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Functions of Saliva

The lubricating ability of saliva depends primarily on its content of mucus. I n the mouth, mixing saliva with food lubricates the ingested material and facilitates the swallowing process. The lubricating effect of saliva is also necessary for speech, as evidenced by the glass of water normally found on the podium of a public speaker. Saliva exerts its effects through a variety of different mechanisms. I t protects the mouth by buffering and diluting noxious substances. Hot solutions of tea, coffee, or soup, for example, are diluted and cooled by saliva. Foul-tasting substances can be washed from the mouth by copious salivation. Similarly, the salivary glands are stimulated strongly before vomiting. The corrosive gastric acid and pepsin that are brought up into the esophagus and mouth are thereby neutralized and diluted by saliva. Dry mouth, or xerostomia, is associated with chronic infections of the buccal mucosa and with dental caries. Saliva dissolves and washes out food particles from between the teeth. Certain specialized constituents of saliva have antibacterial actions. These include the following: a lysozyme, which aacks bacterial cell walls; lactoferrin, which chelates iron and thus prevents the multiplication of organisms that require it for growth; and the binding glycoprotein for immunoglobulin A (I gA), which in combination with I gA forms secretory I gA, which in turn is immunologically active against viruses and bacteria. Various inorganic compounds are taken up by the salivary glands, concentrated, and secreted in saliva. These include fluoride and calcium (Ca2+), which are subsequently incorporated into the teeth. The contributions made by saliva to normal digestion include dissolving and washing away food particles on the taste buds to enable one to taste the next morsel of food eaten. Saliva contains two enzymes, one directed toward carbohydrates and the other toward fat. An α-amylase called ptyalin cleaves internal α-1,4-glycosidic bonds present in starch. Exhaustive digestion of starch by this enzyme, identical to pancreatic amylase, produces maltose, maltotriose, and α-limit dextrins, which contain the α-1,6 branch points of the original molecule. Salivary amylase has a pH optimum of 7, and it is rapidly denatured at
pH 4. However, because a large portion of a meal oen remains unmixed for a considerable length of time in the orad stomach, the salivary enzyme may account for the digestion of as much as 75% of the starch present before it is denatured by gastric acid. I n the absence of salivary amylase, there is no defect in carbohydrate digestion because the pancreatic enzyme is secreted in amounts sufficient to digest all the starch present. The serous salivary glands of the tongue secrete the second digestive enzyme, lingual lipase, which plays a role in the hydrolysis of dietary lipid. Unlike pancreatic lipase,
lingual lipase has properties that allow it to act in all parts of the upper GI tract. Thus the ability of lingual lipase to hydrolyze lipids is not affected by surface-active detergents such as bile salts, medium chain fay acids, and lecithin. I t has an acidic pH optimum and remains active through the stomach and into the intestine.

About dr.Dukagjin Zeqiraj

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