Saliva (commonly referred to as spit, drool or slobber) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which DNA can be extracted), enzymes (such as lingual lipase and amylase), and antimicrobial 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 trapped 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. Saliva has specialized purposes for a variety of animal species beyond predigestion. Certain swifts construct nests with their sticky saliva, such as Aerodramus, whose nest is consumed by humans as bird's nest soup. 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.
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. Medically, constituents of saliva can noninvasively provide important diagnostic information related to oral and systemic diseases.
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. 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 swallowed; it has an optimum pH of 7.4 Lingual lipase, which is secreted by the acinar cells of the sublingual gland; has an optimum pH of around 4.0 so it is not activated until it enters the acidic environment of the stomach Kallikrein, an enzyme that proteolytically cleaves high-molecular-weight kininogen to produce bradykinin; it is secreted by the acinar cells of all three major salivary glands Antimicrobial enzymes that kill bacteria: Lysozyme Salivary lactoperoxidase Lactoferrin Immunoglobulin A Beta defensin Proline-rich proteins (function in enamel formation, Ca2+-binding, microbe killing, and lubrication) Minor enzymes including: salivary acid phosphatases A and 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 (function unknown) 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 1,500 mL per day and researchers generally accept that during sleep the amount drops significantly. 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.
Functions Via the action of amylase and other enzymes, saliva contributes to the digestion of food and to the maintenance of oral hygiene. Saliva limits the growth of bacterial pathogens and is a major factor in sustaining systemic and oral health through the prevention of tooth decay. Saliva also acts as a buffer, reducing acidity of food components by its bicarbonate and phosphate content.
Lubricant Saliva coats the oral mucosa, mechanically protecting it from trauma during eating, swallowing, and speaking. Mouth soreness is common in people with reduced saliva (xerostomia) and food (especially dry food) sticks to the inside of the mouth.. Saliva is also used as a lubricant when fellatio is performed.
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 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 has a role in the sense of taste. It is the liquid medium in which chemicals are carried to taste receptor cells (mostly associated with lingual papillae). People with little saliva may experience dysgeusia (reduced ability to taste or to experience a metallic taste), occurring as an adverse effect of some prescription drugs.
Disease diagnosis Saliva can be used in development of non-invasive biomarkers to diagnose the presence or risk of some diseases, such as dental caries and periodontal disease.
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