The Digestive System PDF

The Digestive System: Introduction The Alimentary Tract  Intestinal movements Propulsive movements/ peristalsis: Food moves forward at an appropriate rate. Mixing movements: Keep contents mixed. Regulated by: Neural control: signals relaxation of stomach/ duodenum ahead of the fo...

The Digestive System: Introduction The Alimentary Tract  Intestinal movements Propulsive movements/ peristalsis: Food moves forward at an appropriate rate. Mixing movements: Keep contents mixed. Regulated by: Neural control: signals relaxation of stomach/ duodenum ahead of the food; control emptying of chyme from stomach. Hormonal: Gastrin triggered by digestive products of meat-stimulates pyloric pump; CCK (cholecystokinin): released from jejunum in response to fatty substances to inhibit stomach emptying. Chemistry: pH, fats, unprocessed protein etc.  Glandular secretion Mucous glands/mucous cells/goblet cells: Activated by local irritation of the epithelia, secrete mucous that acts as a lubricant and also protects surfaces from excoriation (skin wear off). Specialized secretory cells in pits present in invaginations of the epithelia Tubular glands: in stomach and deodenum Salivary glands, pancreas and liver: represent several complex glands providing secretions for digestion and emulsification of food.  Regulation of Glandular secretion Mechanical presence of food-local tactile stimuli, chemical irritation and distention of gut wall. Stimulation of parasympathetic nerves-increase rate of secretion-particularly in upper intestinal tract. Sympathetic nerves-slight increase in secretion of local glands, constriction of blood supply to some glands. Hormonal: GI hormones secreted in the presence of food  Intestinal Mucus Thick secretion composed of water, electrolytes and several glycoproteins. Adhere tightly to food and other articles and form thin film over surfaces. Sufficient body to prevent actual contact of food with mucosa. Slippery: food slides w ease. Causes fecal particles to adhere Resist digestion by GI enzymes. Amphoteric properties: buffer small amounts of acid or alkali.  Swallowing Mastication: Helps to break food particles to smaller pieces- increase surface area for contact with digestive juices. Voluntary phase: initiates swallowing process Pharyngeal phase- involuntary passage of bolus without the compromise of respiration. – Trachea closed, esophagus opened, fast peristalstic wave the forces the bolus down. Respiration is interrupted for only ~6 seconds. Esophageal phase-transport  Salivary Juices Principal glands: Parotid, submandibular and sublingual. 2 types of protein secretion: – serous secretion containing ptyalin (α-amylase) – Mucus secretion containing mucus. Parotid secretes mostly serous type. Submandibular and sublingual: serous and mucus. pH=6.0-7.0, favorable for Ptyalin. Digestion of carbohydrates Starch hydrolysed to disaccharide maltose, polymers of glucose containing 3-9 Glucose molecules. Only 5% digested, 30-40% completed in the esophagus and stomach. When pH falls below 4.0, amylase is no longer active.  Saliva and oral hygiene Flow of saliva washes away pathogenic bacteria. Thiocyanate and proteolytic enzymes (lysozyme) destroy bacteria. Protein antibodies destroy oral bacteria. Regulation of salivary glands Taste stimuli- sour tastes generates more saliva. Tactile stimuli- presence of round objects stimulate. Signals from higher centers of nervous system-response to smell or thought of favorite food. Blood supply to the glands for supply of essential nutrients.  Role of the Stomach Digestion of proteins by pepsin to create proteoses, peptones and polypeptides. Collagen digestion is achieved here. Achieved by the gastric secretions. Motor functions: – Storage of food till it can be processed. – Mixing of food with gastric secretions to form chyme. – Slow emptying of chyme at a rate suitable for proper digestion and images.yourdictionary.com  Gastric secretions 2 types of tubular glands: – Oxyntic/Gastric glands- secrete HCl, pepsinogen, intrinsic factor and mucus. Mucus neck cells-secrete mucus Peptic (chief) cells-secrete pepsinogen Parietal/oxyntic cells- secrete HCl/intrinsic factor – Pyloric Glands- secrete mucus and hormone gastrin.  Secretions from the Oxyntic glands Pepsinogen: Inactive pepsin. Activated by HCl. Pepsin is maximally active at pH=1.8-3.5, complete loss of activity above pH=5.0. Pepsinogen secretion rate is influenced by the amount of acid in the stomach.  Secretions from Pyloric glands Mucous Gastrin: Meats/protein- + Gastrin cells in pyloric glands containing food HCl production + Histamine + Gastrin G-cells of pyloric glands Enterochromaffin-like- cells (ECL) of oxyntic glands Pepsin Protein digestion Pepsinogen  Regulation of stomach emptying Increased food volume in stomach promotes increased emptying. Not because of pressure but due to stretching of the stomach wall. Gastrin enhances activity of pyloric pump and promotes emptying. Duodenal monitoring of: – degree of distension of duodenum – Presence of irritant – Degree of acidity (pH.3.5-4.0) – Degree of osmolality – Presence of certain breakdown products Nervous and hormonal (Cholecystokinin) feedback mechanism of control Events at the Small intestine Completion of digestion of carbohydrates, proteins and fats. Duodenum/Jejunum site of digestion upon the release of pancreatic juices. Ileum specialized for absorption.  Pancreatic secretions Pancreatic juice is secreted in response to chyme in the duodenum. – Presence of fat, protein breakdown products-stimulate secretion of secretin-opposes stomach secretion, slow emptying of the stomach. Multiple enzymes responsible for digesting all types of food: carbohydrates, proteins and fats. Bicarbonate ions important for neutralizing acidity of the chyme. http://www.olivelab.org/the-pancreas- overview.html  Pancreatic Juices Pancreatic acini secrete the pancreatic juices. Bicarbonate ions come from the small ductules and large ducts leading from the acini. Protein digesting enzymes-Trypsin, Chymotrypisn and carboxypetidase. Carbohydrate digestion-pancreatic amylase Fats: Pancreatic lipase (convert fats into FA and monoacyl glycerols), cholesterol estrase, phospholipase (FA into phospholipids) Carbohydrate digestion Amylase digests only α- 1,4 glycosidic bonds formaing maltose units. α -1,6 glycosidic bonds are digested by α- dextrinase on the brush border of enterocytes.  Carbohydrate digestion Pancreatic amylase is similar in action to salivary amylase, but 1000x more powerful. Enterocytes lining the villi of small intestine secrete lactase/sucrase- digestion completed at the brush border. Protein digestion Why isnt the pancreas digested by Trypsin? Trypsin is secreted as the inactive from: Trypsinogen. Trypsinogen is activated by enterokinase present on the surface of intestinal mucosa when chyme comes in contact with it. Chymotrypsin is activated by Trypsin. The same cells that secrete Trypsinogen also secrete a Trypsin inhibitor that protects the pancreas.  Regulation of pancreatic secretions Neural: acetylcholine- stimulate secretion of pancreatic enzymes. Hormonal: – Secretin: released from duodenal mucosa when acidic (pH=4.5-5.0) chyme comes in. Secretin in turn causes secretion of bicarbonate ions and water. – Cholecystokinin (CCK): released from the duodenum/jejunum in presence of peptones/proteoses and long chain fatty acids. Causes secretion of more pancreatic enzymes (no HCO3- or water) and bile.  Secretion of Bile Bile is secreted from liver, but stored in the gall bladder until required. Within the gall bladder, bile gets concentrated. Presence of fatty food triggers release of CCK that stimulates the gall bladder to empty its contents into the duodenum. Bile contains bile salts, bilirubin and some cholesterol. Bile salts aid in fat digestion by emulsifying large fat particles into smaller particle thus creating larger surface area for lipase action. Aid in fat absorption. Excretory role: excrete bilirubin (byproduct of hemoglobin destruction and excess cholesterol. Fat Digestion http://en.wikipedia.org/wiki/File:Micelle.png www.rpi.edu, www.ib.bioninja.com.au Secretions of the Intestine Tactile/chemical/neural/hormonal stimuli stimulate secretion from duodenal glands. Secretions are mainly mucus. Surfaces of enterocytes contain digestive enzymes: several peptidases, sucrase, maltase, lactase and intestinal lipase. The large intestinal glandular cells mainly secrete mucus.  Intestinal Absorption Total fluid to be absorbed is 1.5L+various GI secretions (~7 L). All but ~1.5L is absorbed. Absorptive capacity depends on available surface area. The villi-microvilli-and folds increase surface area by perhaps 1000x Image source: diet777.com  Brunner’s glands (duodenum): in response to tactile stimuli and secretin- secrete mucus. (protect the intestinal wall from the stimach acid) Crypts of Lieberkuhn: – goblet cells secrete mucus – Enterocytes-secrete water and electrolytes over the adjacent villi  Absorption Water is transported mainly by diffusion. Absorption of sodium-co- transport. Absorption of Chloride ions-electrochemical gradient. Absorption of bicarbonate ions-indirectly required for recycling.  Absorption of carbohydrates, proteins and fats Glucose/ Galactose is transported by a sodium co- transporter. Fructose is absorbed by facilitated diffusion. Amino acids are transported by special transporters aided by sodium (similar to glucose). Fats in the form of micelles are soluble in chyme. When these micelles come in contact with the microvilli brush border, lipids easily diffuse from the micelle into the cellular membrane. Movements of small intestine Mixing movements in the small intestine help in mixing of food with enzymes. Chyme is propelled in the small intestine by peristaltic waves. Usually forward movement of chyme is slow to maximize absorption typically taking 3-5 hours. In severe cases of infective diarrhea, there is powerful and rapid peristaltic rush triggered by an irritant that propels the entire intestinal contents into the colon.  Water re-absorption at the Large Intestine About 1500 ml of chyme normally present/day. Most of the water and electrolytes are absorbed leaving only about 100ml of fluid to be excreted in feces. Active absorption of sodium and chloride ions create an osmotic gradient that in turn causes re-absorption of water.