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The Gastro Intestinal System Dr A. Gowran, TCD Ch. 16, Sherwood Digestive System • Digestive tract – Continuous from mouth to anus – Consists of • Mouth • Pharynx • Oesophagus • Stomach • Small intestine – Duodenum – Jejunum – Ileum • Large intestine – Cecum – Appendix – Colon – Rectum • Anus Digestive System • Accessory digestive organs – Salivary glands – Exocrine pancreas – Bilary system • Liver • Gallbladder Digestive System • Primary function – Transfer nutrients, water, and electrolytes from ingested food into body’s internal environment • The Digestive System Performs Four Actions – Digestion – Absorption – Motility – Secretion Digestive System: Digestion – Biochemical breakdown of structurally complex foodstuffs into smaller, absorbable units – Accomplished by enzymatic hydrolysis – Complex foodstuffs and their absorbable units • Carbohydrates → monosaccharides • Proteins → amino acids • Fats → glycerol and fatty acids Digestive System: Absorption • In the small intestine, most absorption is complete – Small units resulting from digestion, along with water, vitamins, and electrolytes are transferred from digestive tract lumen into blood or lymph Digestive System: Motility – Muscular contractions that mix and move forward the contents of the digestive tract – Two types of digestive motility • Propulsive movements – Push contents forward through the digestive tract • Mixing movements – Serve two functions » Mixing food with digestive juices promotes digestion of foods » Facilitates absorption by exposing all parts of intestinal contents to absorbing surfaces of digestive tract Digestive System: Secretions – Consist of water, electrolytes, and specific organic constituents – Secretions are released into digestive tract lumen on appropriate neural or hormonal stimulation – Normally reabsorbed in one form or another back into blood after their participation in digestion Control of Digestive System Function • Digestive motility and secretion are regulated by – Intrinsic controls Autonomous smooth muscle function Intrinsic nerve plexuses & sensory receptors – Extrinsic nerves ANS GI hormones Oral Cavity (Mouth) • Lips – Form opening – Help procure, guide, and contain food in the mouth – Important in speech – Well-developed tactile sensation • Palate – Forms roof of oral cavity (separates mouth from nasal passages) – Uvula (seals off nasal passages during swallowing) • Tongue – Forms floor of oral cavity – Composed of skeletal muscle – Movements aid in chewing and swallowing – Plays important role in speech – Taste buds Oral Cavity • Pharynx – Cavity at rear of throat – Common passageway for digestive and respiratory systems – Tonsils • Within side walls of pharynx • Lymphoid tissue • Teeth – Responsible for chewing (mastication) – First step in digestive process Saliva Salivary secretion is stimulated by cholinergic parasympathetic nerves (Rate of around 1,500 ml per day) Functions of saliva are: prevention of tooth decay: - flushing wastes out - antibacterial environment lubrication of mouth preliminary digestion of starches In absence of saliva: thirst impaired speech and swallowing rapid tooth decay Saliva – Composition • 99.5% H2O • 0.5% electrolytes and protein (amylase, lysozyme) – Functions • Salivary amylase begins digestion of carbohydrates – Lysozyme destroys bacteria – Saliva rinses away material that could serve as food source for bacteria • Solvent for molecules that stimulate taste buds • Aids speech by facilitating movements of lips and tongue • Helps keep mouth and teeth clean • Rich in bicarbonate buffers Swallowing Pharyngoesophageal sphincter Ringlike peristaltic contraction sweeping down the oesophagus Swallowing – Motility associated with pharynx and esophagus – Sequentially programmed all-or-none reflex – Initiated when bolus is voluntarily forced by tongue to rear of mouth into pharynx – Most complex reflex in body – Can be initiated voluntarily but cannot be stopped once it has begun – Process divided into two stages • Oropharyngeal stage • Esophageal stage (moves bolus from mouth through pharynx and into esophagus) Oesophagus Fairly straight muscular tube – Extends between pharynx and stomach – Sphincters at each end • Pharyngoesophageal sphincter – Keeps entrance closed to prevent large volumes of air from entering esophagus and stomach during breathing • Gastroesophageal sphincter – Prevents reflux of gastric contents – Peristaltic waves push food through esophagus – Secretions (mucus) are entirely protective Layers of Digestive Tract Digestive Tract • Wall has same general structure throughout length from esophagus to anus • Four major tissue layers – Mucosa • Innermost layer – Submucosa – Muscularis externa – Serosa • Outer layer © Dr. Richard Kessel & Dr. Randy Kardon/Tissues & Organs/Visuals Unlimited Mucosa • Lines luminal surface of digestive tract • Highly folded surface greatly increases absorptive area • Three layers – Mucous membrane • Serves as protective surface • Modified for secretion and absorption • Contains – Exocrine gland cells – secrete digestive juices – Endocrine gland cells – secrete blood-borne gastrointestinal hormones – Epithelial cells – specialized for absorbing digestive nutrients – Lamina propria • Houses gut-associated lymphoid tissue (GALT) – Important in defense against disease-causing intestinal bacteria – Muscularis mucosa • Sparse layer of smooth muscle Submucosa • Thick layer of connective tissue • Provides digestive tract with distensibility and elasticity • Contains larger blood and lymph vessels • Contains nerve network known as submucosal plexus Muscularis Externa • Major smooth muscle coat of digestive tube • In most areas consists of two layers – Circular layer • Inner layer • Contraction decreases diameter of lumen – Longitudinal layer • Outer layer • Contraction shortens the tube • Contractile activity produces propulsive and mixing movements • Myenteric plexus – Lies between the two muscle layers Serosa • Secretes serous fluid – Lubricates and prevents friction between digestive organs and surrounding viscera • Continuous with mesentery throughout much of the tract – Attachment provides relative fixation – Supports digestive organs in proper place while allowing them freedom for mixing and propulsive movements Stomach © Dr. Fred Hossler/Visuals Unlimited Stomach • J-shaped sac-like chamber lying between esophagus and small intestine • Divided into three sections – Fundus – Body – Antrum • Three main functions – Store ingested food until it can be emptied into S. intestine – Secretes HCl and enzymes that begin protein digestion – Mixing movements convert pulverized food to chyme • Pyloric sphincter – Serves as barrier between stomach and upper part of S. intestine Functions of the stomach • Storage of ingested food • Beginning of protein digestion • Bactericidal activity • Blending of food into chyme • Intermittent release of chyme into intestine • Vitamin B12 absorption Gastric Motility • Four aspects – Filling • Involves receptive relaxation – Enhances stomach’s ability to accommodate the extra volume of food with little rise in stomach pressure – Triggered by act of eating – Mediated by vagus nerve – Storage • Takes place in body of stomach – Mixing • Takes place in antrum of stomach – Emptying • Largely controlled by factors in duodenum Gastric Motility: Emptying • Factors in stomach – Amount of chyme in stomach is main factor that influences strength of contraction • Factors in duodenum – Fat • Fat digestion and absorption takes place only within lumen of small intestine • When fat is already in duodenum, further gastric emptying of additional fatty stomach contents is prevented – Acid • Unneutralized acid in duodenum inhibits further emptying of acidic gastric contents until neutralization can be accomplished – Hypertonicity • Gastric emptying is reflexly inhibited when osmolarity of duodenal contents starts to rise – Distension • Too much chyme in duodenum inhibits emptying of even more gastric contents Gastric Motility: Emptying • Factors trigger either – Neural response • Mediated through both intrinsic nerve plexuses (short reflex) and autonomic nerves (long reflex) • Collectively called enterogastric reflex – Hormonal response • Involves release of hormones from duodenal mucosa collectively known as enterogastrones – Secretin – Cholecystokinin (CCK) Additional factors that that influence gastric motility – Emotions • Sadness and fear – tend to decrease motility • Anger and aggression – tend to increase motility – Intense pain – tends to inhibit motility Gastric Emptying and Mixing as a Result of Antral Peristaltic Contractions Gastric Secretions • Two distinct areas of gastric mucosa that secrete gastric juice – Oxyntic mucosa • Lines body and fundus – Pyloric gland area (PGA) • Lines the antrum • Gastric pits at base of gastric glands • Three types of gastric exocrine secretory cells – Mucous cells • Line gastric pits and entrance of glands • Secrete thin, watery mucus – Chief cells • Secrete enzyme precursor, pepsinogen (Pepsin) – Parietal (oxyntic) cells • Secrete HCl and intrinsic factor Ulcers • Protective mucus layer can be lost • Acid --> damage to gut wall • Mucus loss most commonly caused by H. pylori • Caffeine, alcohol increase acid secretion • Treatment: reduce acid, give antibiotic Small Intestine • Site where most digestion and absorption take place • Three segments – Duodenum – Jejunum – Ileum • Motility Small Intestine • Segmentation – Primary method of motility in small intestine – Consists of ringlike contractions along length of small intestine – Within seconds, contracted segments relax and previously relaxed areas contract – Action mixes chyme throughout small intestine lumen Need for the small intestine • Carbohydrates • Fats – No salivary or gastric breakdown – Salivary amylase • Protein – Gastric pepsin – BUT BREAKDOWN PRODUCTS TOO LARGE TO BE ABSORBED Credit: © Dr. Richard Kessel & Dr. Gene Shih/Visuals Unlimited Small Intestine – Absorbs almost everything presented to it – Most occurs in duodenum and jejunum – Adaptations that increase small intestine’s surface area • Inner surface has permanent circular folds • Microscopic finger-like projections called villi • Brush border (microvilli) arise from luminal surface of epithelial cells – Lining is replaced about every three days LARGE INTESTINE Large Intestine • Primarily a drying and storage organ • Consists of – Colon – Caecum – Appendix – Rectum • Contents received from small intestine consists of indigestible food residues, unabsorbed biliary components, and remaining fluid • Colon – Extracts more water and salt from contents – Faeces – what remains to be eliminated Large Intestine • Taeniae coli – Longitudinal bands of muscle • Haustra – Pouches or sacs Colonoscopy – Actively change location as result of contraction of circular smooth muscle layer • Haustral contractions – Main motility Large Intestine • Mass movements – Massive contractions – Moves colonic contents into distal part of large intestine • Gastrocolic reflex – Mediated from stomach to colon by gastrin and by autonomic nerves – Most evident after first meal of the day – Often followed by urge to defecate • Defecation reflex – Initiated when stretch receptors in rectal wall are stimulated by distension – Causes internal anal sphincter to relax and rectum and sigmoid colon to contract more vigorously – If external anal sphincter (skeletal muscle under voluntary control) is also relaxed, defecation occurs Absorption macromolecules absorbable units Mechanical & Enzymatic 99% of fluid secreted into the GI tract is reabsorbed – only 100ml lost! Most absorption occurs in the Small intestine stomach – lipids soluble substances CHARBOHYDRATE PROTEIN FAT Accessory digestive organs: Liver Body’s major biochemical factory Functions not related to digestion Detoxification Synthesis Stores Excretes Importance to digestive system – secretion of bile salts Liver • Bile – Actively secreted by liver and actively diverted to gallbladder between meals – Stored and concentrated in gallbladder – Consists of: Bile salts Cholesterol Lecithin Bilirubin – After meal, bile enters duodenum • Bile salts – Derivatives of cholesterol – Convert large fat globules into a liquid emulsion – After participation in fat digestion and absorption, most are reabsorbed into the blood Pancreas Pancreas • Mixture of exocrine and endocrine tissue • Elongated gland located behind and below the stomach • Endocrine function – Islets of Langerhans • Found throughout pancreas • Secrete insulin and glucagon • Exocrine function – Secretes pancreatic juice consisting of • Pancreatic enzymes actively secreted by acinar cells that form the acini • Aqueous alkaline solution actively secreted by duct cells that line pancreatic ducts Pancreatic Enzymes • Exocrine secretion is regulated by – Secretin – CCK • Proteolytic enzymes – Digest protein • Trypsinogen - converted to active form trypsin • Chymotrypsinogen – converted to active form chymotrysin • Procarboxypeptidase – converted to active form carboxypeptidase • Pancreatic amylase – Converts polysaccharides into the disaccharide amylase • Pancreatic lipase – Only enzyme secreted throughout entire digestive system that can digest fat
