Saliva is a watery substance formed in the mouths of animals, secreted by the salivary glands. Human saliva comprises 99.5% mostly water, plus electrolytes, mucus, white blood cells, epithelial cells (which can be used to extract DNA), glycoproteins, enzymes (such as amylase and lipase), antimicrobial agents such as secretory IgA and lysozyme. The enzymes found in saliva are essential in beginning the process of digestion of dietary starches and fats. These enzymes additionally play a role in breaking down food particles entrapped within dental crevices, thus protecting teeth from bacterial decay. Furthermore, saliva serves a lubricative function, wetting food and permitting the initiation of swallowing, and protecting the mucosal surfaces of the oral cavity from desiccation.

Various animal species have special uses for saliva that go beyond pre-digestion. Some swifts use their gummy saliva to build nests. Aerodramus nests form the basis of bird's nest soup. Cobras, vipers, and certain additional members of the venom clade hunt with venomous saliva injected by fangs. Some arthropods, such as spiders and caterpillars, produce thread from salivary glands.

Contents

Produced in salivary glands, human saliva is 99.5% water, but it contains a large number of important substances, including electrolytes, mucus, antibacterial compounds and various enzymes.

It is a fluid containing:

Daily salivary output

There is much debate about the amount of saliva that's produced in a healthy person per day; estimates range from 0.75 to 1.5 litres per day while it is generally accepted that throughout sleep the amount drops to nearly zero. In humans, the submandibular gland contributes around 70–75% of secretion, while the parotid gland secretes about 20–25% and small amounts are secreted from the additional salivary glands.

Functions

Saliva contributes to the digestion of food and to the maintenance of oral hygiene. Without normal salivary function the frequency of dental caries, gum disease (gingivitis and periodontitis), and additional oral problems increases significantly.

Lubricant

Saliva coats the oral mucosa, mechanically protecting it from trauma throughout eating, swallowing and speaking. In people with little saliva (xerostomia), soreness of the mouth is quite common, and the food (especially dry food) sticks to the inside of the mouth.

Digestion

The digestive functions of saliva include moistening food and helping to create a food bolus. This lubricative function of saliva allows the food bolus to be passed easily from the mouth into the esophagus. Saliva contains the enzyme amylase, additionally called ptyalin, which is capable of breaking down starch into simpler sugars such as maltose and dextrin that can be further broken down in the small intestine. Only about thirty percent starch digestion takes place in the mouth cavity. Salivary glands additionally secrete salivary lipase (a more potent form of lipase) to start fat digestion. Salivary lipase plays a large role in fat digestion in newborn infants as their pancreatic lipase still needs a few time to develop.

Role in taste

Saliva is quite important in the sense of taste. It is the liquid medium in which chemicals are carried to taste receptor cells (mostly associated with lingual papillae). Persons with little saliva often complain of dysgeusia (i.e. disordered taste, e.g. reduced ability to taste, or having a bad, metallic taste at all times).

Other

  • Saliva maintains the pH of the mouth. Saliva is supersaturated with various ions. Certain salivary proteins prevent precipitation, which would form salts. These ions act as a buffer, keeping the acidity of the mouth within a certain range, typically pH 6.2–7.4. This prevents minerals in the dental hard tissues from dissolving.
  • Saliva secretes carbonic anhydrase (gustin), which is thought to play a role in the development of taste buds.
  • Saliva contains EGF. EGF results in cellular proliferation, differentiation, and survival. EGF is a low-molecular-weight polypeptide first purified from the mouse submandibular gland, but after then found in a large number of human tissues including submandibular gland, parotid gland. Salivary EGF, which seems additionally regulated by dietary inorganic iodine, additionally plays an important physiological role in the maintenance of oro-esophageal and gastric tissue integrity. The biological effects of salivary EGF include healing of oral and gastroesophageal ulcers, inhibition of gastric acid secretion, stimulation of DNA synthesis as well as mucosal protection from intraluminal injurious factors such as gastric acid, bile acids, pepsin, and trypsin and to physical, chemical and bacterial agents.

Production

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.

Sympathetic stimulation of saliva is to facilitate respiration, whereas parasympathetic stimulation is to facilitate digestion.

Parasympathetic stimulation leads to acetylcholine (ACh) release onto the salivary acinar cells. ACh binds to muscarinic receptors, specifically M3, and causes an increased intracellular calcium ion concentration (through the IP3/DAG second messenger system). Increased calcium causes vesicles within the cells to fuse with the apical cell membrane leading to secretion. ACh additionally causes the salivary gland to release kallikrein, an enzyme that converts kininogen to lysyl-bradykinin. Lysyl-bradykinin acts upon blood vessels and capillaries of the salivary gland to generate vasodilation and increased capillary permeability respectively. The resulting increased blood flow to the acini allows production of more saliva. In addition, Substance P can bind to Tachykinin NK-1 receptors leading to increased intracellular calcium concentrations and subsequently increased saliva secretion. Lastly, both parasympathetic and sympathetic nervous stimulation can lead to myoepitheilium contraction which causes the expulsion of secretions from the secretory acinus into the ducts and eventually to the oral cavity.

Sympathetic stimulation results in the release of norepinephrine. Norepinephrine binding to α-adrenergic receptors will cause an increase in intracellular calcium levels leading to more fluid vs. protein secretion. If norepinephrine binds β-adrenergic receptors, it will result in more protein or enzyme secretion vs. fluid secretion. Stimulation by norepinephrine initially decreases blood flow to the salivary glands due to constriction of blood vessels but this effect is overtaken by vasodilation caused by various local vasodilators.

Saliva production might additionally be pharmacologically stimulated by so-called sialagogues. It can additionally be suppressed by so-called antisialagogues.

Human and animal behaviours

Spitting

Spitting is the act of forcibly ejecting saliva or additional substances from the mouth. It is often considered rude and a social taboo in a large number of parts of the world, including Western countries, where it is frequently forbidden by local laws (as it was thought to facilitate the spread of disease). These laws are generally not strictly enforced. In Singapore, the fine for spitting might be as high as SGD$2,000 for multiple offenses, and one can even be arrested. In a few additional parts of the world, expectoration is more socially acceptable (even if officially disapproved of or illegal), and spittoons are still a common appearance in a few cultures. Some animals, including humans in a few cases, use spitting as an automatic defensive move. Camels are well known for doing this, though most tamed camels are trained not to.

Glue to construct bird nests

Many birds in the swift family, Apodidae, produce a viscous saliva throughout nesting season to glue together materials to construct a nest. Two species of swifts in the genus Aerodramus build their nests using only their saliva, the base for bird's nest soup.

Wound licking

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 a few species. NGF hasn't been found in human saliva; however, researchers find human saliva contains such antibacterial agents as secretory IgA, lactoferrin, lysozyme and peroxidase. It hasn't 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 might help to directly remove infective bodies by brushing them away. Therefore, licking would be a way of wiping off pathogens, useful if clean water isn't available to the animal or person.