Functions of the oral mucosa
The oral mucosa has similarities to both skin and intestinal mucosa. It has a protective role during the process of mastication, which exposes the mucosa to compression and shear forces. Areas such as the hard palate and attached gingiva have a horny surface to resist abrasion and are tightly bound to the underlying bone to resist shear forces. The cheek mucosae, on the other hand, are elastic to allow for distension.
The oral cavity contains the greatest variety of micro-organisms present within the human body. The entry into the body of these organisms and any potential toxic waste product is limited by the oral epithelium, which is not, as is often suggested, a highly permeable membrane.
The oral mucosa responds to the senses of pain, touch, and temperature in addition to its unique sense of taste. Some physiological processes are triggered by sensory input from the mouth, such as the initiation of swallowing, gagging and retching.
In some animals the oral mucosa is used to aid thermoregulation, for example, panting in the dog. The human skin possesses sweat glands and a more highly controlled peripheral vasculature, so this role is thought to be minimal, although in sleep, dehydration can result from prolonged breathing through the mouth.
The major salivary glands are the parotid, submandibular (submaxillary) and sublingual glands (Figure 3.3). Minor salivary glands are situated in or immediately below the oral mucosa in the tongue, palate, lips, and cheeks. The major glands are situated some way from the oral cavity, but open into it by a long duct. The parotid salivary glands, the largest of the three, are located between the ear and ascending branch of the lower jaw. Each gland is enclosed in a tissue capsule and is composed of fat tissue and secretory cells and the major duct (Stensen's duct) opens near the second upper molar. The second pair, the submaxillary
Figure 3.3 Position of the salivary glands
Figure 3.3 Position of the salivary glands glands, is located along the side of the lower jawbone. The major duct of each (Wharton's duct) opens into the floor of the mouth at the junction where the front of the tongue meets the mouth's floor. A capsule of tissue also surrounds each of these glands. The third pair, the sublingual glands, is situated beneath the mucous membrane of the floor of the mouth, near the chin region. They are not covered by a capsule and are therefore more dispersed throughout the surrounding tissue. They have many ducts (Rivinus's ducts) that empty near the junction of the tongue and the mouth's floor; several unite to form Bartholin's duct which empties into or near the submaxillary duct. The parotid and submaxillary glands produce watery secretion, while the buccal and sublingual glands produce mainly viscous saliva.
One to two litres of fluid are excreted daily into the human mouth and there is a continuous, low basal secretion of 0.5 ml.min-1 which will rapidly increase to more than 7 ml.min-1 by the thought, smell or taste of food. Control over salivary secretion is exerted primarily via the parasympathetic system. Small amounts of saliva are continually being secreted into the mouth, but the presence of food, or even the smell or thought of it, will rapidly increase saliva flow. Saliva is viscous, colourless and opalescent, hypotonic compared to plasma (between 110 and 220 milliOsmoles per litre), with a specific gravity of about 1003. The pH varies between 7.4 to 6.2 (low to high rates of flow), but the action of bacteria on sugar can reduce the pH to between 3 and 4 around the teeth.
Saliva can be detected in the oral cavity soon after birth. Salivary secretion increases up to the age of 3 to 5 years, but then sharply declines, reaching a steady state by the age of 8 years. In adult females, the flow rate of saliva is somewhat lower than in males2.
160 mOsm kg
200 mOsm kg
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