What Are the Functions of Saliva?
Saliva is the critical component and functional part of the human oral cavity and maintains a dilute aqueous environment in the mouth and comprises both organic and inorganic constituents. Its inherent antimicrobial and cleansing buffering potential protect the tooth enamel and buccal mucosa from bacterial invasions. The saliva also plays a crucial role in food bolus formation on mastication and acts as a lubricant while speaking and swallowing food. Research indicates that human salivary secretions comprise a range of physiologically active substances. Apart from the organic and inorganic components that constitute water as a significant element of salivary secretion, salivary proteins and enzymes are also numerous. Nerve growth factors and regulatory and vasoactive peptides have also been researched as components of human saliva. Salivary biomarkers are also capable of indicating the systemic health conditions of individuals.
How Is Saliva Secreted?
Saliva is secreted primarily by the salivary glands in the mouth or the oral cavity. The salivary glands, like the parotid gland, the sublingual, and the submandibular salivary glands are the major glands for salivary secretion. The minor salivary glands are the lingual, palatal, glossopalatine, and labial or buccal glands. These glands are comprised of multiple secretory units with blind-ending tubules lined by cells. These cells are further capable of secreting and then modifying the salivary content. The secretion of saliva is mainly by our body's ANS (autonomous nervous system).Most salivary secretion content is water with low percentages of organic and inorganic components. These organic and inorganic components are synthesized within the salivary glands and may also be derived from the blood plasma. Therefore, in the initial phase of salivary secretion, the content of saliva would be different or unmodified in the true sense. However, as this saliva eventually passes down the salivary ducts during the secretion phase, it gets modified as the final secretion that also comprises several proteins that may be synthesized in the glands and also secretory granules that may accompany the proteins which are finally discharged into the duct lumen by the phenomenon of exocytosis. The secretion of salivary proteins occurs through several steps that are enlisted as follows:
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Initial amino acid uptake.
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Peptide synthesis and glycosylation pathway (these first two phases occur within the RER- rough endoplasmic reticulum and the cisternae of the golgi body).
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Condensation (occurs within golgi apparatus).
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Exocytosis (the merging of secretory granules with the luminal membranes out into the duct lumen).
What Are the Components of Salivary Secretion?
Sodium and potassium are the two significant cations or inorganic components of salivary secretions. Similarly, the osmotically active anion molecules are chloride and bicarbonate (specifically, bicarbonate plays a crucial role in salivary buffering). While water and other ionic constituents are also inorganic, they are mainly derived by translocation of the blood plasma. Numerous electrolytes present in these secretions are calcium phosphate, thiocyanate, magnesium sulfate, fluoride, and iodine. The inorganic part of saliva is the major constituent that makes up nearly 99 % of the aqueous fluid, while the 1 % constituents are the organic components and ions.
Organic Component - The organic components are responsible for diverse functionalities regarding enzymatic activity, dental tissue protection, and tissue growth control.
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Salivary Amylase: This enzyme is found in the highest concentration in saliva. Salivary amylase is responsible for digestive functions within the stomach after the food bolus is swallowed.
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Lingual Lipase Enzyme: This enzyme is responsible for fat digestion and is active in the stomach's pH.
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Mucous Glycoproteins: These are the glycoproteins found in saliva with a high molecular weight and act as a trap for bacteria protecting the oral soft tissues. Glycoproteins also bind strongly to the tooth enamel, forming the bacterial-resistant pellicle.
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Proline-Rich Glycoproteins: These are the glycoproteins that promote remineralization of the tooth and stabilize the tooth surface. Other functions include the protein's contribution to the oral clearance of microorganisms and reduced microbial load in the oral cavity.
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Secretory Immunoglobulins: IgA (immunoglobulin A) and IgG (immunoglobulin G) are the secretory antibodies that help immensely boost oral immunity. This is the first oral line of defense in the human immune system that helps prevent the colonization of bacteria and contamination by the development of extraoral pathogens in the oral cavity.
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Nerve Growth Factors and Kallikrein: Research is underway to establish the link between the presence of minor amounts of these organic salivary constituents. It may be an essential macromolecule for salivary functionality.
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Lysozymes: These are bactericidal in nature and also have the function of oral immune protectivity.
What Are the Clinical Implications?
The clinical application that impacts the normal oral physiology of salivary flow is due to several oral pathologies that may impact the salivary secretion or even a marked increase in the dental carious lesions. Saliva can act as a crucial buffering solution in the oral cavity and can also prevent the acidification of dental plaque upon tooth enamel. The buffering capacity of saliva is through bicarbonate. Research estimates that at a resting state, the parotid saliva possesses a pH of approximately 5.8 with a bicarbonate concentration close to 0.6 mEq/L (milliequivalents per liter). When there is increased dental plaque or food debris, then the pH of saliva is impacted or turns more acidic. Medical Research indicates that while the salivary flow is highest during meals for mastication and transduction of taste, it is lowest while sleeping and usually of moderate to low flow between two meals. Also, When the salivary flow is extremely low or diminished, it is characterized by dry mouth syndrome or xerostomia. A reduction in the salivary flow in an individual similarly results in the crown of the tooth, with the first layer of enamel being increasingly prone to acidification and, over time, dissolution with the development of carious lesions of the tooth.
Conclusion:
Human saliva is primarily the fluid present in the oral cavity. It is highly beneficial for the transduction of taste and chewing food. It contains essential enzymes, proteins, ions, organic molecules, and immunoglobulins that help maintain oral immune defense.
