As stated earlier, digestion is a process that breaks down food to sizes that can be absorbed by cells. There are two components to the process: physical and chemical. The physical aspects of digestion in the mouth occur when we bite, tear, and chew food. This breaks the food into smaller, but still fairly large, chunks of food. Chemical digestion in the mouth involves saliva. Most of the saliva is secreted by three pairs of salivary glands: the parotid, submandibular, and sublingual. These glands are found under the tongue and in front of the masseter muscle, the muscle that provides most of the force in biting. The parotid glands produce a watery secretion, the sublingual glands produce a very mucoid secretion, and the submandibular glands make a combination of a watery and mucoid secretion. The combination of secretions from these glands makes up normal saliva. If a person is dehydrated, the saliva produced is thick and comes mostly from the sublingual glands.
Saliva has several purposes, including cleansing the mouth, dissolving food to enhance taste, moistening food to form a bolus, and starting the chemical digestion of starches and some lipids.
Two enzymes in the saliva help chemically digest food: amylase and lingual lipase. Amylase starts the breakdown of starch by breaking the complex structure of starch into smaller combinations of glucose units that are separated further along the digestive tube. Because amylase works at an alkaline pH, saliva contains bicarbonate to maintain these conditions.
Lingual lipase is one of the body's forms of lipase that separates triglycerides into its components of glycerol and fatty acids.
A person produces about 1-1.5 quarts (about 1-1.5 liters) of saliva per day. The salivary glands produce saliva when stimulated by the presence of food in the mouth or by certain acidic foods, such as vinegar or citric juices. Sometimes, the thought or smell of food will result in saliva release. If the small or large intestine is irritated by some substance, such as excess acids or bacterial toxins, the salivary glands will release saliva.
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