MAJOR FUNCTIONS OF SALIVA
Salivary function can be organized into 5 major categories that serve to maintain oral health and create an appropriate ecologic balance:
(1) lubrication and protection,
(2) buffering action and clearance,
(3) maintenance of tooth integrity,
(4) antibacterial and antifungal activity,
(5) taste and digestion.
Salivary components work in concert in overlapping, multifunctioning roles, which can be simultaneously beneficial and detrimental.
1. LUBRICATION AND PROTECTION
As a seromucous coating, saliva lubricates and protects oral tissues, acting as a barrier against irritants. These irritants include, but are not limited to, proteolytic and hydrolytic enzymes produced in plaque, potential carcinogens from smoking and exogenous chemicals, and desiccation from mouth breathing. The best lubricating components of saliva are mucins: they have the properties of low solubility, high viscosity, high elasticity, and strong adhesiveness. Any intraoral contact between soft tissues, between soft tissues and teeth, or between soft tissues and prostheses benefits from the lubricating capability of saliva supplied largely by these mucins. Mastication, speech, and swallowing all are aided by the lubricating effects of mucins. Mucins also perform an antibacterial function by selectively modulating the adhesion of microorganisms to oral tissue surfaces, which contributes to the control of bacterial and fungal colonization. As a part of the enamel pellicle, mucins help initiate bacterial colonization by promoting the growth of benign commensal oral flora, forming a protective barrier and lubrication against excessive wear, providing a diffusion barrier against acid penetration, and limiting mineral egress from the tooth surface.
2. BUFFERING ACTION AND CLEARANCE
Saliva dilutes and removes substances (oral debris, noxious agents) from the oral cavity which is referred to as salivary or oral clearance. Both the act of swallowing and the salivary flow rate are important to this process, and these are the principal ways by which oral bacteria and injurious, noxious agents are eliminated from the mouth. Accordingly, a high salivary flow rate results in a high clearance and vice versa. The oral sugar clearance becomes extensively prolonged when the whole unstimulated saliva flow rate is below 0.2 ml/min. Apart from clearing sugars, saliva also clears dietary acids and thereby protects the teeth against erosion.
In general, the higher the flow rate, the faster the clearance and the higher the buffer capacity. Dental caries is probably the most common consequence of hyposalivation. Subjects with impaired saliva flow rate often show high caries incidence or caries susceptibility, also on tooth surfaces that are usually not susceptible to caries. A low flow rate combined with a low or moderate buffer effect clearly indicates poor salivary resistance against microbial attack. On a population level, salivary flow rate and buffer capacity show an inverse correlation with caries susceptibility. It must be emphasized, however, that no linear relationship exists among salivary secretion rate, caries activity, and past experience of caries.
The buffer capacity of both unstimulated and stimulated saliva involves three major buffer systems: the bicarbonate (HCO3), the phosphate, and the protein buffer systems. These systems have different pH ranges of maximal buffer capacity, the bicarbonate and phosphate systems having pK values of 6.1-6.3 and 6.8-7.0, respectively.
3. MAINTENANCE OF TOOTH INTEGRITY
Maintaining tooth integrity is a third function of saliva, one that facilitates the demineralization and remineralization process.
Demineralization occurs when acids diffuse through plaque and the pellicle into the liquid phase of enamel between enamel crystals. Resulting crystalline dissolution occurs at a pH of 5 to 5.5, which is the critical pH range for the development of caries. Dissolved minerals subsequently diffuse out of the tooth structure and into the saliva surrounding the tooth. The buffering capacity of saliva greatly influences the pH of plaque surrounding the enamel, thereby inhibiting caries progression. Plaque thickness and the number of bacteria present determine the effectiveness of salivary buffers.
Remineralization is the process of replacing lost minerals through the organic matrix of the enamel to the crystals. Supersaturation of minerals in saliva is critical to this process. Human salivary secretions are supersaturated with respect to calcium and phosphate, but spontaneous precipitation from saliva to dental enamel does not normally occur.
4. ANTIBACTERIAL AND ANTIFUNGAL ACTIVITY
A fourth function of saliva is its antibacterial activity. Salivary glands are exocrine glands, and, as such, secrete fluid containing immunologic and nonimmunologic agents for the protection of teeth and mucosal surfaces. Immunologic and nonimmunologic antibacterial salivary content come from two different sources — namely, plasma and ductal cells — with different responses to stimulation and different content levels.
5. TASTE AND DIGESTION
A fifth and final function of saliva is to enhance taste and begin the digestive process. The sense of taste is activated during the initial stage of ingestion of food particles allowing for identification of essential nutrients and of harmful and potentially toxic compounds. Taste is a main stimulant for formation of saliva. On the other hand, presence of saliva in the oral cavity is also essential for taste perception, first of all because food particles need to be in solution in order to stimulate taste receptor cells in the taste buds within the lingual papillae (fungiform, foliate, and vallate papillae). Furthermore, taste sensitivity is related to saliva composition as each receptor cells upper surface is bathed by the oral fluids. The hypotonicity of saliva enhances the tasting capacity of salty foods and nutrient sources. This enhanced tasting capability depends on the presence of gustin, which binds zinc. Saliva has an early, limited role in total digestion by beginning the breakdown of starch with amylase, a major component of parotid saliva that initially dissolves sugar. The contribution of saliva to starch breakdown is limited because most of the digestion of starch results from pancreatic amylase, not salivary amylase. Salivary enzymes also initiate fat digestion. More importantly, saliva serves to lubricate the food bolus, which aids in swallowing.
Further functions of saliva are:
• Temperature regulation: evaporative cooling is clearly of importance in dogs, which have very poorly developed sweat glands.
• Production of growth factors and other regulatory peptides.
A considerable volume of saliva is produced over a day:
° 0.5 to 1.5 liter of fluid is secreted in a day
° This represents about 1/5 of the total plasma volume
° This fluid is not lost as most of it is swallowed and reabsorbed by the gut
° Parotis, submandibularis, sublingualis are the major salivary glands
SALIVA COMPONENTS
INORGANIC
- Calcium
- Phosphate
- Hydrogen Carbonate
- Other Ions (Fluoride,Thiocyanate)
ORGANIC
- Mucins
- Proline-Rich Proteins
- Amylase
- Lipase
- Peroxidase
- Lysozyme
- Lactoferrin
- sIgA
- Histatins
- Statherin
- Blood group substances,
- Sugar, Steroid Hormones,Aminoacids, Ammonia, Urea
