Saliva and Salivation: Composition, Sources & pH

Saliva and Salivation Overview

• Saliva is a colourless, slightly sticky solution produced by the salivary glands
• Humans produce 1-1.5 litres of saliva every 24 hours
• Saliva can be serous or mucous in consistency

Saliva Sources

• Saliva amounts vary from major and minor salivary glands
• Submandibular glands produce 60-65% of total saliva volume
• Parotid glands produce 25% of total saliva volume
• Sublingual glands produce 5-10% of total saliva volume
• Minor salivary glands produce less than 10% of total saliva volume

Saliva Composition

• Water makes up 99% of saliva, while organic/inorganic substances make up 1%
• Saliva composition is difficult to assess, varying with stimuli like food or wax
• 'Whole' saliva in the mouth differs from 'pure' saliva in glands
• Crevicular fluid from inflamed gingival pockets becomes part of 'whole' saliva in periodontal disease
• Food debris, epithelial cells, and bacteria are also present in saliva

pH of Saliva

• Resting pH varies between 6.7-7.4
• Can vary depending on diet and time of day
• Low pH is linked to more caries, while higher pH is linked to more calculus

Organic Components of Saliva

• Saliva contains several hundred different proteins, many with antibacterial properties
• Mucin and enzymes are proteins found in saliva
• Immunoglobulins like IgA, IgG, and IgM aid in bacterial disposal by clumping them together, preventing surface clinging, and rendering the bacteria unable to harm
• Agglutination is the term of the action of immunoglobulin action in conjunction with serous saliva for bacterial removal

Enzymes in Saliva

• Lysozyme destroys harmful bacteria via the lysis of cell walls
• Lactoferrin binds free iron, inhibiting bacterial metabolism
• Lactoperoxidase also inhibits bacterial metabolism
• Amylase begins the breakdown of cooked starches
• Ribonuclease catalyzes the endonucleolytic cleavage of RNA
• Phosphatase is involved in calculus formation & remineralisation of caries

Other Organic Components

• Mucin is a group of proteins that give saliva its viscous/sticky consistency and there exist several different mucin proteins
• Muco-proteins contain more than 4% carbohydrate derivatives
• Glyco-proteins contain less than 4% carbohydrate derivatives
• Urea can neutralise acids
• Uric acid and ammonia (formed from urea, raises pH) are by-products of cell metabolism

Cells, Debris and Hormones in Sliva

• Constant turnover of desquamated epithelial cells as worn/dead cells are replaced
• White blood cells are present from the gingival crevice, especially during inflammation
• Commensal bacterial cells exist in symbiotic equilibrium and prevent pathogenic bacteria from thriving.
• Food debris and hormones like oestrogen and progesterone are also present

Inorganic Components of Saliva

• Mainly consists of mineral salts which are known as ions when in solution
• The main component ions maintain the pH levels and have a buffering effect
• Calcium phosphate ions play a major role regarding caries incidence and calculus formation
• Other ions present:
  • Sodium, potassium, magnesium, chloride
  • Fluoride (remineralisation, anti-bacterial, topical effect resulting in teeth being more resistant to decay)
  • Bicarbonate (buffer), sulphate, hydrogen

Gas Components of Saliva

• Oxygen and Nitrogen
• Carbon Dioxide levels are higher in the salivary glands and its precipitation (release) may be related to calculus formation

Functions of Saliva

• Glyco-proteins lubricate and waterproof the soft tissues of the oral cavity, which helps:
• protect against abrasion
• aid speech
• maintain a healthy mucosa (stops it cracking and bacteria invading)
• Also forms the salivary pellicle (barrier to acid attacks)
• Serous saliva cleanses by removing food and bacterial debris

Other Functions of Saliva

• Ions maintain pH, preventing bacterial colonisation, neutralising acids and alkalis
• Higher bicarbonate ion levels occur with higher salivary flow when eating
• Salivary amylase initiates cooked starch breakdown and bolus formation
• Taste sensation is possible through saliva as solution, the lack of saliva effects xerostomia patients
• Proteins and Ig's destroy/prevent harmful bacteria colonisation

Tooth Integrity

• Ions in saliva aid maturation of newly erupted teeth enamel
• Aids enamel repair from acid attacks

Saliva and Caries

• Saliva generally has a protective function and patients that have impaired saliva flow often have rapid
• deposition of plaque with rampant caries and severe periodontal disease

Viscosity

• The salivary pellicle, formed by glyco-proteins, protects against acid but also encourages adherence of plaque bacteria

Supersaturation of Inorganic Ions

• Acts as a buffer to control acid in solution and continuously aids control of the effects of demineralisation and remineralisation

Serous Saliva

• Cleansing effect may reduce fermentable carbohydrate available for conversion to acid

Quantity

• Changes between day and night (1-1.5 litres during the day, 10ml at night)
• Is affected by stimulation, drugs, illness and radiation

Ionic Seesaw

• Prolonged exposure to high levels of sugars can exhaust the saliva buffering system
• As a result the pH level falls
• The immediate effect is increased free energy of ions and the enamel/plaque interface
• The migration of ions is increased from both enamel and plaque
• A pH fall below 6.0 the solubility limit of plaque fluid increases reducing calcium; and phosphate ions
• When the solubility limit exceeds calcium and phosphate ion products, these enter the enamel under a concentration gradient
• Demineralisation begins when mineral ions leave enamel to enter plaque below pH 5.5

Buffering Action

• If the pH rises, the capacity to hold saturation results in mineral ions returning to the enamel
• Therefore enamel remineralises
• Repeated pH fluctuation results in a seesaw of ion shift between enamel and plaque fluid

Mechanism of salivation

• A reflex mechanism stimulated by the sensory inputs: sight, smell, taste.
• Controlled by the sympathetic and parasympathetic nerve supply to the salivary glands

Sympathetic / Parasympathetic Nerves

• The two nerves have opposite effects on the output from salivary glands
• Sympathetic stimulation reduces flow causing dry mouth, which results from vasoconstriction
• Parasympathetic stimulation increases flow when relaxed, causing mouth watering, resulting from vasodilation

Secretomotor Supply to the Salivary Glands

• Sympathetic Supply
• Internal carotid artery to the sphenopalatine ganglion
• Branches run with the greater palatine, lesser palatine and pharyngeal nerves of the trigeminal nerve maxillary division

Parasympathetic Supply

• Superior salivary nucleus of the brain runs with the facial (7th) nerve, via the greater petrosal nerve
• Nerve of the pterygoid canal to the sphenopalatine ganglion where there is a synapse.
• Post-synaptic fibres - ganglion along with branches of the maxillary division of the trigeminal nerve greater palatine, lesser palatine and pharyngeal nerves) to the hard palate, soft palate and pharyngeal glands.

Submandibular and sublingual glands

• Sympathetic supply is the facial artery via the submandibular ganglion and the lingual nerve branch on the mandibular division
• Parasympathetic is the superior salivary nucleus of the brain via the facial (7th) nerve
• Chorda tympani branch to the submandibular ganglion, synapse present
• Post ganglionic fibres run with the lingual nerve, a branch of the mandibular division of the trigeminal nerve

Parotid Glands

• Sympathetic:
  • Middle meningeal artery via the otic ganglion
  • Auriculotemporal nerve (a branch of the mandibular division)
• Parasympathetic:
  • Via the glossopharyngeal (9th) nerve
  • Tympanic plexus of the middle ear and the lesser petrosal nerve to the otic ganglion, including a synapse
  • Post-ganglionic fibres run with the auriculotemporal branch of the mandibular division of the trigeminal (5th) nerve

Taste nerve supply

• From the taste buds via the lingual nerve from mandibulary division
• Tongue's anterior two-thirds travel via the submandibular ganglion
• It follows the Chorda tympani branch of the facial nerve to the nucleus solitarius in the brain
• From the posterior third
• The tongue sends via the glossopharyngeal cranial nerve
• These signals reach the nucleus solitarius of the brain.

Salivary Flow Factors

• Factors which might cause an increase in saliva flow.
  • Mastication
  • Irritants
  • Gingival conditions
  • Hunger
  • Smell/sight of food
  • Teething in babies
• Factors which might cause a reduction in saliva flow
  • Drugs
  • Diabetes
  • Alcohol
  • Fear
  • Radiotherapy
  • Surgery
• Medical conditions impact saliva production, resulting in either:
  • Hypersalivation is the excess production of saliva
  • Xerostomia is dry mouth due to a saliva reduction or absence

Saliva in dentistry

• Saliva in dentistry must be carefully managed
• Saliva can be a route of transmission for many viruses
• COVID-19
  • Hep B
  • Hep C
  • Herpes simplex (type 1)
  • Epstein-Barr virus
  • Tuberculosis
• Cross-infection control procedures must be followed