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Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Digestive System • Primary function – Transfer nutrients, water, and electrolytes from ingested food into body’s internal environment • Four functions – Motility – Secretion – Digestion – Absorption Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Digestive System • Digestive 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Digestive System • Absorption – Small units resulting from digestion, along with water, vitamins, and electrolytes are transferred from digestive tract lumen into blood or lymph Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Digestive System • Digestive tract – Continuous from mouth to anus – Consists of • • • • • Mouth Pharynx Esophagus Stomach Small intestine • Accessory digestive organs – Salivary glands – Exocrine pancreas – Bilary system • Liver • Gallbladder – Duodenum – Jejunum – Ileum • Large intestine – – – – Cecum Appendix Colon Rectum • Anus Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Digestive System Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Layers of Digestive Tract Wall Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Digestive System Function • Digestive motility and secretion are regulated by – Autonomous smooth muscle function – Intrinsic nerve plexuses – Extrinsic nerves – Gastrointestinal hormones Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Summary of Pathways Controlling Digestive System Activities Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Mouth (Oral Cavity) • 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Mouth (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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Mouth (Oral Cavity) • Teeth – Functions of chewing • Grind and break food into smaller pieces to make swallowing easier and increase food surface area on which salivary enzymes can act • Mix food with saliva • Stimulate taste buds Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Mouth (Oral Cavity) • Saliva – Produced largely by three major pairs of salivary glands – Composition • 99.5% H2O • 0.5% electrolytes and protein (amylase, mucus, lysozyme) – Functions • • • • Salivary amylase begins digestion of carbohydrates Facilitates swallowing by moistening food Mucus provides lubrication Antibacterial action – 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Control of Salivary Secretion Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Pharynx and Esophagus • 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) Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Pharynx and Esophagus • Esophagus – 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Peristalsis in the Esophagus Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 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 small intestine – Secretes hydrochloric acid (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 small intestine Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Stomach Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gastric Emptying and Mixing as a Result of Antral Peristaltic Contractions Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gastric 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gastric 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 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 – Parietal (oxyntic) cells • Secrete HCl and intrinsic factor Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gastrointestinal Secretions Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning HCl Secretion • Functions of HCl – Activates pepsinogen to active enzyme pepsin and provides acid medium for optimal pepsin activity – Aids in breakdown of connective tissue and muscle fibers – Denatures protein – Along with salivary lysozyme, kills most of the microorganisms ingested with food Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Phases of Gastric Secretion • Cephalic phase – Refers to increased secretion of HCl and pepsinogen that occurs in response to stimuli acting in the head before food reaches stomach • Gastric phase – Begins when food actually reaches the stomach – Presence of protein increases gastric secretions • Intestinal phase – Inhibitory phase – Helps shut off flow of gastric juices as chyme begins to empty into small intestine Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gastric Mucosal Barrier • Enables stomach to contain acid without injuring itself Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Liver • Largest and most important metabolic organ in the body • Body’s major biochemical factory • Importance to digestive system – secretion of bile salts Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Liver • Functions not related to digestion – Metabolic processing of the major categories of nutrients – Detoxifying or degrading body wastes and hormones, drugs, and other foreign compounds – Synthesizes plasma proteins – Stores glycogen, fats, iron, copper, and many vitamins – Activates vitamin D – Removes bacteria and worn-out red blood cells – Excretes cholesterol and bilirubin Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Small Intestine • Site where most digestion and absorption take place • Three segments – Duodenum – Jejunum – Ileum • Motility includes – Segmentation – Migrating motility complex Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Segmentation Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Small Intestine • Segmentation – initiated by pacemaker cells in small intestine which produce basic electrical rhythm (BER) – Circular smooth muscle responsiveness is influenced by distension of intestine, gastrin, and extrinsic nerve activity – Functions • Mixing chyme with digestive juices secreted into small intestine lumen • Exposing all chyme to absorptive surfaces of small intestine mucosa • Migrating motility complex – Sweeps intestines clean between meals Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Small Intestine • Secretion – Juice secreted by small intestine does not contain any digestive enzymes – Synthesized enzymes act within brush-border membrane of epithelial cells • Enterokinase • Disaccharidases • aminopeptidases Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Small Intestine • Digestion – Pancreatic enzymes continue carbohydrate and protein – Brush-border enzymes complete digestion of carbohydrates and protein – Fat is digested entirely within small intestine lumen by pancreatic lipase Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Small Intestine • Absorption – 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 – Products of fat digestion undergo transformations that enable them to be passively absorbed • Eventually enter lymph Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Large Intestine • Primarily a drying and storage organ • Consists of – Colon – Cecum – 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 – Feces – what remains to be eliminated Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Large Intestine Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Large Intestine • Taeniae coli – Longitudinal bands of muscle • Haustra – Pouches or sacs – Actively change location as result of contraction of circular smooth muscle layer • Haustral contractions – Main motility – Initiated by autonomous rhythmicity of colonic smooth muscle cells Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 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 Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gastrointestinal Hormones • Gastrin – Release is stimulated by presence of protein in stomach – Secretion inhibited by accumulation of acid in stomach – Functions • Acts in several ways to increase secretion of HCl and pepsinogen • Enhances gastric motility, stimulates ileal motility, relaxes ileocecal sphincter, induces mass movements in colon • Helps maintain well-developed, functionally viable digestive tract lining Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gastrointestinal Hormones • Secretin – Presence of acid in duodenum stimulates release – Functions • Inhibits gastric emptying in order to prevent further acid from entering duodenum until acid already present is neutralized • Inhibits gastric secretion to reduce amount of acid being produced • Stimulates pancreatic duct cells to produce large volume of aqueous NaHCO3 secretion • Stimulates liver to secrete NaCO3 rich bile which assists in neutralization process • Along with CCK, is trophic to exocrine pancreas Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gastrointestinal Hormones • CCK – Functions • Inhibits gastric motility and secretion • Stimulates pancreatic acinar cells to increase secretion of pancreatic enzymes • Causes contraction of gallbladder and relaxation of sphincter of Oddi • Along with secretin, is trophic to exocrine pancreas • Implicated in long-term adaptive changes in proportion of pancreatic enzymes in response to prolonged diet changes • Important regulator of food intake Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gastrointestinal Hormones • GIP – Glucose-dependent insulinotrophic peptide – Stimulates insulin release by pancreas Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Esophageal Phase Gastric Mixing Oropharyngeal Stage Segmentation