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Chapter 18 Lecture Outline. Functions of GI Tract 18-5.

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Presentation on theme: "Chapter 18 Lecture Outline. Functions of GI Tract 18-5."— Presentation transcript:

1 Chapter 18 Lecture Outline

2 Functions of GI Tract 18-5

3  Is movement of food through GI tract by means of:  Ingestion--taking food into mouth  Mastication--chewing food and mixing it with saliva  Deglutition--swallowing food  Peristalsis--rhythmic wave-like contractions that move food through GI tract Motility 18-6

4  Includes release of exocrine and endocrine products into GI tract  Exocrine secretions include: HCl, H 2 O, HCO 3 -, bile, lipase, pepsin, amylase, trypsin, elastase, and histamine  Endocrine includes hormones secreted into stomach and small intestine to help regulate GI system  e.g. gastrin, secretin, CCK, GIP, GLP-1, guanylin, VIP, and somatostatin Secretion 18-7

5  Is passage of digested end products into blood or lymph Absorption 18-8 Digestion Refers to breakdown of food molecules into smaller subunits

6  Includes temporary storage and subsequent elimination of indigestible components of food Storage and Elimination 18-9 Immune Barrier Includes physical barrier formed by tight junctions between cells of small intestine And cells of the immune system that reside in connective tissue just below epithelium

7 Structure of Digestive System 18-10

8 Digestive System continued  Organs include oral cavity, pharynx, esophagus, stomach, and small and large intestine  Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas 18-12

9 Layers of GI Tract 18-13

10  Is the absorptive and 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, and thin layer of muscle Mucosa 18-14

11  Is a thick, highly vascular layer of connective tissue where absorbed molecules enter blood and lymphatic vessels  Contains glands and nerve plexuses (submucosal plexus) that carry ANS activity to muscularis mucosae of small and large int. Submucosa 18-15

12  Is responsible for segmental contractions and peristaltic movement through GI tract  Has an inner circular and outer longitudinal layer of smooth muscle  Activity of these layers moves food through tract while pulverizing and mixing it  Myenteric plexus between these layers is major nerve supply to GI tract  Includes fibers and ganglia from both Symp and Parasymp systems Muscularis 18-16

13 Serosa  Is outermost layer; serves to bind and protect  Consists of areolar connective tissue covered with layer of simple squamous epithelium 18-17

14 From Mouth to Stomach 18-19

15  Mastication (chewing) mixes food with saliva which contains salivary amylase  An enzyme that catalyzes partial digestion of starch From Mouth to Stomach 18-20

16  Deglutition (swallowing) begins as voluntary activity  Oral phase is voluntary and forms a food bolus  Pharyngeal and esophageal phases are involuntary and 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

17  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 18-23

18 Stomach 18-24

19  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


21 Stomach continued  Is enclosed by gastroesophageal sphincter on top and pyloric sphincter on bottom  Is divided into 3 regions:  Fundus  Body  Antrum 18-26

22  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 18-27

23 Stomach continued  Gastric mucosa has gastric pits in its folds  Cells that line folds deeper in the mucosa, are exocrine gastric glands 18-28

24  Gastric glands contain cells that secrete different products that form gastric juice  Goblet cells secrete mucus  Parietal cells secrete HCl and intrinsic factor (necessary for B 12 absorption in intestine)  Chief cells secrete pepsinogen (precursor for pepsin) Stomach continued 18-29

25 HCl in Stomach  Is produced by parietal cells which pump 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 HCO 3 - 18-31


27  Is secreted in response to the hormone gastrin; and 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

28  Makes gastric juice very acidic which denatures proteins to make them more digestible  Converts pepsinogen into pepsin  Pepsin is more active at low pHs HCl in Stomach continued 18-33

29  Both HCL and pepsin can damage lining and produce a peptic ulcer  1st line of defense is the adherent layer of mucus  = a stable gel of mucus coating the gastric epithelium  Contains bicarbonate for neutralizing HCL  Is a barrier to actions of pepsin  Gastric epithelial cells contain tight junctions to prevent HCL and pepsin from penetrating the surface  Gastric epithelial cells are replaced every 3 days Protection of Stomach Against HCL and Pepsin 18-34

30 Digestion and Absorption in Stomach  Proteins are partially digested by pepsin  Carbohydrate digestion by salivary amylase is soon inactivated by acidity  Alcohol and aspirin are the only commonly ingested substances that are absorbed 18-35

31 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  And also proton pump inhibitors such as Prilosec  Acute gastritis is an inflammation that results in acid damage due to histamine released by inflammation  Is why histamine receptor blockers such as Tagamet and Zantac can treat gastritis 18-36

32 Small Intestine 18-37

33  Is longest part of GI tract; approximately 3m long  Duodenum is 1st 25cm after pyloric sphincter  Jejunum is next 2/5 of length  Ileum is last 3/5 of length; empties into large intestine Small Intestine (SI) 18-38

34  Absorption of digested food occurs in SI  Facilitated by long length and tremendous surface area Small Intestine (SI) continued 18-39

35  Surface area increased by foldings and projections  Large folds are plicae circulares  Microscopic finger- like projections are villi  Apical hair-like projections are microvilli Small Intestine (SI) continued 18-40

36 Small Intestine (SI) continued  Each villus is covered with columnar epithelial cells interspersed with goblet cells  Epithelial cells at tips of villi are exfoliated and replaced by mitosis in crypts of Lieberkuhn  Inside each villus are lymphocytes, capillaries, and central lacteal 18-41

37 Small Intestine (SI) continued  A carpet of hair-like microvilli project from apical surface of each epithelial cell  Create a brush border 18-42

38 Intestinal Contractions and Motility  2 major types of contractions occur in SI:  Peristalsis is weak and slow and 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 (shown in diagram) 18-44

39 Large Intestine 18-48

40  Has no digestive function but absorbs H 2 O, electrolytes, B and K vitamins, and folic acid  Internal surface has no villi or crypts and is not very elaborate  Contains large population of microflora  = 10 13 to 10 14 commensal bacteria of 400 species  which produce folic acid and vitamin K and ferment indigestible food to produce fatty acids  And reduce ability of pathogenic bacteria to infect LI  antibiotics can negatively affect commensals Large Intestine (LI) or Colon 18-49

41  Extends from ileocecal valve at end of SI to anus  Outer surface bulges to form pouches (haustra) Large Intestine (LI) or Colon continued 18-50

42  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 and water reabsorption stimulated by aldosterone  LI can also secrete H 2 O via active transport of NaCl into intestinal lumen Fluid and Electrolyte Absorption in LI 18-51

43 Liver 18-53

44 Structure of Liver  Liver is the largest internal organ  Hepatocytes form hepatic plates that are 1–2 cells thick  Plates are separated by sinusoids which are fenestrated and permeable even to proteins  Contain phagocytic Kupffer cells 18-54

45 Structure of Liver continued  A damaged liver can regenerate itself from mitosis of surviving hepatocytes  In some cases, such as alcohol abuse or viral hepatitis, regeneration does not occur  Can lead to liver fibrosis and ultimately cirrhosis 18-55

46  Food absorbed in SI is delivered 1st to liver  Capillaries in digestive tract drain into the hepatic portal vein which carries blood to liver  Hepatic vein drains liver  Liver also receives blood from the hepatic artery Hepatic Portal System 18-56

47 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 and artery which open into sinusoids 18-57

48 Liver Lobules continued  Bile is secreted by hepatocytes in bile canaliculi  Empty into bile ducts which flow into hepatic ducts that carry bile away from liver 18-58

49 Enterohepatic Circulation  Is recirculation of compounds between liver and intestine  Many compounds are released in bile, reabsorbed in SI, and returned to liver to be recycled  Liver excretes drug metabolites into bile to pass out in feces 18-59


51 Detoxication of Blood  Liver can remove hormones, drugs, and 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 18-64

52 Gallbladder and Pancreas 18-68

53  Is a sac-like organ attached to inferior surface of liver  Stores and concentrates bile continuously produced by liver  When SI is empty, sphincter of Oddi in common bile duct closes and bile is forced into gallbladder  Expands as it fills with bile  When food is in SI, sphincter of Oddi opens, gall bladder contracts, and bile is ejected thru cystic duct into common bile duct then to duodenum Gallbladder 18-69

54 Pancreas  Is located behind stomach  Has both endocrine and exocrine functions  Endocrine function performed by islets of Langerhans  Secretes insulin and glucagon  Exocrine secretions include bicarbonate solution and digestive enzymes  These pass in pancreatic duct to SI  Exocrine secretory units are acini 18-70

55 18-71 Pancreatic juice and bile are secreted into the duodenum

56 18-72 The Pancreas is Both an Exocrine and Endocrine Gland (a)A photomicrograph of the endocrine and exocrine portions of the pancreas (b)An illustration depicting the exocrine acini, where acinar cells prod. Inactive enzymes stored as zymogen granules which are secreted via ducts into duodenum

57  Contains water, bicarbonate, and 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 18-73

58 The Activation of Pancreatic Juice Enzymes  Most pancreatic enzymes are produced in inactive form (zymogens)  Trypsin is activated by brush border enzyme, enterokinase  Trypsin in turn activates other zymogens 18-74

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