliters per minute (Orchardson 2001). While sleeping, salivary flow drops to almost zero.

Contents

Human saliva includes:

• Water (98 to 99 percent).
• 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 cause lysis in bacteria.
  • lingual lipase (EC3.1.1.3). Lingual lipase has a pH optimum ~4.0 so it is not activated until 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. This is a newly researched pain-killing substance found in human saliva.

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 enzymes (salivary lipase) to start fat digestion (Maton et al. 1993).

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 can help to heal wounds 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 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 wiping off pathogens, useful if clean water is not available to the animal or person.

The mouth of animals is the habitat of many bacteria, some pathogenic. Some diseases, such as herpes, can be transmitted through the mouth. 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.

Cleaning

Saliva is an effective cleaning agent used in art conservation. Cotton swabs coated with saliva are rolled across a paintings surface to delicately remove thin layers of dirt that may accumulate.

Stimulation

The production of saliva is stimulated both by the sympathetic nervous system and the parasympathetic.

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

References

ISBN links support NWE through referral fees

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• Orchardson, R. 2001. Saliva. In C. Blakemore, and S. Jennett, The Oxford Companion to the Body. New York: Oxford University Press. ISBN 019852403X.

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