The functions of saliva fall into three general categories: digestion, lubrication, and protection. Saliva is produced in large volumes, relative to the weight of the salivary glands, by an active process. Unlike the process in the other gastrointestinal glands, the secretion of saliva is almost totally under neural control. Both branches of the autonomic nervous system stimulate salivary secretion, although the para-sympathetic system provides a much stronger input. The healthy adult secretes approximately 1 L of saliva per day.
The digestive actions of saliva are the results of two enzymes, one directed toward carbohydrates and the other toward fat. Saliva contains an a-amylase, called ptyalin, that cleaves internal a-l,4-glycosidic bonds. The enzyme is identical to pancreatic amylase, and the products of exhaustive digestion are maltose, malto-triose, and a-limit dextrins, which contain the a-1,6 branch points of the starch molecule. The pH optimum for the enzyme is 7, and it is rapidly denatured when exposed to gastric juice at a pH of less than 4. However, because a large portion of a meal often remains unmixed in the orad stomach, salivary amylase is able to act within this mass of ingested material and digest up to 75% of the starch before being mixed with acid. Without salivary amylase, there is no deficiency in carbohydrate digestion, because the pancreatic enzyme is secreted in sufficient amounts to digest all of the starch present in the small intestine.
Lingual lipase is secreted by salivary glands of the tongue and begins the digestion of triglycerides. Its acidic pH optimum allows it to remain active throughout the stomach and into the proximal duodenum.
Although this is not strictly a digestive function, saliva dissolves many dietary constituents. This process of solubilization increases the sensitivity of the taste buds. Saliva also washes food particles from the taste buds, so that subsequently ingested material can be tasted.
The lubricating properties of saliva are due primarily to its mucus content. Chewing mixes saliva with ingested material, thoroughly lubricating it and facilitating the swallowing process. The lubricated bolus moves more easily down the esophagus. Lubrication provided by saliva is also necessary for speech.
Saliva protects the mouth by diluting and buffering harmful substances. Hot solutions of tea, coffee, or soup, for example, are diluted by saliva and their temperatures lowered. Foul-tasting substances can eventually be washed out of the mouth by copious salivation. Before vomiting, salivation is stimulated strongly. This saliva dilutes and neutralizes corrosive gastric juice, preventing damage to the mouth and esophagus. Dry mouth, or xerostomia, is associated with chronic infections of the buccal mucosa and with dental caries. Saliva dissolves and washes food particles from between the teeth. Saliva also contains a number of organic substances that are bacteriocidal. These include a lysozyme, which attacks bacterial cell walls; the binding glycoprotein for immuno-globulin A (IgA), which together with IgA forms secretory IgA, which in turn is immunologically active against bacteria and viruses; and lactoferrin, which chelates iron, preventing access by organisms that require iron for growth. Various compounds such as fluoride and calcium phosphate are taken up by the salivary glands and secreted in the saliva in concentrated amounts. These in turn may be incorporated into the teeth.
The major salivary glands are three paired structures that deliver their secretions into the mouth through ducts. The largest of these are the parotid glands, located between the angle of the jaw and the ear; the submaxillary glands are located below the angle of the jaw; and the sublingual glands, as their name implies, are found below the tongue. The parotid glands are made up only of serous cells and secrete a watery fluid. The other two pairs are mixed glands containing cells that secrete mucin glycoprotein as well. Smaller salivary glands occur within the mucosa of the tongue, lips, palate, and other areas of the buccal cavity.
The microscopic structure of the salivary glands is similar to that of the pancreas and analogous to a bunch of grapes. A single grape corresponds to the acinus, which is the blind end of a branching duct system and is made up of a group of cells called acinar cells (Fig. 1). The acinar cells secrete the initial salivary fluid, consisting of electrolytes, mucus, and enzymes. From the acinus, saliva passes relatively unchanged through a short, intercalated duct and into the striated duct. The striated duct is lined by columnar epithelial cells that function like renal tubule cells to modify the inorganic composition of saliva. The combination of the acinus, intercalated duct, and striated duct represent the secretory unit or salivon of the salivary gland. The basement membranes of the acini and intercalated ducts are covered in part by specialized contractile cells called myoepithelial cells. These cells are shaped somewhat like stars, and the motile extensions contain actin and myosin. Contraction of the myoepithelial cells occurs
when salivary secretion is stimulated, and results in the rapid expulsion of saliva into the mouth. Contraction of these cells expels saliva from the acinus, shortens and widens the intercalated duct, and prevents distension of the acinus. Similar cells can be found in the mammary gland and pancreas.
The salivary glands receive a high blood flow proportional to their weight. The direction of the arterial flow is opposite to the flow of saliva through the salivon. Separate capillary beds, the vessels of which appear to be parallel to each other and the salivons, supply the ductules and the acini. These capillaries are extremely permeable, allowing rapid movement of water and other molecules across their basement membranes. The rate of blood flow through resting salivary tissue is approximately 20 times that through muscle. This, in part, accounts for the ability of these glands to produce prodigious amounts of saliva relative to their weight.
The salivary glands are innervated by both components of the autonomic nervous system. Parasympathetic innervation is delivered by the facial and glossopharyngeal nerves, whereas sympathetic innervation is from thoracic spinal nerves via the superior cervical ganglion. The autonomic nervous system regulates the secretion, blood flow, and growth of the salivary glands.
Saliva is primarily a mixture of water, electrolytes, and some organic compounds. The major characteristics of saliva are (1) its large volume relative to the mass of the salivary glands, (2) its high potassium concentration, (3) its low osmolarity, and (4) the specialized organic materials it contains.
Saliva can be elaborated in large volumes compared with the secretions of similar organs. Maximal secretory rates may be as high as 1 mL per gram of gland per minute, which is comparable to the secretory rate for the entire pancreas. On the basis of tissue weight, this amounts to a 50-fold higher rate of secretion.
At all secretory rates except the highest, saliva is significantly hypotonic to plasma. As the rate of secretion increases, the osmolarity of saliva increases and approaches isotonicity at maximal rates.
The concentrations of electrolytes in saliva vary with the rate of secretion (Fig. 2). The K+ concentration is always greater than that found in plasma, indicating that the salivary glands secrete K+ against its electrochemical gradient. As the flow rate of saliva increases, the K+ concentration in the fluid decreases, plateaus, and remains relatively constant at higher flow rates. In
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