Gastrointestinal Physiology PDF

Gastrointestinal Physiology By the end of this lecture, you should be able to: 1. Discuss general functions of the digestive system , the basic digestive processes and regulation of gut functions. 2. Outline functions of saliva. 3. Discuss functions of the stomach and its protective mechanism...

Gastrointestinal Physiology By the end of this lecture, you should be able to: 1. Discuss general functions of the digestive system , the basic digestive processes and regulation of gut functions. 2. Outline functions of saliva. 3. Discuss functions of the stomach and its protective mechanisms. 4. Discuss digestive roles of pancreatic juice and bile. 5. Describe briefly the digestion and absorption of carbohydrate, protein and fat. 6. Recognize functions of the large intestine. 7. Give a brief account on gut hormones.  Components of the Digestive system It consists of : 1-Alimentary canal (starts at the mouth and ends at the anus) 2-Accessory digestive glands (located outside the gastrointestinal tract, however, they empty their secretions into the tract) 1. salivary glands. 2. Liver. 3. Gallbladder. 4. exocrine pancreas. 3- Glands in the wall of the stomach and small intestine  General functions of the Gut 1. Food digestion and absorption. Digestion is the breakdown of complex molecules into simple ones to facilitate their absorption. Digestion needs the secretion of digestive juices and wall motility. 2. Excretion of waste products (undigested & unabsorbed food) 3. Immune function. The gut wall contain lymphoid tissue which participate in immune reactions  Structure of the digestive tract wall 1) Mucosa: it lines the luminal surface; it contains exocrine and endocrine cells. 2) The submucosa: This is formed of connective tissue, blood vessels, and lymph vessels. 3) The muscularis externa: inner circular and outer longitudinal layer with myenteric plexus in between. 4) Serosa: outer protective layer Function of digestive system is performed by 4 basic processes:- 1. Secretion 2. Motility 3. Digestion 4. Absorption There are exocrine glands located along the wall of the gut as well as the three accessory glands (salivary glands, pancreas and liver) that secrete digestive juices that contain water , electrolytes, mucus and different enzymes. Movement of food in the digestive tract is accomplished by smooth muscle contraction ( involuntary control). Types of Gut motility 1. Propulsive (peristaltic) movement. 2. Mixing (segmenting) movement.  1- Propulsive movement, seen in the esophagus & the intestine: Function: forward movement of food at an appropriate rate for digestion and absorption 2-Mixing movement: Segmentation contractions are the mixing movement in the small and large intestine Function: mixing food with the digestive juices , thus helping digestion and absorption.  Digestion is accomplished by enzymes that hydrolyze certain bonds in the food molecule to release simple molecules. Absorption occurs in the small intestine. Undigested & unabsorbed materials are excreted to the outside by the process of DEFECATION. Gut functions are controlled by :- I)-Neural regulation: 1-Enteric nervous system ( The brain of the gut) It is a network of neurons arranged in the wall of the gut. It can regulate gut functions independent of the autonomic nervous system 2-Autonomic nerves (Extrinsic innervation): a-Sympathetic. b-Parasympathetic (vagal & sacral) Sympathetic and parasympathetic nerves regulate gut function by modulating the activity of the enteric nervous system. II )-Hormonal regulation: The mucosa of the gut contain endocrine cells that secrete hormones that circulate in the blood. These hormones reach the gut and regulate its motility and secretion.  The salivary secretion It is secreted by three pairs of salivary glands 1) Parotid 2) Submandibular (=submaxillary) 3) Sublingual  Functions of saliva 1-Digestive function: Saliva contains salivary amylase that starts digestion of starch→ di and trisaccharides as well as polysaccharides. 2-Moisten the buccal cavity: helps articulation (aiding speech) by facilitating the movement of the lips and tongue. Acts as a solvent for molecules that stimulate the taste buds. 3-Defensive function: Saliva contains antibacterial enzymes (lyzozymes), thiocyanate ions, immunoglobulins (IgA) and lactoferrin (animals lick their wounds). 4-Keeps oral pH at 7 and antagonizes its change. The salivary buffers are (bicarbonate and mucin).This is important since acidity increases Ca2+ solubility leading to its loss from the enamel of teeth.  Swallowing (deglutition) It is the mechanism by which the food is transported from the mouth, through the pharynx and esophagus to the stomach. Phases of deglutition: a) The buccal phase: voluntary contraction of the muscles of the tongue that push the food backwards into the pharynx. b) The pharyngeal phase: it is an involuntary phase that starts by stimulating the specific pressure receptor in the pharynx that send impulses to the swallowing centre in the brain. c) Esophageal phase: it is an involuntary peristaltic waves that conduct food from the pharynx to the stomach. The Stomach  Functions of the stomach 1-Storage of food. 2-Mixing : ingested food is mixed with gastric secretion and produce a thick liquid mixture called chyme. 3-Secretion of gastric juice that contains HCl and enzymes (pepsin) that begin protein digestion and the intrinsic factor (which helps absorption of Vit B12) 4-Gastric HCl is important because: A. Activates pepsin. B. It helps absorption of IRON & Ca⁺⁺ because it causes the pH of the chyme to be acidic which favors absorption of these minerals. C. HCl cause STERILIZATION of the ingested food: it kills the majority of ingested microrganisms. 5-Piecemeal evacuation: evacuation of stored food in the stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine.  Protective Mechanisms Of The Stomach 1. Mucus protective barrier. THICK ALKALINE MUCUS LAYER which prevent diffusion of gastric acid, to the epithelial surface. 2. Pepsinogen is secreted in the inactive form and activated by HCl in gastric lumen. 3. Rapid turnover of the mucosal barrier. The Pancreas  Pancreatic juice has IMPORTANT DIGESTIVE FUNCTION because it contains digestive enzymes for digestion of: 1. Carbohydrate (pancreatic amylase). 2. Protein (trypsin, chymotrypsin and carboxypeptidase). 3. Triglycerides (Pancreatic lipase) Pancreatic juice contains sodium bicarbonate to neutralize the acid chyme to protect the intestinal mucosa.  Hormonal regulation of pancreatic secretion 1) Secretin: it is secreted in response to the entry of acid into the duodenum. 2) Cholecystokinin (CCK):it is secreted in response to the entry of fat and protein into the duodenum. The Liver Liver continuously synthesize &secretes bile. Between meals, the sphincter of Oddi is closed ,so the secreted bile is moved into the gall bladder to be stored and concentrated. During meals, bile is secreted from the gall bladder & liver into the duodenum.  Liver functions 1. Secretory function (Bile formation ) Synthesis & secretion of bile salts. 2. Synthesis of plasma proteins. 3. Metabolic functions: 1. Carbohydrate metabolism: gluconeogenesis, glycogenesis , glycogenolysis. It is essential for glucose homeostasis. 2. Protein metabolism. synthesis of non –essential amino acids. 3. Fat metabolism: 1. Synthesis of cholesterol, lipoproteins & phospholipids. 2. Lipogenesis , Oxidation of fatty acids to supply energy. 5. Synthesis of 25- hydroxy cholecalciferol. 6. Storage function: Glycogen, Vitamin A (10 months), D (3-4 months), B12 (12 months), Iron as ferritin, copper. 7. Vascular Function: 1. Blood storage: 10% of total blood volume. Also, after meals, the liver accommodates blood drained by the gastrointestinal tract 2. Blood filtration: 99% of bacteria that enter the portal blood from the intestine, small blood clots become phagocytosed by Kupffer cells that line hepatic sinusoids. 3. Removal of old red blood cells by kupffer cells. 8. Detoxification of drugs & hormones (thyroxin, steroid).  Functions of bile 1. Neutralization of HCl: Share in HCl neutralization in the duodenum due to its high bicarbonate content. 2. Digestive function: Bile contains bile acids & salts, which are essential for digestion and absorption of fat and fat-soluble vitamins in the small intestine. Bile salts cause FAT EMULSIFICATION (i.e., breakdown of big fat globules into small ones to become dissolved in water), thus increasing the surface area of fat droplets available for pancreatic lipase.  Fatty acids Big fat globule PL  Big fat globule PL  Functions of the small intestine In the small intestine, chyme delivered by the stomach is mixed with intestinal juice, pancreatic juice and bile. Functions of the small intestine:- 1. Complete digestion of food. This function involves. intestinal juice contains digestive enzymes that complete digestion of carbohydrates and proteins. (peptidases and disaccharidases). 2. Absorption of digested food.  Overview of digestion in the small intestine I. Carbohydrate digestion is completed by disaccharidases (maltase, lactase & sucrase) in the intestinal juice. II. Protein digestion is completed by aminopeptidase in the intestinal juice. III. Fat digestion is completed in the lumen of the small intestine by pancreatic lipase into fatty acids & monoglycerides.  Large Intestine Functions: 1-Absorption: Na+, Cl-, water 2-Defecation: to remove the undigested and unabsorbed material. It is the function of rectum and anal canal. 3-Synthesis of vitamins (vitamin K and several members of vitamin B) by bacteria living in the large intestine.