Saliva is a mixed secretion of the parotid, submaxillary, sublingual, and buccal glands. The saliva of man and many other animals is a viscous, colourless, cloudy and opalescent liquid.
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The average pH value is 6 8 with a range of 5*6 to 7″6 and specific gravity is 1 -002 to 1 ‘008. It is constantly secreted-in small quantities to keep the buccal cavity moist, but when food is present the rate of secretion is increased because the saliva not only moistens the food, lubricating its subsequent passage through the alimentary canal but also begins the process of digestion.
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It contains 98.5 to 99% water and 1 to 1.5% of a dense residue which includes the following:
(i) Cellular components:
The cellular components of saliva are desquamated epithelial cells of the oral mucosa, leukocytes and numerous bacteria. Yeast cells and sometimes protozoans like Amoeba salivaricus are also found.
(ii) Inorganic components:
Inorganic components of saliva include chlorides, sulphates, carbonates of sodium, potassium, calcium and magnesium and traces of ammonia as well.
The saliva of man and some animals also has potassium sulphocyanate about 0 01%.
(iii) Organic components:
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Organic components of the saliva are chiefly mucin (which gives saliva its viscosity and lubricating properties) and enzymes which digest the food.
Some other organic substances, such as urea, cholesterol, amino-acids, citrates and vitamins, have also been found in saliva. Glucose is practically absent.
Enzymes of saliva:
Saliva contains a large number of enzymes such as amylase, lysosome acid phosphotase, aldolase, cholesterase, lysozyme, maltase, catalase, lipase, urease and protease of these probably only two namely amylase and lysozyme are of physiological importance.
Amylase:
The saliva of cat, dog and some other animals lack salivary amylase, while the saliva of other animals such as man, pig and rats has strong amylase activity.
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Salivary amylase was once named ptyalin, to distinguish it from pancreatic amylase (diastase), but these two enzymes are now known to be identical. It hydrolyses starch and glycogen to maltose, isomaltose, dextrin and some glucose.
Lysozyme:
This enzyme is polysaccharidase which hydrolyses certain complex polysaccharides present in the cell wall of many different species of bacteria (e.g., Micrococcus lysodeikticus), thereby killing and dissolving them. Its action is partially retarted by the presence of mucin.