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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 18 The Digestive System 18-1
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 18 Outline Functions of GI Tract Structure of Digestive System From Mouth to Stomach Stomach Small Intestine Large Intestine Liver Gall Bladder & Pancreas Control & Phases of Digestion Digestion & Absorption of Food Types 18-2
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Overview Inside gastrointestinal (GI) tract, food is broken down by hydrolysis into molecular monomers Absorption of monomers occurs in small intestine 18-3
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig 18.1 18-4
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Functions of GI Tract 18-5
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Is movement of food through GI tract by means of: Ingestion--taking food into mouth Mastication--chewing food & mixing it with saliva Deglutition--swallowing food Peristalsis--rhythmic wave-like contractions that move food through GI tract Motility 18-6
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Includes release of exocrine & endocrine products into GI tract Exocrine secretions include: HCl, H 2 0, HC0 3 -, bile, lipase, pepsin, amylase, trypsin, elastase, & histamine Endocrine includes hormones secreted into stomach & small intestine to help regulate GI system E.g. gastrin, secretin, CCK, GIP, GLP-1, guanylin, VIP, & somatostatin Secretion 18-7
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Is passage of digested end products into blood or lymph Absorption 18-8
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Includes temporary storage & subsequent elimination of indigestible components of food Storage and Elimination 18-9
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Structure of Digestive System 18-10
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Digestive System Is composed of GI tract (alimentary canal) & accessory digestive organs GI tract is 30 ft long; extends from mouth to anus 18-11
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Digestive System continued Organs include oral cavity, pharynx, esophagus, stomach, & small & large intestine Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, & pancreas Fig 18.2 18-12
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Layers of GI Tract Are called tunics The 4 tunics are mucosa, submucosa, muscularis, & serosa Fig 18.3 18-13
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Is the absorptive & secretory layer lining lumen of GI tract In places is highly folded with villi to increase absorptive area Contains lymph nodules, mucus-secreting goblet cells, & thin layer of muscle Mucosa Fig 18.3 18-14
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Is a thick, highly vascular layer of connective tissue where absorbed molecules enter blood & lymphatic vessels Contains glands & nerve plexuses (submucosal plexus) that carry ANS activity to muscularis mucosae Submucosa Fig 18.3 18-15
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Is responsible for segmental contractions & peristaltic movement through GI tract Has an inner circular & outer longitudinal layer of smooth muscle Activity of these layers moves food through tract while pulverizing & mixing it Myenteric plexus between these layers is major nerve supply to GI tract Includes fibers & ganglia from both Symp & Parasymp systems Muscularis 18-16
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Serosa Is outermost layer; serves to bind & protect Consists of areolar connective tissue covered with layer of simple squamous epithelium Fig 18.3 18-17
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parasympathetic effects, arising from vagus & spinal nerves, stimulate motility & secretions of GI tract Sympathetic activity reduces peristalsis & secretory activity GI tract contains an intrinsic system that controls its movements--the enteric nervous system GI motility is influenced by paracrine & hormonal signals Regulation of GI Tract 18-18
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From Mouth to Stomach 18-19
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mastication (chewing) mixes food with saliva which contains salivary amylase An enzyme that catalyzes partial digestion of starch From Mouth to Stomach 18-20
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Deglutition (swallowing) begins as voluntary activity Oral phase is voluntary & forms a food bolus Pharyngeal & esophageal phases are involuntary & cannot be stopped To swallow, larynx is raised so that epiglottis covers entrance to respiratory tract A swallowing center in medulla orchestrates complex pattern of contractions required for swallowing From Mouth to Stomach continued 18-21
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Esophagus connects pharynx to stomach Upper third contains skeletal muscle Middle third contains mixture of skeletal & smooth Terminal portion contains only smooth Passes through diaphragm via esophageal hiatus From Mouth to Stomach continued 18-22
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Peristalsis propels food thru GI tract = wave-like muscular contractions After food passes into stomach, the gastroesophageal sphincter constricts, preventing reflux From Mouth to Stomach continued Fig 18.4 18-23
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Stomach 18-24
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Is most distensible part of GI tract Empties into the duodenum Functions in: storage of food; initial digestion of proteins; killing bacteria with high acidity; moving soupy food mixture (chyme) into intestine Stomach 18-25
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Stomach continued Is enclosed by gastroesophageal sphincter on top & pyloric sphincter on bottom Is divided into 3 regions: Fundus Body Antrum Fig 18.5 18-26
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Inner surface of stomach is highly folded into rugae Contractions of stomach churn chyme, mixing it with gastric secretions Eventually these will propel food into small intestine Stomach continued Fig 18.5 18-27
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Stomach continued Gastric mucosa has gastric pits in its folds Cells that line folds deeper in the mucosa, are exocrine gastric glands Fig 18.7 18-28
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gastric glands contain cells that secrete different products that form gastric juice Goblet cells secrete mucus Parietal cells secrete HCl & intrinsic factor (necessary for B 12 absorption in intestine) Chief cells secrete pepsinogen (precursor for pepsin) Stomach continued Fig 18.7 18-29
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Enterochromaffin- like cells secrete histamine & serotonin G cells secrete gastrin D cells secrete somatostatin Stomach continued Fig 18.7 18-30
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. HCl in Stomach Is produced by parietal cells which AT H + into lumen via an H + / K + pump (pH ≈1) Cl - is secreted by facilitated diffusion H+ comes from dissociation of H 2 CO 3 Cl- comes from blood side of cell in exchange for HC0 3 - Fig 18.8 18-31
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Is secreted in response to the hormone gastrin; & ACh from vagus These are indirect effects since both stimulate release of histamine which causes parietal cells to secrete HCl HCl in Stomach continued 18-32
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Makes gastric juice very acidic which denatures proteins to make them more digestible Converts pepsinogen into pepsin Pepsin is more active at low pHs Fig 18.9 HCl in Stomach continued 18-33
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Digestion & Absorption in Stomach Proteins partially digested by pepsin Carbohydrate digestion by salivary amylase is soon inactivated by acidity Alcohol & aspirin are only commonly ingested substances absorbed 18-34
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gastric and Peptic Ulcers Peptic ulcers are erosions of mucous membranes of stomach or duodenum caused by action of HCl In Zollinger-Ellison syndrome, duodenal ulcers result from excessive gastric acid in response to high levels of gastrin Helicobacter pylori infection is associated with ulcers Antibiotics are useful in treating ulcers Acute gastritis is an inflammation that results in acid damage due to histamine released by inflammation Why histamine receptor blockers such as Tagamet & Zantac can treat gastritis 18-35
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Include: Impermeability of parietal & chief cells to HCl A layer of alkaline mucus containing HC0 3 - Tight junctions between adjacent epithelial cells Rapid rate of cell division (entire epithelium replaced in 3 days) Prostaglandins (PGs) inhibit gastric secretions Which is why PG blockers such as NSAIDs can cause ulcers Protective Mechanisms of Stomach 18-36
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Small Intestine 18-37
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Is longest part of GI tract; approximately 3m long Duodenum is 1st 25cm after pyloric sphincter Jejunum is next 2/5s Ileum is last 3/5s; empties into large intestine Small Intestine (SI) Fig 18.10 18-38
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Absorption of digested food occurs in SI Facilitated by long length & tremendous surface area Small Intestine (SI) continued 18-39
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Surface area increased by foldings & projections Large folds are plicae circulares Microscopic finger- like projections are villi Apical hair-like projections are microvilli Small Intestine (SI) continued Fig 18.10 18-40
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Small Intestine (SI) continued Each villus is covered with columnar epithelial cells interspersed with goblet cells Epithelial cells at tips of villi are exfoliated & replaced by mitosis in crypts of Lieberkuhn Inside each villus are lymphocytes, capillaries, & central lacteal Fig 18.12 18-41
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Small Intestine (SI) continued A carpet of hair-like microvilli project from apical surface of each epithelial cell Create a brush border 18-42
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Attached to microvilli are brush border enzymes that are not secreted into lumen Enzyme active sites are exposed to chyme Intestinal Enzymes 18-43
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Intestinal Contractions and Motility 2 major types of contractions occur in SI: Peristalsis is weak & slow & occurs mostly because pressure at pyloric end is greater than at distal end Segmentation is major contractile activity of SI Is contraction of circular smooth muscle to mix chyme Fig 18.14 18-44
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Occur automatically via endogenous pacemaker activity Contractions are driven by graded depolarizations called slow waves Intestinal Contractions & Motility continued Fig 18.15 18-45
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slow waves are produced by non- neuronal interstitial cells of Cajal (ICC) Conducted to smooth muscle via gap junctions Slow waves spread from 1 smooth muscle cell to another thru nexuses Intestinal Contractions & Motility continued Fig 18.16 18-46
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. When slow waves exceed threshold, trigger APs in smooth muscle by opening V-gated Ca 2+ channels Influx of Ca 2+ produces depolarization phase of AP & stimulates contraction Repolarization via K + efflux Contractions are modified by ANS activity ACh from Parasymp increases amplitude & duration of slow waves NE & Epi from Symp decrease activity of intestines Intestinal Contractions & Motility continued 18-47
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Large Intestine 18-48
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Has no digestive function but absorbs H 2 0, electrolytes, B & K vitamins, & folic acid Internal surface has no villi or crypts & is not very elaborate Contains large population of microflora LI bacteria produce folic acid & vitamin K & ferment indigestible food to produce fatty acids Large Intestine (LI) or Colon 18-49
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Extends from ileocecal valve at end of SI to anus Outer surface bulges to form pouches (haustra) Chyme from SI enters cecum, then passes to ascending colon, transverse colon, descending colon, sigmoid colon, rectum, & anal canal Large Intestine (LI) or Colon continued Fig 18.17 18-50
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. SI absorbs most water but LI absorbs 90% of water it receives Begins with osmotic gradient set up by Na + /K + pumps Water follows by osmosis Salt & water reabsorption stimulated by aldosterone LI can also secrete H 2 0 via AT of NaCl into intestinal lumen Fluid & Electrolyte Absorption in LI 18-51
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Defecation After electrolytes & water have been absorbed, waste material passes to rectum, creating urge to defecate Defecation reflex begins with relaxation of external anal sphincter allowing feces to enter anal canal Longitudinal rectal muscles contract to increase rectal pressure; internal anal sphincter relaxes Excretion is aided by contractions of abdominal & pelvic muscles which push feces from rectum 18-52
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Liver 18-53
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Structure of Liver Liver largest internal organ Hepatocytes form hepatic plates that are 1–2 cells thick Plates separated by sinusoids which are fenestrated & permeable even to proteins Contain phagocytic Kupffer cells Fig 18.20 18-54
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Food absorbed in SI is delivered 1st to liver Capillaries in digestive tract drain into hepatic portal vein which carries blood to liver Hepatic vein drains liver Liver also receives blood from hepatic artery Hepatic Portal System 18-55
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Liver Lobules Are functional units formed by hepatic plates In middle of each is central vein At edge of each lobule are branches of hepatic portal vein & artery which open into sinusoids Fig 18.20 18-56
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Liver Lobules Bile is secreted by hepatocytes in bile canaliculi Empty into bile ducts which flow into hepatic ducts that carry bile away from liver Fig 18.21 18-57
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Enterohepatic Circulation Is recirculation of compounds between liver & intestine Many compounds are released in bile, reabsorbed in SI, & returned to liver to be recycled Liver excretes drug metabolites into bile to pass out in feces Fig 18.22 18-58
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 18-59
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Amounts to 250–1500 ml/day Bile pigment (bilirubin) is produced in spleen, bone marrow, & liver Is a derivative of heme groups (minus iron) from Hb Carried in blood attached to albumin Free bilirubin combines with glucuronic acid to form conjugated bilirubin that is secreted into bile Converted by intestinal bacteria to urobilinogen 30-50% of urobilogen is absorbed by intestine & enters hepatic vein Thus enters enterohepatic circulation to be recycled or filtered by kidneys & excreted in urine Bile Production and Secretion 18-60
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metabolism of Heme and Bilirubin Fig 18.23 18-61
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Bile Acids Are formed in major breakdown pathway for cholesterol Are mostly cholic & chenodeoxycholic acids Form bile salts by combining with glycine or taurine Bile salts aggregate as micelles 95% of bile acids are absorbed by ileum Fig 18.25 18-62
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Detoxification of Blood Liver can remove hormones, drugs, & other biologically active molecules from blood by: Excretion into bile Phagocytosis by Kupffer cells Chemical alteration of molecules E.g. ammonia is produced by deamination of amino acids in liver Liver converts it to urea which is excreted in urine 18-63
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Liver conjugates steroid hormones & xenobiotics with groups that make them anionic Which can be transported into bile or urine by multispecific organic anion transport carriers & excreted Cytochrome P450 enzymes are involved in hepatic metabolism of steroids & drugs Detoxification of Blood continued 18-64
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Secretion of Glucose, Triglycerides & Ketones Liver helps regulate blood glucose by removing it from blood or releasing it to blood Removes it via glycogenesis & lipogenesis Or produces it via glycogenolysis & gluconeogenesis Can convert free fatty acids into ketone bodies (ketogenesis) that can be used for energy during fasting 18-65
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Production of Plasma Proteins Albumin & most of plasma globulins are produced by liver Albumin makes up 70% of total plasma protein & contributes most to colloid osmotic pressure of blood Globulins transport cholesterol & hormones, inhibit trypsin, & are involved in blood clotting Constitute many of the clotting factors 18-66
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Gall Bladder & Pancreas 18-67
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Is a sac-like organ attached to inferior surface of liver Stores & concentrates bile continuously produced by liver When SI is empty, sphincter of Oddi in common bile duct closes & bile is forced into gallbladder Expands as it fills with bile When food is in SI, sphincter of Oddi opens, gall bladder contracts, & bile is ejected thru cystic duct into common bile duct then to duodenum Gallbladder Fig 18.26 18-68
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pancreas Is located behind stomach Has both endocrine & exocrine functions Endocrine function performed by islets of Langerhans Secretes insulin & glucagon Exocrine secretions include bicarbonate solution & digestive enzymes These pass in pancreatic duct to SI Exocrine secretory units are acini 18-69
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig 18.26 18-70
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig 18.28 18-71
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Contains water, bicarbonate, & digestive enzymes Digestive enzymes include amylase for starch, trypsin for proteins, and lipase for fats Brush border enzymes are also required for complete digestion Pancreatic Juice 18-72
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pancreatic Juice Most pancreatic enzymes are produced in inactive form (zymogens) Trypsin is activated by brush border enzyme, enterokinase Trypsin activates other zymogens Fig. 18.29 Fig 18.30 18-73
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 18-74
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Control & Phases of Digestion 18-75
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Neural & endocrine mechanisms modify activity of GI system Vagus nerve is heavily involved in regulating & coordinating digestive activities GI tract is both an endocrine gland & target for action of hormones Hormones include secretin, gastrin, CCK, & GIP Neural and Endocrine Regulation 18-76
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gastric motility & secretion occur automatically Waves of contraction are initiated spontaneously by pacesetter cells & secretion occurs in absence of hormonal & neural input ANS & hormonal effects are superimposed on automatic activity Extrinsic control of gastric function is divided into cephalic, gastric, & intestinal phases Regulation of Gastric Function 18-77
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cephalic Phase Refers to control by brain of vagus activity Stimulated by sight, smell, & taste of food Activation of vagus: Stimulates chief cells to secrete pepsinogen Directly stimulates G cells to secrete gastrin Directly stimulates ECL cells to secrete histamine Indirectly stimulates parietal cells to secrete HCl Continues into 1 st 30 min of a meal 18-78
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Arrival of food in stomach stimulates gastric phase Gastric secretion stimulated by distension of stomach & chemical nature of chyme Gastric Phase 18-79
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Short polypeptides & amino acids stimulate G cells to secrete gastrin & chief cells to secrete pepsinogen Gastrin stimulates ECL cells to secrete histamine which stimulates parietal cell secretin of HCl This is a positive feedback mechanism: As more HCl & pepsinogen are secreted, more polypeptides & amino acids are released Gastric Phase continued Fig 18.31 18-80
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Secretion of HCl is also regulated by a negative feedback mechanism: HCl secretion decreases if pH < 2.5; at pH 1 gastrin secretion stops D cells stimulate secretion of somatostatin which inhibits gastrin secretion Gastric Phase continued Fig 18.31 18-81
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Begins with inhibition of gastric activity when chyme enters SI Arrival of chyme in SI is detected by sensory neurons of vagus This causes a neural reflex that inhibits gastric motility & secretion Fat in chyme stimulates SI to secrete enterogasterones--hormones that inhibit gastric motility & secretion Enterogasterones include somatostatin, CCK, & GLP-1 Intestinal Phase 18-82
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Submucosal & myenteric plexuses contain as many neurons as spinal cord Includes preganglionic Parasymp axons, ganglion cell bodies, postganglionic Symp axons; & afferent intrinsic & extrinsic sensory neurons; interneurons, & glia Peristalsis is controlled by enteric NS Enteric Nervous System 18-83
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. For peristalsis: ACh & substance P stimulate smooth muscle contraction above bolus NO, VIP, & ATP stimulate smooth muscle relaxation below bolus Insert fig. 18.31 Enteric Nervous System continued Fig 18.32 18-84
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Paracrine Regulators of Intestine ECL cells release serotonin & motilin in response to pressure & chemical stimuli in SI Serotonin stimulates intrinsic afferents which activate motor neurons in intrinsic NS Motilin stimulates contraction in duodenum & stomach antrum Guanylin, from ileum & colon, stimulates production of cGMP which inhibits absorption of Na + & causes secretion of Cl - & H 2 0 18-85
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Intestinal Reflexes Can be mediated by enteric NS & paracrines; & regulated by ANS & hormones Gastroileal reflex refers to increased motility of ileum & movement of chyme thru ileocecal sphincter in response to increased gastric activity Ileogastric reflex decreases gastric motility in response to distension of ileum Intestino-intestinal reflex causes relaxation of rest of intestine when any part is overdistended 18-86
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Secretion of pancreatic juice & bile is stimulated by secretin & bile Secretin is secreted in response to duodenal pH < 4.5 Stimulates release of HC0 3 - into SI by pancreas & into bile by liver CCK is secreted in response to fat & protein content of chyme in duodenum Stimulates production of pancreatic enzymes Enhances secretin Stimulates contraction of sphincter of Oddi Secretion of Pancreatic Juice 18-87
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Digestion & Absorption of Food Types 18-88
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Digestion & Absorption of Carbohydrates Most carbohydrates are ingested as starch-- a polymer of glucose Salivary amylase begins starch digestion Pancreatic amylase converts starch to oligosaccharides Oligosaccharides hydrolyzed by SI brush border enzymes Fig 18.33 18-89
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Begins in stomach when pepsin digests proteins to form polypeptides In SI, endopeptidases (trypsin, chymotrypsin, elastase) cleave peptide bonds in interior of polypeptides SI exopeptidases (carboxypeptidase, aminopeptidase) cleave peptide bonds from ends of polypeptides Digestion & Absorption of Protein 18-90
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Protein digestion in SI results in free amino acids, dipeptides, & tripeptides Which are transported into SI cells where di- & tripeptides are broken down to amino acids Which are secreted into blood Digestion & Absorption of Protein continued Fig 18.34 18-91
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Occurs in SI Arrival of lipids in duodenum causes secretion of bile Fat is emulsified by bile salt micelles Forms tiny droplets of fat dissolved in bile salt micelles Greatly increases surface area for fat digestion Digestion & Absorption of Lipids 18-92
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pancreatic lipase hydrolyzes triglycerides to free fatty acids & monglycerides Phospholipase A breaks down phospholipids into fatty acids & lysolecithin Digestion & Absorption of Lipids continued Fig 18.35 18-93
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Products of fat digestion dissolve in micelles forming mixed micelles which move to brush border Digestion & Absorption of Lipids continued Fig 18.36 18-94
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Free fatty acids, monoglycerides, & lysolecithin leave micelles & enter epithelial cells Inside epithelial cells, they are resynthesized into triglycerides & phospholipids Digestion & Absorption of Lipids continued Fig 18.37 18-95
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Triglycerides & phospholipids combine with a protein to form small particles called chylomicrons Which are secreted into central lacteals of SI villi Digestion & Absorption of Lipids continued Fig 18.37 18-96
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. In blood, endothelial lipoprotein lipase hydrolyzes triglycerides to free fatty acids & glycerol for use in cells Cholesterol-containing remnants are taken up by liver Transport of Lipids 18-97
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cholesterol & triglycerides from liver form VLDLs which are secreted & take triglycerides to cells Once triglycerides are removed, VLDLs become LDLs LDLs transport cholesterol to organs & blood vessels HDLs transport excess cholesterol back to liver High ratio of HDL-cholesterol to total cholesterol is believed to confer protection against atherosclerosis Transport of Lipids continued 18-98
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