Saliva (commonly referred to as spit) is an extracellular fluid
produced and secreted by salivary gland
s in the mouth
. In human
s, saliva is 98% water
, white blood cells
, epithelial cells
(from which DNA
can be extracted), enzymes
(such as amylase
agents such as secretory IgA
, and lysozymes
The enzymes found in saliva are essential in beginning the process of digestion
of dietary starches and fats. These enzymes also play a role in breaking down food particles entrapped within dental crevices, thus protecting teeth from bacterial decay.
Saliva also performs a lubricating function, wetting food
and permitting the initiation of swallowing
, and protecting the oral mucosa
from drying out
Various animal species
have special uses for saliva that go beyond predigestion. Some swift
s use their gummy saliva to build nests. ''Aerodramus
form the basis of bird's nest soup
Cobras, vipers, and certain other members of the venom clade
hunt with venomous saliva injected by fang
s. Some caterpillar
s produce silk fiber
from silk proteins stored in modified salivary glands.
Produced in salivary gland
s, human saliva comprises 99.5% water, but also contains many important substances, including electrolyte
compounds and various enzyme
* Water: 99.49%
** 2–21 mmol/L sodium
(lower than blood plasma
** 10–36 mmol/L potassium
(higher than plasma)
** 1.2–2.8 mmol/L calcium
(similar to plasma)
** 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
(mmol/L concentration is usually higher than plasma, but dependent variable according to dietary iodine intake)
(mucus in saliva mainly consists of mucopolysaccharides
* Antibacterial compounds (thiocyanate
, hydrogen peroxide
, and secretory immunoglobulin A
* Epidermal growth factor
* Various enzyme
s; most notably:
(EC18.104.22.168), or ptyalin, secreted by the acinar cells of the parotid and submandibular glands, starts the digestion of starch before the food is even swallowed; it has a pH
optimum of 7.4
** Lingual lipase
, which is secreted by the acinar cells of the sublingual gland; has a pH
optimum around 4.0 so it is not activated until entering the acidic environment of the stomach
, an enzyme that proteolytically cleaves high-molecular-weight kininogen to produce bradykinin
, which is a vasodilator; it is secreted by the acinar cells of all three major salivary glands
enzymes that kill bacteria:
-rich proteins (function in enamel
-binding, microbe killing and lubrication)
** Minor enzymes including: salivary acid phosphatase
s A+B, N-acetylmuramoyl-L-alanine amidase
, NAD(P)H dehydrogenase (quinone)
, superoxide dismutase
, glutathione transferase
, class 3 aldehyde dehydrogenase
, glucose-6-phosphate isomerase
, and tissue kallikrein
* Cells: possibly as many 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 a foul odor
, a pain-killing substance found in human saliva
, a protein which binds to Vitamin B12
to protect it against degradation in the stomach, before it binds to intrinsic factor
Daily salivary output
There is much debate about the amount of saliva that is produced in a healthy person
. Production is estimated at 1500ml per day and is generally accepted that during sleep the amount drops significantly.
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 other salivary glands.
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 other oral problems increases significantly.
Saliva coats the oral mucosa
mechanically protecting it from trauma during eating, swallowing, and speaking. Mouth soreness is very common in people with reduced saliva (xerostomia
) and food (especially dry food) sticks to the inside of the mouth.
The digestive functions of saliva include moistening food and helping to create a food bolus
. The lubricative function of saliva allows the food bolus to be passed easily from the mouth into the esophagus. Saliva contains the enzyme amylase, also 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. About 30% of starch digestion takes place in the mouth cavity. Salivary glands also secrete salivary lipase
(a more potent form of lipase) to begin fat digestion. Salivary lipase
plays a large role in fat digestion in newborn infants as their pancreatic lipase still needs some time to develop.
Role in taste
Saliva is very important in the sense of taste
. It is the liquid medium in which chemicals are carried to taste receptor cells (mostly associated with lingual papilla
e). People 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). A rare condition identified to affect taste is that of '''Saliva Hypernatrium'
'', or excessive amounts of sodium in saliva that is ''not'' caused by any other condition (e.g., ''Sjögren syndrome
''), causing everything to taste 'salty'.
* 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 since then found in many human tissues including submandibular gland, parotid gland. Salivary EGF, which seems also regulated by dietary inorganic iodine
, also 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.
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 fluid-like.
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 receptor
s, 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 also causes the salivary gland to release kallikrein
, an enzyme that converts kininogen
. 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 the production of more saliva. In addition, Substance P
can bind to Tachykinin NK-1 receptor
s leading to increased intracellular calcium concentrations and subsequently increased saliva secretion. Lastly, both parasympathetic and sympathetic nervous stimulation can lead to myoepithelium
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 receptor
s 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 may also be pharmacologically stimulated by the so-called sialagogue
s. It can also be suppressed by the so-called antisialagogue
is the act of forcibly ejecting saliva or other substances from the mouth. In many parts of the world, it is considered rude and a social taboo
, and has even been outlawed in many countries. In Western countries, for example, it has often been outlawed for reasons of public decency and attempting to reduce the spread of disease; however, these laws are often not strictly enforced. In Singapore
, the fine for spitting may be as high as SGD$2,000 for multiple offenses, and one can even be arrested. In some other parts of the world, such as in China
, expectoration is more socially acceptable (even if officially disapproved of or illegal), and spittoon
s are still a common appearance in some cultures. Some animals, even humans in some cases, use spitting as an automatic defensive maneuver. Camel
s are well known for doing this, though most domestic camels are trained not to.
Because saliva can contain large amounts of virus
copies in infected individuals (for example, in people infected with SARS-CoV-2
), spitting in public places can pose a health hazard to the public.
Glue to construct bird nests
Many birds in the swift
family, Apodidae, produce a viscous saliva during 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
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
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 mucin
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.
In Pavlov's experiment, dogs were conditioned to salivate in response to a ringing bell, this stimulus is associated with a meal or hunger. Salivary secretion is also associated with nausea. Saliva is usually formed in the mouth through an act called gleeking
, which can be voluntary or involuntary.
Making alcoholic beverages
Some old cultures chewed grains to produce alcoholic beverages, such as chicha
A number of commercially available saliva substitutes exist.
*Basic reproduction number