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Salivary Ion Secretion Mechanisms

Saliva contains high concentrations of potassium and bicarbonate ions but low concentrations of sodium and chloride ions. This special ion composition arises from two active transport processes in salivary ducts. Sodium is actively reabsorbed and potassium secreted in its place. Excess sodium reabsorption creates a negative charge pulling chloride into ducts. Meanwhile, bicarbonate is secreted into duct lumens in exchange for chloride. These processes result in saliva with lower sodium and chloride but higher potassium and bicarbonate than plasma. During high salivation, duct modification is reduced, increasing sodium/chloride while decreasing potassium enrichment versus plasma levels.

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321 views5 pages

Salivary Ion Secretion Mechanisms

Saliva contains high concentrations of potassium and bicarbonate ions but low concentrations of sodium and chloride ions. This special ion composition arises from two active transport processes in salivary ducts. Sodium is actively reabsorbed and potassium secreted in its place. Excess sodium reabsorption creates a negative charge pulling chloride into ducts. Meanwhile, bicarbonate is secreted into duct lumens in exchange for chloride. These processes result in saliva with lower sodium and chloride but higher potassium and bicarbonate than plasma. During high salivation, duct modification is reduced, increasing sodium/chloride while decreasing potassium enrichment versus plasma levels.

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bhumiparikh
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Secretion of Ions in Saliva. Saliva contains especially large quantities of potassium and bicarbonate ions.

Conversely, the concentrations of both sodium and chloride ions are several times less in saliva than in plasma. One can understand these special concentrations of ions in the saliva from the following description of the mechanism for secretion of saliva. Figure 642 shows secretion by the submandibular gland, a typical compound gland that contains acini and salivary ducts. Salivary secretion is a two-stage operation: the first stage involves the acini, and the second, the salivary ducts. The acini secrete a primary

secretion that contains ptyalin and/or mucin in a solution of ions in concentrations not greatly different from those of typical extracellular fluid. As the primary secretion flows through the ducts, two major active transport processes take place that markedly modify the ionic composition of the fluid in the saliva. First, sodium ions are actively reabsorbed from all the salivary ducts and potassium ions are actively secreted in exchange for the sodium. Therefore, the sodium ion concentration of the saliva becomes greatly reduced, whereas the potassium ion concentration becomes increased. However, there is excess

sodium reabsorption over potassium secretion, and this creates electrical negativity of about -70 millivolts in the salivary ducts; this in turn causes chloride ions to be reabsorbed passively.Therefore, the chloride ion concentration in the salivary fluid falls to a very low level, matching the ductal decrease in sodium ion concentration. Second, bicarbonate ions are secreted by the ductal epithelium into the lumen of the duct. This is at least partly caused by passive exchange of bicarbonate for chloride ions, but it may also result partly from an active secretory process. The net result of these transport processes is that

under resting conditions, the concentrations of sodium and chloride ions in the saliva are only about 15 mEq/ L each, about one seventh to one tenth their concentrations in plasma. Conversely, the concentration of potassium ions is about 30 as in plasma; and the concentration of bicarbonate ions is 50 to 70 mEq/L, about two to three times that of plasma. During maximal salivation, the salivary ionic concentrations change considerably because the rate of formation of primary secretion by the acini can increase as much as 20-fold. This acinar secretion then flows through the ducts so rapidly that the ductal

reconditioning of the secretion is considerably reduced. Therefore, when copious quantities of saliva are being secreted, the sodium chloride concentration rises only to one half or two thirds that of plasma, and the potassium concentration rises to only four times that of plasma.mEq/L, seven times as great

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