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Several types of glands provide the different types of secretions in the alimentary tract.

Effect of Contact of Food with the Epithelium--Function of Enteric Nervous Stimuli.
The mechanical presence of food in a particular segment of the gastrointestinal tract usually causes the glands of that region and often of adjacent regions to secrete moderate to large quantities of juices.
Part of this local effect, especially the secretion of mucus by mucous cells, results from direct contact stimulation of the surface glandular cells by the food.
In addition, local epithelial stimulation also activates the enteric nervous system of the gut wall.
The types of stimuli that do this are:
(1) tactile stimulation,
(2) chemical irritation,
and (3) distention of the gut wall.
The resulting nervous reflexes stimulate both the mucous cells on the gut epithelial surface and the deep glands in the gut wall to increase their secretion.

Autonomic Stimulation of Secretion
Parasympathetic Stimulation.
Stimulation of the parasympathetic nerves to the alimentary tract almost invariably increases the rates of alimentary glandular secretion.
This is especially true of the glands in the upper portion of the tract innervated by the glossopharyngeal and vagus parasympathetic nerves, such as the salivary glands, esophageal glands, gastric glands, pancreas, and Brunner's glands in the duodenum.
It is also true of the glands of the distal portion of the large intestine, innervated by the pelvic parasympathetic nerves. Secretion in the remainder of the small intestine and in the first two thirds of the large intestine occurs mainly in response to local neural and hormonal stimuli in each segment of the gut.

Sympathetic Stimulation.
Stimulation of sympathetic nerves in some parts of the gastrointestinal tract causes a slight to moderate increase in secretion by some of the local glands.
Conversely, sympathetic stimulation also results in constriction of the blood vessels that supply the glands.
Therefore, sympathetic stimulation can have a dual effect:
First, sympathetic stimulation alone usu- ally slightly increases secretion.
But,second, if parasympathetic or hormonal stimulation is already causing copious secretion by the glands, superimposed sympathetic stimulation usually reduces the secretion, sometimes significantly so, mainly because of vasoconstrictive reduction of the blood supply.

Regulation of Glandular Secretion by Hormones.
In the stomach and intestine, several different gastrointestinal hormones help regulate the volume and character of the secretions.
These hormones are liberated from the gastrointestinal mucosa in response to the presence of food in the lumen of the gut.
The hormones then are absorbed into the blood and carried to the glands, where they stimulate secretion.
This type of stimulation is particularly valuable to increase the output of gastric juice and pancreatic juice when food enters the stomach or duodenum. Also, hormonal stimulation of the gallbladder wall causes it to empty its stored bile into the duodenum.
Other hormones that are still of doubtful effectiveness have been postulated to stimulate secretion by the glands of the small intestine.
Chemically, the gastrointestinal hormones are polypeptides or polypeptide derivatives.

Salivary Glands; Characteristics of Saliva. The principal glands of salivation are the parotid, submandibular, and sublingual glands; in addition, there are many small buccal glands. The daily secretion of saliva normally ranges between 800 and 1500 milliliters, by the average value of 1000 milliliters.

Saliva contains two major types of protein secretion:

(1) a serous secretion that contains ptyalin (an c-amylase), which is an enzyme for digesting starches,
and (2) mucous secretion that contains mucin for lubricating and for surface protective purposes.

The parotid glands secrete entirely the serous type of secretion, and the submandibular and sublingual glands secrete both the serous type and mucus. The buccal glands secrete only mucus.

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 com- position 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 concentration of the saliva becomes greatly reduced, whereas the potassium ion concentration becomes increased. However, there is excess of 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 exchange of bicarbonate for chloride ions, but it may also result partly from an active secre- tory 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 mEqfL, seven times as great as its concentration in plasma; and the concentration of bicarbonate ions is 50 to 70 mEqfL, 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 secreted, the sodium chloride concentration rises to about one half to two thirds that of plasma, and the potassium concentration falls to only four times that of plasma.

In the presence of excess aldosterone secretion, sodium and chloride reabsorption and potassium secretion become greatly increased, so that the sodium chloride concentration in the saliva is sometimes reduced almost to zero while potassium concentration increases even higher than the normal seven times plasma potassium level.

Salivation can also be stimulated or inhibited by nervous signals arriving in the salivatory nuclei from higher centers of the central nervous system. For instance, when a person smells or eats favorite foods, salivation is greater than when disliked food is smelled or eaten. The appetite area of the brain, which partially regulates these effects, is located in proximity to the parasympathetic centers of the anterior hypothalamus, and it functions to a great extent in response to signals from the taste and smell areas of the cerebral cortex or amygdala.
Salivation also occurs in response to reflexes originating in the stomach and upper intestines--particularly when irritating foods are swallowed or when a person is nauseated because of some gastrointestinal abnormality.
The swallowed saliva presumably helps to remove the irritating factor in the gastrointestinal tract by diluting or neutralizing the irritating substances.

Sympathetic stimulation can also increase salivation a moderate amount, but much less so than does parasympathetic stimulation. The sympathetic nerves originate from the superior cervical ganglia and travel along the blood vessels to the salivary glands.

A secondary factor that also affects salivary secretion is the blood supply to the glands because secretion always requires adequate nutrition.
The parasympathetic nerve signals that induce copious salivation also moderately dilate the blood vessels.
In addition, salivation itself directly dilates the blood vessels, thus providing increased nutrition as it is needed by the secreting cells.
Part of this additional vasodilator effect is caused by kallikrien secreted by the activated salivary cells, which in turn acts as an enzyme to split one of the blood proteins, an alpha2-globulin, to form bradykinin, a strong vasodilator.