Q. Discuss the Secretion, Composition and Functions of Saliva [GI Phys.]
Saliva is the secretion associated with the mouth. It is produced largely by three major pairs of salivary glands that are located outside of the oral cavity and discharge saliva through short ducts into the mouth.
Secretion of saliva
Saliva contains a serous secretion and a mucus secretion: The principal glands that of salivation are the parotid, submandibular, and sublingual glands; in addition, there are many tiny buccal glands. Daily secretion of saliva normally ranges between 800 and 1500 milliliters.
Saliva contains two major types of protein secretions:
- a serous that contains ptyalin which is an enzyme for digesting starches, and
- mucus secretion that contains mucin for lubricating and for surface protective purposes
The parotid glands secrete almost entirely the serous type of secretion, whereas the submandibular and sublingual glands secrete both serous secretion and mucus. The buccal glands secrete only mucus.
Salivary secretion is a two-stage process: The first stage involves the acini, and the second the salivary ducts.
The acini secrete a primary secretion that contains ptyalin and/or mucin in a solution of ions in concentrations not greatly different from those of typical extracellular fluid. As the primary secretion flows through the ducts, two major active transport processes take place that markedly modify the ionic composition of the fluid in the saliva.
First, sodium ions are actively reabsorbed from all salivary ducts and potassium ions are actively secreted in exchange for sodium. Therefore, the sodium ion concentration of the saliva becomes greatly reduced, whereas the potassium ion concentration becomes increased.
However, there is excess sodium reabsorption over potassium secretion, and this creates electrical negativity of about -70 milliVolts in the salivary ducts; this in turn causes chloride ion to be reabsorbed passively.
Therefore, the chloride ion concentration in the salivary fluid falls to a very low level, matching the ductal decrease in sodium ion concentration.
Second, bicarbonate ions are secreted by the ductal epithelium into the lumen of the duct. This is at least partly caused by passive exchange of bicarbonate for chloride ions, but it may also result partly from an active secretory process.
The net result of this transport processes is that under resting conditions, the concentrations of sodium and chloride ions in the saliva are only about 15 mEq/L each, about one-seventh to one-tenth their concentrations in plasma. Conversely, the concentration of potassium ions is about 30 mEq/L, seven times as great as in plasma, and the concentration of bicarbonate ions is 50 to 70 mEq/L, about two to three times that of plasma.
During maximal salivation, the salivary ionic concentrations changes considerably because the rate of formation of primary secretion by the acini can increase as much as 20-fold. This acinar secretion then flows through the ducts so rapidly that the ductal reconditioning of the secretion is considerably reduced.
Therefore, when copious quantities of saliva are being secreted, the sodium chloride concentration is about one-half or two-thirds that of plasma, and the potassium concentration rises to only four times that of plasma.
Composition of Saliva
Whole saliva is a mixed secretion with daily output of between 1.0 and 2.0litres
- Water: > 99%
- Electrolytes: Inorganic solid 0.2 % i.e. chlorides, carbonates, phosphates and potassium thiocyanate
- Salivary α- amylase: Organic solid 0.3 % i.e. mucin, urea & enzymes
- Lingual lipase
- Blood group antigens
Functions of Saliva
- Saliva begins the digestion of carbohydrate in the mouth through action of salivary amylase, an enzyme that breaks polysaccharides down into maltose, a disaccharide consisting of two glucose molecules.
- Saliva facilitates swallowing by moistening food particles, thereby holding them together, and by providing lubrication through the presence of mucus, which is thick and slippery
- Saliva exert some antibacterial action by means of a two-fold effect – first by lysozyme, an enzyme that lyses, or destroys, certain bacteria by breaking down their cell walls, and second by rinsing away material that may serve as a food source for bacteria
- Saliva serves as a solvent that dissolves molecules that stimulate the taste buds. Only molecules in solution can react with taste bud receptors. If you dry your tongue, and then drop some sugar on it – you cannot taste the sugar until it is moistened.
- Saliva aids speech by facilitating movement of the lips and tongue. It is exceedingly difficult to talk when the mouth feels dry
- Saliva plays an important role in oral hygiene by helping keep the mouth and teeth clean. The constant flow of saliva helps to flush away food residues, shed epithelial cells, and foreign particles. Saliva’s contribution in this regard in apparent to anyone who has experienced a foul taste in the mouth when salvation is suppressed for a while, such as during a fever, or states of prolonged anxiety.
- Saliva is rich in bicarbonate buffers, which neutralize acids in food as well as acids produced by bacteria in the mouth, thereby helping t prevent dental caries (cavities)