Saliva

Saliva (commonly referred to as spit) is an

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.^[2] Saliva also performs a lubricating function, wetting food and permitting the initiation of swallowing, and protecting the oral mucosa from drying out.^[3]

The process of breaking down dietary carbohydrates and lipids starts with the enzymes present in saliva. In addition to keeping teeth free from bacterial decay, these enzymes aid in the breakdown of food particles stuck in dental fissures. In addition to lubricating food and enabling the start of swallowing, saliva also serves to prevent the mouth mucosa from drying up.Saliva has specialized purposes for a variety of animal species beyond predigestion [3]. Certain swifts construct nests with their sticky saliva. The foundation of bird's nest soup is an aerodramus nest.[4] Venomous saliva injected by fangs is used by cobras, vipers, and certain other members of the venom clade to hunt. Some caterpillars use modified salivary glands to store silk proteins, which they then use to make silk fiber.^[4]

Composition

Produced in salivary glands, human saliva comprises 99.5% water, but also contains many important substances, including electrolytes, mucus, antibacterial compounds and various enzymes.^[1] Medically, constituents of saliva can noninvasively provide important diagnostic information related to oral and systemic diseases.^[5]

• Water: 99.5%
• 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 (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
  • Iodine (mmol/L concentration is usually higher than plasma, but dependent variable according to dietary iodine intake)
• Mucus (mucus in saliva mainly consists of mucopolysaccharides and glycoproteins)
• Antibacterial compounds (thiocyanate, hydrogen peroxide, and secretory immunoglobulin A)
• Epidermal growth factor (EGF)
• Saliva eliminates caesium, which can substitute for potassium in the cells.^[6][7]
• Various enzymes; most notably:
  • α-amylase (EC3.2.1.1), 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
  • vasodilator
    ; it is secreted by the acinar cells of all three major salivary glands
  • Antimicrobial enzymes that kill bacteria:
    • Lysozyme
    • Salivary lactoperoxidase
    • Lactoferrin^[8]
    • Immunoglobulin A^[8]
  • Proline-rich proteins (function in enamel formation, Ca²⁺-binding, microbe killing and lubrication)^[8]
  • Minor enzymes including: salivary
    glutathione transferase, class 3 aldehyde dehydrogenase, glucose-6-phosphate isomerase, and tissue kallikrein (function unknown)^[8]
• 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
  .
• Opiorphin, a pain-killing substance found in human saliva
• Haptocorrin, a protein which binds to vitamin B₁₂ to protect it against degradation in the stomach, before it binds to intrinsic factor.

Daily salivary output

Experts debate the amount of saliva that a healthy person produces. Production is estimated at 1500ml per day and researchers generally accept that during sleep the amount drops significantly.^[3][9] In humans, the submandibular gland contributes around 70 to 75% of secretions, while the parotid gland secretes about 20 to 25%; small amounts are secreted from the other salivary glands.^[10]

Functions

Saliva contributes to the digestion of food and to the maintenance of oral hygiene. Without normal salivary function the frequency of

Lubricant

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.

Digestion

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.^[12]

Role in taste

Saliva is very important in the

), causing everything to taste 'salty'.

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.^[13]
• Saliva contains EGF. EGF results in cellular proliferation, differentiation, and survival.^[14] 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.^[15]

Production

The production of saliva is stimulated both by the

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

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

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

.

Behaviour

Spitting

Carrollton section of New Orleans

Spitting 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 sometimes been outlawed. In some countries, for example, it has been outlawed for reasons of public decency and attempting to reduce the spread of disease. These laws may not strictly enforced, but in Singapore, the fine for spitting may be as high as SGD$2,000 for multiple offenses, and one can even be arrested.^{[citation needed]} In China, expectoration is more socially acceptable (even if officially disapproved of or illegal), and spittoons are still a common appearance in some cultures. Some animals, even humans in some cases, use spitting as an automatic defensive maneuver. Camels are well known for doing this, though most domestic camels are trained not to.

Spitting by an infected person (for example, one with

is a health hazard to the public.

Glue to construct bird nests

Many birds in the

Wound licking

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