Figure 3.4 Influence of flow on saliva composition
Saliva is primarily composed of water, mucus, proteins, mineral salts, and amylase3. As it circulates in the mouth cavity it picks up food debris, bacterial cells, and white blood cells. The composition of saliva depends upon the rate at which the different cell types contribute to the final secretion. The two types of secretion are mucous secretion, which is thick due to a glycoprotein called mucin, and watery secretion which contains salivary amylase. The major ions are Na+, K+, Cl- and HCO3-. In the ducts of the salivary glands, sodium and chloride are reabsorbed, but potassium and bicarbonate are secreted (Figure 3.4) and hence the electrolyte balance is altered depending upon the rate of flow of saliva.
Apart from the enzyme a-amylase, ptyalin is also present. This enzyme begins to hydrolyse polysaccharides such as glycogen and starch to smaller saccharides. The enzyme acts at an optimum pH of 6.9, but is stable within the range 4 to 11 and hence it will continue to act until the food is acidified by gastric acid. The time of contact in the mouth is too short for digestion to occur but the enzyme may prevent accumulation of starchy material in the gaps between teeth. Lingual lipase is responsible for hydrolysis of triglycerides. It is extremely hydrophobic and its digestive action continues in the stomach. A variety of esterases, mainly carboxylesterases are also present in the saliva and these may reduce the concentration of ester prodrugs or drugs containing susceptible ester groups4.
Saliva lubricates and moistens the inside of the mouth to help with speech and to change food into a liquid or semisolid mass that can be tasted and swallowed more easily. The salivary film thickness is estimated to be between 0.07 and 0.10 mm5. It also helps to control the body's water balance; if water is lacking, the salivary glands become dehydrated, leaving the mouth dry producing a sensation of thirst thus stimulating the need to drink. The flow of saliva helps to wash away the bacteria and the food particles which act as their nutrient, into the acidic environment of the stomach where they are digested. Saliva also contains thiocyanate, and protein antibodies and lysozyme which destroy bacteria. In the absence of saliva, oral ulcerations occur and dental caries becomes extremely prevalent. This condition, xerostomia, can be treated with artificial saliva formulations, which are based on materials such as hydroxypropylmethylcellulose and more recently, pig gastric mucin.
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