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1. 23 The Digestive System: Part C

2. Pancreas • Location • Mostly retroperitoneal, deep to the greater curvature of the stomach • Head is encircled by the duodenum; tail abuts the spleen

3. Pancreas • Endocrine function • Pancreatic islets secrete insulin and glucagon • Exocrine function • Acini (clusters of secretory cells) secrete pancreatic juice • Zymogen granules of the secretory cells (Acinar) contain digestive enzymes

4. Small duct Acinar cells Basement membrane Zymogen granules Rough endoplasmic reticulum (a) Figure 23.26a

5. Pancreatic Juice • Watery alkaline solution (pH 8) neutralizes chyme and allows pancreatic secreted enzymes to work • The epithelial cells lining the small pancreatic ducts secrete the electrolytes (primarily HCO3–) • The bicarbonate is made in the epithelial cells – for every bicarbonate secreted a H+ is returned to the blood – thus the alkaline tide in the venous blood return from the stomach is balanced by the acidic venous blood from the pancreas

6. Pancreatic Juice • Acinar cells produce the enzyme rich secretion • Enzymes • Amylase, lipases, nucleases are secreted in active form but require ions or bile for optimal activity • Proteases secreted in inactive form • Protease activation in duodenum • Trypsinogen is activated to trypsin by brush border enzyme enteropeptidase • Procarboxypeptidase and chymotrypsinogen are activated by trypsin

7. Stomach Pancreas Epithelial cells Membrane-bound enteropeptidase Trypsinogen (inactive) Chymotrypsinogen (inactive) Procarboxypeptidase (inactive) Trypsin Chymotrypsin Carboxypeptidase Figure 23.27

8. Regulation of Bile Secretion • Gallbladder contraction is stimulated by • Cholecystokinin (CCK) from intestinal cells exposed to proteins and fat in chyme • Vagal stimulation (minor stimulus) • CKK also causes the hepatopancreatic sphincter to relax

9. Regulation of Bile Secretion • Bile secretion is stimulated by • Bile salts in enterohepatic circulation – the more bile salts in the enterohepatic circulation the more bile is secreted. • Secretin from intestinal cells exposed to HCl and fatty chyme

10. Regulation of Pancreatic Secretion • Bile and pancreatic secretions are regulated by the same factors (neural and hormonal) • CCK induces the secretion of enzyme-rich pancreatic juice by acini • Secretin causes secretion of bicarbonate-rich pancreatic juice by duct cells • Vagal stimulation also causes release of pancreatic juice (minor stimulus)

11. Slide 1 4 1 Chyme enter- ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly. 5 CCK (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum. 2 CCK (red dots) and secretin (yellow dots) enter the bloodstream. 3 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3–-rich pancreatic juice. 6 During cephalic and gastric phases, vagal nerve stimulation causes weak contractions of gallbladder. Figure 23.28

12. 1 Chyme enter- ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. Figure 23.28, step 1

13. 1 Chyme enter- ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. 2 CCK (red dots) and secretin (yellow dots) enter the bloodstream. Figure 23.28, step 2

14. 1 Chyme enter- ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. 2 CCK (red dots) and secretin (yellow dots) enter the bloodstream. 3 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3–-rich pancreatic juice. Figure 23.28, step 3

15. 4 1 Chyme enter- ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly. 2 CCK (red dots) and secretin (yellow dots) enter the bloodstream. 3 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3–-rich pancreatic juice. Figure 23.28, step 4

16. 4 1 Chyme enter- ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly. 5 CCK (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum. 2 CCK (red dots) and secretin (yellow dots) enter the bloodstream. 3 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3–-rich pancreatic juice. Figure 23.28, step 5

17. 4 1 Chyme enter- ing duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly. 5 CCK (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum. 2 CCK (red dots) and secretin (yellow dots) enter the bloodstream. 3 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3–-rich pancreatic juice. 6 During cephalic and gastric phases, vagal nerve stimulation causes weak contractions of gallbladder. Figure 23.28, step 6

18. Digestion in the Small Intestine • Chyme from stomach contains • Partially digested carbohydrates and proteins • Undigested fats (minimally worked on by salivary lipase and gastric lipase)

19. Requirements for Digestion and Absorption in the Small Intestine • The intestine must get slow delivery of hypertonic acidic chyme from the stomach • 3cc or less per peristaltic wave and 3 waves per minute so 9 cc or less per minute into small intestine from stomach • If the hypertonic chyme was delivered to the small intestine too quickly it would pull in too much water from the bloodstream • Additionally the chyme is quite acidic – thus ulcer formation if it enters the small intestine too fast

20. The small intestine only provides brush border enzymes – most digestive chemicals in the small intestine come from the liver and pancreas • Delivery of bile, enzymes, and bicarbonate from the liver and pancreas • Intestinal motility mixes chyme with pancreatic, bile and intestinal juices as a result of its segmentation waves • The small intestine mainly uses segmentation waves – peristaltic waves begin after most of the materials have been absorbed

21. Motility of the Small Intestine • Segmentation Waves • Make the intestinal contents appear as if they are being massaged- the chyme is moved back and forward in the lumen a few centimeters at a time by alternating contraction and relaxation of rings of smooth muscle. • Initiated by intrinsic pacemaker cells located in circular muscles – but unlike stomach pacemaker cells which have only one rhythm – the pacemakers in the duodenum depolarize more frequently (12 -14 contractions per minute) than those in ileum ( 8 or 9 contractions per minute)

22. Mixes and moves contents slowly and steadily toward the ileocecal valve – giving plenty time to complete digestion and absorption • The intensity of the waves is altered by long and short reflexes • The segmentation waves wane in the late intestinal (fasting) phase after most of the small intestinal contents have been absorbed • Once the segmentation waves wane the peristaltic waves begin and a result of secretion of motilin from the duodenal mucosae

23. Motility of the Small Intestine • Peristalsis • Initiated by motilin in the late intestinal phase • As the motilin blood level rises peristaltic waves are initiated in the proximal duodenum every 90 – 120 minutes and sweep slowly along the intestines – dying out in approximately 2 feet from its initiation area. • The next wave starts distal to the previous wave thus termed the MMC ( migrating motility complex) • A complete trip from duodenum to ileum takes approximately two hours • The process repeats itself thus meal remnants, bacteria, sloughed off mucosal cells and debris are moved to the large intestine

24. This housekeeping function is critical for preventing the overgrowth of bacteria that migrate from the large intestine. As food enters the stomach with the next meal, peristalsis is replaced by segmentation

25. Motility of the Small Intestine • The local enteric neurons coordinate intestinal motility and it depends on which neurons are activated or inhibited • Cholinergic sensory neurons may activate the myenteric plexus • Causes contraction of the circular muscle proximally and of longitudinal muscle distally • Forces chyme along the tract

26. Motility of the Small Intestine • Most of the time the ileocecal sphincter is closed. Two mechanisms open it • The stomach initiates a gastroileal reflex – a long reflex that enhances the force of segmentation in the ileum • Gastrin increases the motility of the ileum and relaxes the ileocecal valve • Ileocecal valve flaps close when chyme exerts backward pressure

27. Microvilli Absorptive cell (b) Figure 23.3b

28. From mouth (a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally. Figure 23.3a

29. Large Intestines • Approximately twice the diameter of the small intestines – 3 inches wide • Approximately 5 feet long • Function (absorb most of the remaining water from indigestible food residues and temporarily store the residues before elimination as feces)

30. Functions of the Large Intestine • Vitamins, water, and electrolytes are reclaimed • Major function is propulsion of feces toward the anus • Colon is not essential for life

31. Large Intestine • Unique features • Teniae coli • Three bands of longitudinal smooth muscle in the muscularis • Haustra • Pocketlike sacs caused by the tone of the teniae coli • Epiploic appendages • Fat-filled pouches of visceral peritoneum

32. Large Intestine • Regions • Cecum (pouch with attached vermiform appendix) • Colon • Rectum • Anal canal

33. Left colic (splenic) flexure Right colic (hepatic) flexure Transverse mesocolon Transverse colon Epiploic appendages Superior mesenteric artery Descending colon Haustrum Ascending colon Cut edge of mesentery IIeum Teniae coli IIeocecal valve Sigmoid colon Cecum Vermiform appendix Rectum Anal canal External anal sphincter (a) Figure 23.29a

34. Colon • Ascending colon and descending colon are retroperitoneal • Transverse colon and sigmoid colon are anchored via mesocolons (mesenteries)

35. Greater omentum Transverse colon Transverse mesocolon Descending colon Jejunum Mesentery Sigmoid mesocolon Sigmoid colon Ileum (c) Figure 23.30c

36. Liver Lesser omentum Pancreas Stomach Transverse mesocolon Duodenum Transverse colon Mesentery Greater omentum Jejunum Ileum Visceral peritoneum Parietal peritoneum Urinary bladder Rectum (d) Figure 23.30d

37. Rectum and Anus • Rectum • Three rectal valves stop feces from being passed with gas • Anal canal • The last segment of the large intestine • Sphincters • Internal anal sphincter—smooth muscle • External anal sphincter—skeletal muscle

38. Rectal valve Rectum Hemorrhoidal veins Levator ani muscle Anal canal External anal sphincter Internal anal sphincter Anal columns Pectinate line Anal sinuses Anus (b) Figure 23.29b

39. Large Intestine: Microscopic Anatomy • Mucosa of simple columnar epithelium except in the anal canal (stratified squamous) • Abundant deep crypts with goblet cells • Extensive mucus eases passage of feces and protects the intestinal wall from irritating acids and gases released by resident bacteria in the colon • Low folds give anal columns and anal sinuses are between the folds – the sinuses exude mucus when defecate

40. The horizontal tooth-shaped line that parallels the inferior margins of the anal sinuses is called the pectinate line. Superior to this line, the mucosa is innervated by visceral sensory fibers and is relatively insensitive to pain. The area inferior to this line is innervated by somatic sensory fibers – thus very sensitive to pain. • Superficial venous plexuses of the anal canal form hemorrhoids if inflamed

41. Bacterial Flora • 10 million different types • Enter from the small intestine or anus • Metabolize some host products (mucin, heparin, and hyaluronic acid) • Ferment some indegestible carbohydrates (cellulose, xylan, and others • Release irritating acids and a mixture of gases (dimethyl sulfide, H2, N2, CH4, and CO2) • Dimethyl sulfide is quite odorous • About 500 ml of gas is produced each day • Synthesize Vitamin B complex vitamins and Vitamin K

42. Most bacteria exist peacefully with their host in the large intestine – but an elegant system keeps them from breaching the mucosal barrier • The epithelial cells of the gut mucosa respond to specific bacterial components by releasing chemicals that recruit immune cells, particularly dendritic cells into the mucosa. • The dendritic cells pry open the tight junctions between the epithelial cells and send extensions into the lumen of sample the microbial antigens • They then migrate to the nearby lymphoid follicles (MALT) where they present antigens to T cells. • An IgA antibody response restricted to the gut lumen is triggered that prevents the bacteria from straying into tissues deep to the mucosa.

43. Motility of the Large Intestine • Haustral contractions (occur every 30 minutes or so) • Slow segmenting movements that occur mainly in the transverse and descending colon • Haustra sequentially contract in response to distension

44. Mass Movements • Long slow-moving but powerful contractions that move over large areas of the colon- three to four times a day and force the contents towards the rectum. • Typically, they occur during or just after eating, which indicates the presence of food in the stomach activates the gastrocolic reflex in the colon. • Of the 500 cc of fluid entering the cecum only about 150cc becomes feces.

45. Motility of the Large Intestine • Gastrocolic reflex • Initiated by presence of food in the stomach • Activates three to four slow powerful peristaltic waves per day in the colon (mass movements)

46. Defecation • Mass movements force feces into rectum • Distension initiates spinal defecation reflex • Parasympathetic signals • Stimulate contraction of the sigmoid colon and rectum • Relax the internal anal sphincter • Conscious control allows relaxation of external anal sphincter

47. 1 Distension, or stretch, of the rectal walls due to movement of feces into the rectum stimulates stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons. Impulses from cerebral cortex (conscious control) Sensory nerve fibers Voluntary motor nerve to external anal sphincter 2 A spinal reflex is initiated in which parasympathetic motor (efferent) fibers stimulate contraction of the rectal walls and relaxation of the internal anal sphincter. Sigmoid colon Stretch receptors in wall Involuntary motor nerve (parasympathetic division) Rectum External anal sphincter (skeletal muscle) Internal anal sphincter (smooth muscle) If it is convenient to defecate, voluntary motor neurons are inhibited, allowing the external anal sphincter to relax so that feces may pass. 3 Figure 23.31

48. Chemical Digestion • Catabolic • Enzymatic • Hydrolysis

49. Chemical Digestion and Absorption of Carbohydrates • Digestive enzymes • Salivary amylase, pancreatic amylase, and brush border enzymes (dextrinase, glucoamylase, lactase, maltase, and sucrase)

50. Bonding Carbohydrate monomers together • Monosaccharides bond together by the removal of a water molecule (dehydration synthesis) to form a covalent bond between the two monosaccharides known as a “glycosidic bond” • When bond two monosaccharides together termed a disaccharide, when join 3 – 10 together termed an Oligosaccharide – more than 10 together – termed a polysaccharide