Secretory functions of GIT The glands of the GIT have 2 main functions: 1- Secretion of digestive enzymes from the mouth to the ileum. 2- Secretion of mucus by the mucus secreting glands from the mouth to the anus, for lubrication of the content & protection of the mucosa.

Anatomical types of glands 1- Goblet cells, they are single cells present in all parts of GIT & secret mucus. 2- Invagination of the mucosa down into the submucosa forming pits, in the small intestine they are called Crypts of Lieberkühn 3- In the stomach & duodenum there are deep tubular glands. 4- Associated glands like salivary glands, pancreas & liver.

Stimulation of glandular secretion 1- By local contact of food with the epithelium, this is partly mediated by the enteric NS. 2- Autonomic stimulation; parasympathetic stimulation increases the secretion of salivary, esophageal, gastric, pancreatic & duodenal glands & the glands in the distal portion of large intestine. But secretions of the jejunum, ileum & The proximal 2/3rd of large intestine are controlled by local neural & hormonal stimuli.

Stimulation of the sympathetic system causes mild increase in the glandular secretion but because it reduces the blood supply to the glands, so it reduces the secretions of already stimulated gland. 3- Hormonal control.

Mucus It is thick secretion compose of water, electrolytes & glycoproteins. Advantages of Mucus: 1. Lubrication of food. 2. Protection of mucosa from irritation. 3. Easier passage of food. 4. Has buffering properties & can neutralize weak acids or bases.

Saliva It is secreted by the 2 parotids, 2 submandibular, 2 sublingual & many small glands in the buccal mucosa. Its daily secretion is about 800 – 1500 ml (about 0.5 ml / min.) The pH of saliva is 6.0 – 7.0 It contains 2 types of secretions: 1- Serous secretion: contains ptylin or α- amylase which digest starch. 2- Mucus secretion: containing mucin for lubrication & surface protection. Saliva is almost of the mucus type & secreted all the time except during sleep.

Importance of saliva for oral hygiene: The parotid secretion is purely serous, while others are mixed & the small buccal glands secret only mucus. Importance of saliva for oral hygiene: 1. Keeps the oral tissue moisten all the time. 2. Washes bacteria & remnants of food. 3. Contains substances that kill bacteria like lysozymes. 4. Contains immunoglobulins. 5. Contains enzymes for digestion of food. In the absence of saliva the mucosa of the mouth becomes dry with ulceration, infection & dental carries.

Neural control of salivary secretions 1- Parasympathetic stimulation increases salivary secretion. Sensory stimuli from the tactile & taste sensations (specially sour taste) travel to the superior & inferior salivatory nuclei at the junction between the pons & medulla. 2- Salivation can be stimulated or inhibited by nervous signals arriving the salivatory nuclei from the higher center, e.g when the person smells food.

3- Salivation increases in response to irritant foods in the stomach & intestine, especially when the person is nauseated, the swallowed saliva helps to dilute or neutralize the irritating substance. 4- Sympathetic stimulation also increases salivation moderately, but lesser than parasympathetic stimulation. The sympathetic nerves arise from superior cervical ganglia to the salivary glands.

Esophageal secretions It is entirely mucoid in character, they protect the mucosa & provides lubrication for swallowing.

Gastric secretions The mucosa of the stomach contains 3 types of glands: 1- The entire surface of the stomach is lined by mucus secreting cells. 2- The oxyntic or gastric glands, they are present at the fundus & body of the stomach, they secret HCl, pepsinogen, intrinsic factor & mucus. 3- The pyloric glands, they are present at the antrum, they secret mucus, some pepsinogen & gastrin.

Oxyntic (gastric) glands These are tubular glands containing 3 types of cells: 1- Mucus neck cells secreting mucus & some pepsinogen. 2- Peptic or chief cells secrete large quantities of pepsinogen. 3- The parietal cells (oxyntic cells) secretes acid & intrinsic factor.

Pepsinogen: is a protein which has no proteolytic activity, but when it becomes in contact with acid it is converted to pepsin which is an active proteolytic enzyme acts in acidic media at pH of 1.8 – 3.5. Intrinsic factor: is secreted with HCl & it is important for the absorption of vit. B12. In chronic gastritis the acid producing cells are destroyed & the patient develops megaloblastic anemia due to B12 deficiency.

Pyloric glands They secrete: 1- Mucus by neck mucus cells. 2- Little pepsinogen by some peptic cells. 3- Gastrin hormone by the G cells.

Surface mucus cells These cells secrete thick alkaline mucous secretion which forms a gel like layer of about 1 mm thickness to protect the mucosal cells & lubricate the food.

Regulation of gastric acid secretion Ach, histamine & gastrin are the basic stimulators of the glands of the stomach. Ach increases the secretion of all the cells & glands, i.e it increases HCl from the parietal cells, pepsinogen by the peptic cells & mucus by the mucus secreting cells. Gastrin & histamine stimulates mainly acid secretion.

Stimulation of acid secretion 1- Nervous stimulation: 2 types of reflexes are initiated; A- The long vagovagal reflexs. They are transmitted from the gastric mucosa to the brain stem & then back to the stomach through the vagus N. B- Short reflexes. They are originated locally & are transmitted entirely through the local enteric NS.

Types of stimuli that can initiate these reflexes are: 1) Distention of the stomach. 2) Tactile stimuli on the surface gastric mucosa. 3) Chemical stimuli like AA, peptides & acids that are already secreted by the gastric glands.

2- Role of gastrin: Both the nerve signals from the vagus & enteric reflexes cause the antral mucosa to secrete gastrin hormone, which is carried by the blood to the parietal cells to secrete HCl, also they are carried to the peptic cells to secrete pepsinogen but to lesser extent. 3- Role of histamine: Histamine is a very important co-factor for the action of Ach & gastrin, it acts through H2 receptors, so drugs that block these receptors (like Cimetidin) causes great reduction in acid secretion even in the presence of Ach & gastrin.

Regulation of pepsinogen 1- Release of Ach from the nerve endings stimulates the peptic cells to secrete pepsinogen. 2- Acid in the stomach stimulates peosinogen secretion.

Phases of gastric secretion 1- The cephalic phase: It results in secretion of about 20% of gastric secretion. It occurs in response to sighting, smelling or tasting food even before it enters the stomach. The stimuli originates from the cerebral cortex or from the appetite center in the hypothalamus, then transmitted through the vagus to the stomach.

2- The gastric phase: It accounts for 70% of gastric secretion. It occurs in response to excitation of the long vagovagal reflex, local enteric reflex & gastrin mechanism. 3- The intestinal phase: It occurs due to the presence of food in the duodenum, which leads to release of gastrin.

Pancreatic secretions The pancreas is a compound gland (endocrine & exocrine). The endocrine portion secretes insulin & glucagon. The exocrine portion secretes digestive enzymes, bicarbonate & water, they pass through the pancreatic duct to the common bile duct which opens into the duodenum through the ampulla of vater which is surrounded by the sphincter of Oddi.

Exocrine secretions 1- Enzymes secreted by the acini: a- Proteolytic enzymes: The most important are trypsin, chymotrypsin & carboxypeptidase, & less important are elastases & nucleases. They are concerned with the digestion of proteins.

b- Pancreatic amylases: They are concerned with digestion of starch, glycogen & other CHO except cellulose. c- Pancreatic lipases: They are concerned with the digestion of fat.

The pancreatic enzymes are secreted in an inactive forms (trypsinogen, chymotrypsinogen & procarboxypeptidase), they are activated when they enter the duodenum, they don’t digest the pancreatic cells because the pancreas secretes trypsin inhibitor with the secretion of trypsin. Trypsinogen is activated by enterokinases secreted by the intestinal mucosa when it contacts the chyme. Chymotrysinogen & procarboxypeptidase are activated by trypsin.

2- Bicarbonate & water: They are secreted by the ducts & ductules. They have 2 important functions; a- They neutralize the acidic chyme which enters the duodenum from the stomach. b- They provide appropriate pH for the action of pancreatic enzymes in the duodenum.