Why is saliva and a good salivary flow so important?
Saliva is made up of the secretions of the three pairs of salivary glands these are: the parotid, sub-mandibular, sublingual and hundreds of minor salivary glands within the submucosa and some gingival crevicular fluid. These glandular secretions- saliva, constantly bathe the teeth and oral tissues; its presence is vital to the maintenance of healthy teeth and soft oral tissues.
The basic secretory units of salivary glands are clusters of cells called acinar cells, they resembles a many-lobed “berry”, such as a raspberry (acinus Latin for “berry”) hence the name. One of the most important functions of saliva is its buffering ability. This buffering system maintains the acid-base equilibrium by neutralising organic acids present in food, and acids produced by bacteria in caries. This buffering action reduces the H+ ion (released by acids) concentration in the oral cavity. Bicarbonate present in saliva is mainly responsible for this.
Bicarbonate is concentrated within acinar cells and released following a secretory stimulus. Carbon dioxide inside cells is converted to bicarbonate (HCO3-) and H+ by carbonic anhydrase. Carbonic anhydrase is an enzyme that assists rapid inter-conversion of carbon dioxide and water into carbonic acid, protons and bicarbonate ions. This chemical reaction is represented below.
CO2+ H2O <——>H2CO3 <—–> HCO3– + H+
How does the carbonic acid/bicarbonate buffer work?
The pH of the oral cavity is maintained at about 6.3 to ensure the maintenance of the integrity of the tooth structure. Once food is in the mouth, two important events occur:
(1) Drop in pH
(2) Rise in bicarbonate concentration
The drop in pH is caused by increased concentrations of H+ within plaque, due to acid being produced by bacteria when they ferment carbohydrates. The buffering capacity of saliva is due mainly to the presence of bicarbonate ions, secreted within the ducts. The concentration of bicarbonate is determined by the stimulation of saliva and by carbonic anhydrase (secreted by the serous acinar cells in the parotid and sub-mandibular glands). In stimulated saliva, such as when eating, saliva flow rate increases dramatically. More bicarbonate is produced as a by-product of cell metabolism which diffuses into dental plague and helps neutralise the increased amount of acid (H+) produced by oral microbes.
The following equilibrium exists in saliva:
Direction of reaction
<—————————————————
Carbonic Anhydrase
CO2 + H2O <——-> H2CO3<—–>HCO3 – + H+ {from secreted (lactic acid
bicarbonate) from bacteria}
Carbon dioxide is present in the form of bicarbonate in stimulated saliva. A rise in bicarbonate concentration conveniently removes the increased amounts of H+ produced by bacteria, such that the above equilibrium shifts to the left to produce more carbonic acid. However, the concentration of carbonic acid in the mouth is maintained at remarkably constant levels; this means that the excess carbonic acid must be removed. The increase in carbonic acid concentration causes the equilibrium to shift left further such that more CO2 is produced. The enzyme carbonic anhydrase catalyses this reaction by driving the conversion of carbonic acid to carbon dioxide and water. Because the partial pressure of CO2 in saliva is now higher than that in the atmosphere and the mouth is an open system, CO2 diffuses out from the saliva. (Bardow et al., 2008, p.198).
As the concentration of carbonic acid falls, more bicarbonate ions bind to hydrogen ions to form carbonic acid, establishing a new equilibrium. In this way, excess H+ produced within plaque is effectively neutralised and removed, reducing the risk of erosion of teeth. The concentration of HCO3– in saliva is not constant, but varies with the flow rate in such a way that unstimulated saliva contains only a few mmole/liter (mM), whereas stimulated saliva contains much higher levels of HCO3–, depending on the stimulus intensity.
Saliva’s Protective Function
Saliva limits damages from trauma such as: mechanical, chemical, thermal and biological. Saliva participates in the process of remineralization and slows down demineralization. At pH 6.8 – 7.2 saliva is over saturated with calcium phosphate solution therefore after slight demineralization, lost mineral components can return from saliva to hard tissues of tooth.
The equilibrium between saliva and enamel is represented by the following equation:
Ca10 (PO4)6 (OH)2 <——–>10Ca2+ + 6(PO4)3– + 2OH–
The acidifying of environment (e.g. as a result of carbohydrates’ fermentation) increases solubility, and decreases saturation of saliva with calcium phosphates and saliva become an unsaturated solution.
Severe reduction of salivary output not only results in a rapid deterioration in oral health but also has a detrimental impact on the quality of life for the sufferer. Patients suffering from dry mouth can experience difficulty with eating, swallowing, speech, the wearing of dentures, trauma to and ulceration of the oral mucosa, taste alteration, poor oral hygiene, a burning sensation of the mucosa, oral infections including candida and rapidly progressing dental caries.
Dry mouth and Polypharmacy
The sensation of dry mouth is becoming increasingly common in developed countries where adults are living longer. Polypharmacy (the use of a large number of medications- five or more) is very common among the older adult population, and many commonly prescribed drugs cause a reduction in salivary flow. Since polypharmacy is a consequence of having several underlying medical conditions, it is much more common in elderly patients. With increasing age the mix of conventionally prescribed and complementary medicines changes from minimising risk factors (lipid lowering, bone density etc.) to treating disease. With increasing frailty, older people tend to take more medication making polypharmacy a major cause of dry mouth. (Australian Institute of Health and Welfare 2012. Dementia in Australia. Cat. no. AGE 70. Canberra: AIHW). Xerostomic drugs (Xerostomia, subjective sensation of dry mouth), especially those with anticholinergic effects; (anticholinergic drugs inhibit the transmission of nerve impulses by opposing the actions of the neurotransmitter acetylcholine) can cause xerostomia; producing symptoms of dry mouth by reducing saliva volume or by altering the threshold of perceiving a dry mouth. Hyposalivatory drugs reduce salivary flow. Xerostomia also occurs in Sjögren’s syndrome (where immune cells infiltrate and destroy the mucous-producing glands of the body, the specific cause of Sjögren’s is unknown; however, there appears to be a genetic influence occurring more often in families that have other autoimmune diseases. Other factors, as viral infections and stress also appear to be able to trigger the disease.
A 24-hour survey of medicine usage of Australians aged fifty years or older found 43.3% use five(5) or more conventional or complementary medicines while 10.7% use ten (10) or more. Polypharmacy (taking five or more medicines) increases with age with five or more medicines taken by 32.2% of 50-
64 year olds, 49.4% of 65-74 year olds and 66.0% for those over 70 years (Silva M, Hopcraft M, Morgan M. Dental caries in Victorian nursing homes. Aust Dent J. 2014; (59): 321–328.)
In addition to specific diseases of the salivary glands, salivary flow is usually severely impaired following radiotherapy in the head and neck area for cancer treatment in both children and adults of all ages. Clearly oral dryness is a problem which faces an increasingly large proportion of the population. An understanding of saliva and its role in oral health will help to promote awareness among health care workers of the problems arising when the quantity or quality of saliva is decreased.