Functional Anatomy of the Salivary Glands
There are three pairs of major salivary glands: parotid, submandibular, and sublingual. In addition, many smaller glands are found on the tongue, lips, and palate. These glands are the typical tubuloalveolar structures of glands located in the GI tract (
Fig. 28.1 ). The acinar portion of the gland is classified according to its major secretion: serous (“watery”), mucous, or mixed. The parotid gland produces mainly serous secretion, the sublingual gland secretes mainly mucus, and the submandibular gland produces a mixed secretion.
Cells in the secretory end pieces, or acini, are called acinar cells and are characterized by basally located nuclei, abundant rough endoplasmic reticulum, and apically located secretory granules that contain the enzyme amylase and other secreted proteins. There are also mucous cells in the acinus; the granules in these cells are larger and contain the specialized glycoprotein mucin. There are three kinds of ducts in the gland that transport secretions from the acinus to the opening in the mouth and also modify the secretion: intercalated ducts drain acinar fluid into larger •BOX 28.1FunctionsofSalivaandChewing
Disruption of food to produce smaller particles Formation of a bolus for swallowing Initiation of starch and lipid digestion Facilitation of taste Production of intraluminal stimuli in the stomach Regulation of food intake and ingestive behavior Cleansing of the mouth and selective antibacterial action Neutralization of refluxed gastric contents Mucosal growth and protection in the rest of the GI tract Aid in speech 522 SECTION6Berne & Levy Physiology • Fig.28.1General structure of tubuloalveolar secretory glands (e.g., salivary glands, pancreas) associ-ated with the digestive tract. Demilune of serous cells Serous cell Basement membrane Intercalated duct Mucous cell Salivary duct (secretory) ducts, the striated ducts, which then empty into even larger excretory ducts. In addition, a single large duct from each gland drains saliva to the mouth. The ductal cells lining the striated ducts, in particular, modify the ionic composition and osmolarity of saliva.
Composition of Saliva
The important properties of saliva are a large flow rate relative to the mass of gland, low osmolarity, high K+ concentration, and organic constituents, including enzymes (amylase, lipase), mucin, and growth factors. The latter are not important in the integrated response to a meal but are essential for long-term maintenance of the lining of the GI tract.
The inorganic composition is entirely dependent on the stimulus and the rate of salivary flow. In humans, salivary secretion is always hypotonic. The major components are − , Ca++ , Mg++
Na+ , K+ , HCO3 , and Cl− . Fluoride can be secreted in saliva, and fluoride secretion forms the basis of oral fluoride treatment for prevention of dental caries. The concentration of ions varies with the rate of secretion; the flow rate of salivary secretion is stimulated during the postprandial period.