Human Physiology BIOL3205 Digestive System Lecture Notes PDF

Human Physiology BIOL3205 Digestive system Prof. Chi Bun Chan School of Biological Sciences 5N10 Kadoorie Biological Sciences Building [email protected] 39173823  Lecture outline Basic anatomy of the digestive s...

Human Physiology BIOL3205 Digestive system Prof. Chi Bun Chan School of Biological Sciences 5N10 Kadoorie Biological Sciences Building [email protected] 39173823  Lecture outline Basic anatomy of the digestive system Transportation of food along the digestive tract Digestive juices Digestion Control of digestive secretion and motility Disorders of the digestive system  Human digestive system Food is the energy source from which the cells generate ATP for various cellular activities (e.g. muscle contraction) building blocks for repairing, regeneration, and growth Into organic molecules that can be used by the body Transfers nutrients, H2O, and electrolytes from the external environment into the internal environment The digestive system consist of Digestive tract (or gastrointestinal tract, GI tract) – mouth, pharynx, esophagus, stomach, small intestine, large intestine and anus. Accessory digestive glands – salivary gland, exocrine pancreas, and the biliary system (liver and bile duct)  Digestive activities Digestive processes include: Motility – muscular contractions that mix and move forward the contents within the GI tract Secretion – discharge of fluids that contain enzymes and other substances into the lumen of GI tract Digestion – chemically breakdown of the food into smaller molecules by enzymes in the lumen of the GI tract Absorption – Transportation of molecules from the GI tract lumen into the bloodstream  Most of the GI tract has similar structure General structure of the Contains 4 layers Mucosa – Mucous membrane contains digestive tract epithelial cells (enterocytes), exocrine cells, and endocrine cells Submucosa – connective tissues to enable it to tolerate a large degree of stretch without damaging the tissue Muscularis externa – smooth muscle (circular and longitudinal) to propel the luminal contents through the GI tract and mix the contents with secretion Serosa – Connective tissues that suspends the GI tract from the inner wall of the abdominal cavity – Secretes watery lubricating fluid that prevents friction with the surrounding organs  Movement of the bolus along the GI tract Peristalsis – a wave of contraction in the Contraction muscularis externa that travels longitudinally down the tract Relaxation Caused by the coordinated activity between circular and longitudinal muscle layers Proximal circular muscle contracts, longitudinal muscle relaxes, → decrease the diameter Food bolus Distal segment circular relaxes, longitudinal muscle contracts → increase the diameter Gravity is not required  Characteristics of the stomach Divided into fundus, body, and antrum Function of the stomach pulverizes food into smaller particles (digestion) and mixes with gastric juices (secretion from the stomach), forming a mixture called chyme store ingested food until it can be emptied into the small intestine at a rate for appropriate digestion and absorption Propulsion of chyme (semifluid mass) into the small intestine is called gastric emptying Extremely acidic environment (pH 1.5-2.0) Begins protein digestion by secreting zymogen Pepsinogen → pepsin No absorption except alcohol  Stomach secretions Stomach is a secretory organ that produces gastric juice and hormones for digestion Both the fundus and the body contains gastric pits, which contains cell for gastric secretion (endocrine and exocrine) Cells in the stomach Mucous cell – production of mucus (gastric mucosal barrier) for protection Chief cells – secrete pepsinogen, which will convert into pepsin (proteolytic enzyme) Parietal cells – production of HCl Endocrine cells (e.g. G cells, D cells )  Gastric juice Secreted 2L of gastric juice every day Gastric juice is mainly comprised of H2O, digestive enzymes (pepsinogen), mucus, and HCl - Enzymes: digestion - Mucus: protection (mechanical and chemical) - Acid: denature the protein, activate the proteolytic enzyme (pepsinogen), kill the microorganism.  Gastric ulcers and Helicobacter pylori Ulcers are erosions of the GI tract linings that are deep enough to penetrate the muscularis mucosae (the lowest portion of mucosa) Most vulnerable areas: lower regions of the stomach and uppermost portion of the duodenum Chronic, rhythmic and periodic gnawing or burning in the stomach area May cause hemorrhage and peritonitis (http://2.bp.blogspot.com) H. pylori is the caused of >80% of all gastric ulcer prevalence of H. pylori infection was found to be 58.4% in Hong Kong Mechanism: Secretion of toxin that causes inflammation and weakens the gastric mucosal barrier (http://www.flickr.com/photos/nsf_beta/4822021538/in/set-72157621768317570) Digestive functions of pancreas Pancreas contains both endocrine (insulin and glucagon) and exocrine (pancreatic juice) tissues Exocrine gland forms acini Pancreatic juice contains * Alkaline solution (HCO3-) – neutralize the acid * Proteolytic zymogen (trypsinogen, chymotrypsinogen, and procarboxypeptidase) – protein digestion * Pancreatic amylase – carbohydrate digestion * Pancreatic lipase – triglyceride digestion  Small intestine Divided into duodenum (~30 cm), jejunum ( ~1 m) and ileum (~ 3.5m). Performs motility, secretion, digestion, and absorption Absorption is completed within the first 20% of the intestine length Special structural organization to increase surface area Inner surface is thrown into the circular fold Epithelial cells (enterocytes) on the circular fold are further folded into finger-like projections called villi. Each epithelial cell has about 3000-6000 microvilli, which form the brush border on the surface of a villus Highly increases the surface area of mucosa (600 fold) Each villus contains a network of capillaries (connect to hepatic portal vein) and lacteal Structure of the small intestine (Mucus secretion)  Motility of intestine Peristalsis Contraction Movement of the small intestine contains brief periods of peristalsis, which are interspersed with relatively longer segmentation Relaxation Ring-like contraction of circular smooth muscle along the intestine length Between the contractile segments are relaxed areas containing a small bolus of chime Do not sweep along the length of the intestine Peristalsis → movement of chyme in a single T1: Contraction direction Segmentation → mixing of the chyme in both T2: Relaxation directions Segmentation Dual functions - mixing the chyme with digestive (pancreatic) juices T3: Contraction - exposing all the chyme to the absorption surface Digestion and absorption Digestion generally involves the hydrolysis of macromolecules to smaller units Carbohydrates → monosaccharides by amylase Proteins → amino acid by protease Triglyceride → monoglyceride and fatty acid by lipase (http://bio1151.nicerweb.com/Locked/media/ch05/hydrolysis.html) Digestion of proteins in stomach and small intestine Stomach Small intestine Peptide a.a. Peptide a.a.  Digestion of lipids 90% of consumed lipids are triglyceride Triglycerides are digested into free fatty acids and monoglycerides by lipase Digestion of lipids mainly occurs in the lumen of the small intestine by the pancreatic lipase Because of its hydrophobic nature, lipids do not mix readily with the stomach content but form a large droplet of oil Efficient digestion of lipid requires the presence of bile salt through the process of emulsification  Action of bile salt Cholesterol Bile salts are cholesterol derivatives that are synthesized by the hepatocytes and are stored in the gallbladder Secreted into the duodenum Contains both hydrophobic and hydrophilic groups Contact with fat globule with their hydrophobic sides face inwards Endow the oil droplet with a polar coating Mix ready with the water and increase the surface area for the lipase reaction  Digestion and absorption of carbohydrates Polysaccharides from plant → starch and cellulose (dietary fiber) Polysaccharides from animals → glycogen Polysaccharides is hydrolyzed into disaccharides (maltose, sucrose, SGLT and lactose) by amylase (saliva and pancreatic juice) Glucose is absorbed by carrier- mediated transport (symporter) via the sodium and glucose co- GLUT Na+-K+ ATPase transporter (SGLT) Leave the cells by facilitated diffusion via glucose transporter (GLUT) (p.75, Human Physiology: from cells to systems, Sherwood)  Lactose intolerance Lactose is a disaccharide sugar that contains galactose and glucose Broken down by lactase, a membrane-bound enzyme in the intestine epithelium 20-25% of adults are believed to be lactase deficient (genetic basis) Non-digested lactose changes the osmotic gradient of the chime and stimulates bacterial growth, which results in the production of gas (http://www.foodintolerances.org/lactose-food-table.aspx) and other waste products that irritate the lining of the intestine Symptoms of lactose intolerance include loose stools, abdominal bloating and pain, flatulence, nausea, and borborygmi No cure – lactose-free dietary product, pill of lactase  Fluid absorption and diarrhea GI tract secreted ~ 7L of solution per day Entire plasma ~ 2.75 L Absorption of the solution has to be closely parallel to the secretion to prevent excess dehydration 95% of the fluid is re-absorbed in the small intestine Absorption of water is passive and is driven by the osmotic gradient across the mucosal epithelium Most of the secreted substances (electrolyte, bile acid, etc.) are recycled Diarrhea (watery stool): a loss of fluid and electrolyte balance Caused by bacteria (e.g. vibrio cholera) or virus-induced intestinal motility, presence of particles (e.g. lactose) in the lumen that disrupt the osmotic gradient, microorganism toxins that induce secretion of intestinal fluid Oral rehydration therapy; NaCl, KCl, Na citrate and glucose  Water absorption in small intestine Sodium is absorbed from the intestinal lumen most prominently by cotransport with glucose and amino acids Absorbed sodium is rapidly exported from the cell via sodium pumps a high osmolality in the small intercellular spaces between adjacent enterocytes. Water diffuses in response to the osmotic gradient established by sodium Water, as well as sodium, then diffuses into capillary blood within the villus.  Oral rehydration therapy (http://www.ebmedicine.net/topics.php?paction=showTopicSeg&topic_id=174&seg_id=3445)  The colon consists of 4 regions: ascending Colon colon, transverse colon, descending colon, and sigmoid colon For reducing the volume of chyme by absorbing water and ions from the chyme Storage of feces Secrets mucus Ascending and transverse colon contract simultaneously 3-4 times a day (mass movement) Contains ~2000 species of bacteria called microbiota Promote colonic motility, help maintain colonic mucosal integrity, aid immune function, systemic metabolism, etc. Composition of gut microbiota can be altered by consumption of prebiotics or probiotics Disorders caused by microbiota disruption  Functions of components of the digestive system Digestive organ Motility Secretion Digestion Absorption Mouth and salivary gland Chewing Saliva Carbohydrate - (Mucus, amylase, lysosome, NaHCO3) Pharynx and esophagus Swallowing Mucus - - Peristalsis Stomach Gastric emptying Gastric juice (Mucus, HCl, Pepsin) Carbohydrate - Mixing Hormone (Gastrin) Protein Exocrine pancreas - Pancreatic juice - - (Amylase, protease, lipase, NaHCO3) Liver - Bile - - (Bile salt, bilirubin) Small intestine Peristalsis Mucus Carbohydrate Carbohydrate Segmentation Hormones (cholecystokinin, secretin) Protein Protein Lipid Lipid H2O Electrolytes Large intestine Mass movement Mucus - H2O Electrolytes  Gastrointestinal activity regulation Designed to maximize the uptake of nutrients Regulation of GI activity involves the autonomic nervous system, enteric nervous system, and GI hormones Conditions in the lumen of the tract is monitored by mechanoreceptors, chemoreceptors, and osmoreceptors Signals can come from Cephalic phase Gastric phase Intestinal phase Regulates (1) Motility (gastric and intestinal) (2) Secretion (gastric and pancreatic) Short reflex and long reflex pathways  Cephalic, gastric and intestinal phase signals CCK: Cholecystokinin GIP: Gastric inhibitory polypeptide GLP: Glucagon-like peptide 1 Integration of short and long term reflexes in the digestive system Long reflex – traveling from receptors to the CNS and then to the intrinsic nerve plexuses, which relay information to the effector Involves sympathetic (reduction of GI activity) and parasympathetic (enhances GI activity) nerves E.g. Gastric juice secretion Short reflex - stimulation in the lumen of the GI tract that triggers a response without any involvement of CNS Signals travel from receptors to the enteric nervous system and then directly to the effecters within the GI tract E.g. Gastric emptying  Long reflex pathways Long reflex – traveling from receptors to the CNS and then to the intrinsic nerve plexuses, which relay information to the effector Involves sympathetic (reduction of GI activity) and parasympathetic (enhances GI activity) nerves  Short reflex pathways Short reflex - stimulation in the lumen of the GI tract that triggers a response without any involvement of CNS Signals travel from receptors to the enteric nervous system and then directly to the effecters within the GI tract (http://www.austincc.edu/apreview/PhysText/Digestive.html)  Regulation of GI motility Stomach motility Intestine motility Signal Effector Signal Effector Cephalic phase - Emotion - Long reflex - Emotion - Long reflex (sympathetic and (sympathetic and parasympathetic) parasympathetic) Gastric phase - Distension - Short reflex - Distension - Short reflex - Hormone (Gastrin) - Gastrin Intestinal phase - Distension - Short reflex - Distension - Short reflex Chemical - Lumen acidity - Hormones: - Osmolarity Cholecystokinin (CCK) - Fat content After the lecture, you should be able to explain Basic anatomy of the digestive system Motility of the GI tract Composition of digestive juices Mechanism of chemical digestion Regulation of digestive system secretion and motility