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.

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 ptyalin 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.

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.

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.

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.

Liver functions Storage of glycogen, iron, vit. B12, A & D. Filtration of blood. Conversion of galactose & fructose into glucose (gluconeogenesis). Fat metabolism: oxidation of FA to supply energy, formation of lipoproteins, synthesis of cholesterol (80% converted into bile salts) & phospholipids & conversion of proteins & CHO into fat. Protein metabolism: deamination of AA, formation of urea for removal of ammonia, formation of PP, synthesis of the non-essential AA.

6. formation of clotting factors. 7. Catabolism & excretion of drugs & hormones like thyroxine, cortisol, aldosterone, estrogen, & it is one of the major routes for excretion of Ca++.

Jaundice (Icterus) Increase concentration of free or conjugated bilirubin > 2 mg / dL causes jaundice. The skin, sclera & mucous membranes become yellow.

Causes of jaundice 1- Excess production of bilirubin (hemolytic anemia). 2- Decrease uptake of bilirubin by the hepatocytes. 3- Defect in conjugation. 4- Disturbance in secretion of the conjugated bilirubin into the bile canaliculi. 5- Intrahepatic or extrahepatic bile duct obstruction. 1, 2, 3------- unconjugated bilirubin is increased. 4, 5----------- conjugated bilirubin is increased. Conjugated= direct , unconjugated= indirect

Biliary secretions Functions of the bile: The liver secretes about 600 – 1200 ml of bile / day. Functions of the bile: 1- It plays important role in fat digestion & absorption. This is the function of bile acids which are present in the bile. 2- The bile serves as a mean for excretion of waste products from the blood like bilirubin & excess cholesterol synthesized by the liver.