بسم الله الرحمن الرحيم
Lecture -6- Medical Physiology (GIT system)
2nd stage Dr. Noor Jawad
Secretory Functions of the Alimentary Tract
Objectives of our lecture:
Functions of salivary secretions?
Phases of gastric secretion?
Throughout the gastrointestinal tract, secretory glands subserve two primary functions:
digestive enzymes are secreted in most areas of the alimentary tract, from the mouth to the distal end of the ileum.
mucous glands located from the mouth to the anus provide mucus for lubrication and protection of all parts of the alimentary tract.
SECRETION OF SALIVA
Saliva Contains a Serous Secretion and a Mucus Secretion. The principal glands of salivation are the parotid, submandibular, and sublingual glands; in addition, there are many tiny buccal glands. Daily secretion of saliva normally ranges between 800 and 1500 milliliters, as shown by the average value of 1000 milliliters in Table below.
Saliva contains two major types of protein secretion: (1) a serous secretion that contains ptyalin (an α-amylase), which is an enzyme for digesting starches, and (2) mucus secretion that contains mucin for lubricating and for surface protective purposes.
The parotid glands secrete almost entirely the serous type of secretion, whereas the submandibular and sublingual glands secrete both serous secretion and mucus.
The buccal glands secrete only mucus. Saliva has a pH between 6.0 and 7.0, which is a favorable range for the digestive action of ptyalin.
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. 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 mEq/L, seven times as great as in plasma, and the concentration of bicarbonate ions is 50 to 70 mEq/L, about two to three times that of plasma.
Therefore, when copious quantities of saliva are being secreted, the sodium chloride concentration is about one half or two thirds that of plasma, and the potassium concentration rises to only four times that of plasma.
Regulation of salivary secretion
Salivation can also be stimulated or inhibited by nervous signals arriving in the salivatory nuclei from higher centers of the central nervous system.
Sympathetic stimulation can also increase salivation a slight amount—much less so than parasympathetic stimulation.
The sympathetic nerves originate from the superior cervical ganglia and travel along the surfaces of the blood vessel walls to the salivary glands.
A secondary factor that also affects salivary secretion is the blood supply to the glands because secretion always requires adequate nutrients from the blood. The parasympathetic nerve signals that induce copious salivation also moderately dilate the blood vessels. In addition, salivation directly dilates the blood vessels, thus providing increased salivatory gland nutrition as needed by the secreting.
Function of Saliva for Oral Hygiene.
Under basal awake conditions, about 0.5 milliliter of saliva, almost entirely of the mucous type, is secreted each minute; however, during sleep, little secretion occurs. This secretion plays an exceedingly important role for maintaining healthy oral tissues. The mouth is loaded with pathogenic bacteria that can easily destroy tissues and cause dental caries. Saliva helps prevent the deteriorative processes in several ways:
1. The flow of saliva helps wash away pathogenic bacteria, as well as food particles that provide their metabolic support.
2. Saliva contains several factors that destroy bacteria. One of these is thiocyanate ions and another is several proteolytic enzymes—most important, lysozyme—that (a) attack the bacteria, (b) aid the thiocyanate ions in entering the bacteria where these ions in turn become bactericidal, and (c) digest food particles, thus helping further to remove the bacterial metabolic support.
3. Saliva often contains significant amounts of antibodies that can destroy oral bacteria, including some that cause dental caries.
GASTRIC SECRETION
CHARACTERISTICS OF THE GASTRIC SECRETIONSIn addition to mucus-secreting cells that line the entire surface of the stomach, the stomach mucosa has two important types of tubular glands: oxyntic glands (also called gastric glands) and pyloric glands.
The oxyntic (acid-forming) glands secrete hydrochloric acid, pepsinogen, intrinsic factor, and mucus.
The pyloric glands secrete mainly mucus for protection of the pyloric mucosa from the stomach acid. They also secrete the hormone gastrin.
The oxyntic glands are located on the inside surfaces of the body and fundus of the stomach—the proximal 80 percent of the stomach. The pyloric glands are located in the antral portion of the stomach—the distal 20 percent of the stomach.
Secretions from the Oxyntic (Gastric) Glands A typical stomach oxyntic gland. It is composed of three types of cells: (1) mucous neck cells, which secrete mainly mucus; (2) peptic (or chief) cells, which secrete large quantities of pepsinogen; and (3) parietal (or oxyntic) cells, which secrete hydrochloric acid and intrinsic factor.
Phases of Gastric Secretion Gastric secretion
is said to occur in three “phases” (as shown in Figure): a cephalic phase, a gastric phase, and an intestinal phase.
1.Cephalic Phase.
The cephalic phase of gastric secretion occurs even before food enters the stomach, especially while it is being eaten. It results from the sight, smell, thought, or taste of food, and the greater the appetite, the more intense is the stimulation. Neurogenic signals that cause the cephalic phase of gastric secretion originate in the cerebral cortex and in the appetite centers of the amygdala and hypothalamus.
They are transmitted through the dorsal motor nuclei of the vagi and thence through the vagus nerves to the stomach. This phase of secretion normally accounts for about 30 percent of the gastric secretion associated with eating a meal.
2. Gastric Phase.
Once food enters the stomach, it excites (1) long vagovagal reflexes from the stomach to the brain and back to the stomach, (2) local enteric reflexes, and (3) the gastrin mechanism, all of which cause secretion of gastric juice during several hours while food remains in the stomach. The gastric phase of secretion accounts for about 60 percent of the total gastric secretion associated with eating a meal and therefore accounts for most of the total daily gastric secretion of about 1500 milliliters.
Intestinal Phase.
The presence of food in the upper portion of the small intestine, particularly in the duodenum, will continue to cause stomach secretion of small amounts of gastric juice, probably partly because of small amounts of gastrin released by the duodenal mucosa. This secretion accounts for about 10 percent of the acid response to a meal.
