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PowerPoint® Lecture Slides prepared by Barbara Heard, Atlantic Cape Community College CHAPTER 23 The Digestive System: Part C © Annie Leibovitz/Contact Press Images © 2013 Pearson Education, Inc. Pancreas • Endocrine function – Pancreatic islets secrete insulin and glucagon • Exocrine function – Acini (clusters of secretory cells) secrete pancreatic juice • To duodenum via main pancreatic duct • Zymogen granules of acini cells contain proenzymes © 2013 Pearson Education, Inc. Figure 23.26a Structure of the enzyme-producing tissue of the pancreas. Small duct Acinar cell Basement membrane Zymogen granules Rough endoplasmic reticulum Duct cell One acinus © 2013 Pearson Education, Inc. Figure 23.26b Structure of the enzyme-producing tissue of the pancreas. Acinar cells Pancreatic duct © 2013 Pearson Education, Inc. Pancreatic Juice • 1200 – 1500 ml/day • Watery alkaline solution (pH 8) neutralizes chyme • Electrolytes (primarily HCO3–) • Enzymes – Amylase, lipases, nucleases secreted in active form but require ions or bile for optimal activity – Proteases secreted in inactive form © 2013 Pearson Education, Inc. Figure 23.27 Activation of pancreatic proteases in the small intestine. Stomach Pancreas Epithelial cells Membrane-bound enteropeptidase Trypsinogen (inactive) © 2013 Pearson Education, Inc. Trypsin Chymotrypsinogen (inactive) Chymotrypsin Procarboxypeptidase (inactive) Carboxypeptidase Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice. 1 Chyme enter -ing duodenum causes duodenal enteroendocrine cells to release cholecystokinin (CCK) and secretin. 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. © 2013 Pearson Education, Inc. Slide 1 4 Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate Liver to produce bile more rapidly. 5 CCK (via blood stream) causes gallbladder to contract and Hepatopancreatic Sphincter to relax. Bile Enters duodenum. 6 During cephalic and gastric phases, vagal Nerve stimulates gallbladder to contract weakly. CCK secretion Secretin secretion Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice. 1 Chyme enter -ing duodenum causes duodenal enteroendocrine cells to release cholecystokinin (CCK) and secretin. 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. © 2013 Pearson Education, Inc. Slide 6 4 Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate Liver to produce bile more rapidly. 5 CCK (via blood stream) causes gallbladder to contract and Hepatopancreatic Sphincter to relax. Bile Enters duodenum. CCK secretion Secretin secretion Digestion in the Small Intestine • Chyme from stomach contains – Partially digested carbohydrates and proteins – Undigested fats • 3–6 hours in small intestine – Most water absorbed – ~ All nutrients absorbed • Small intestine, like stomach, no role in ingestion or defecation © 2013 Pearson Education, Inc. Figure 23.23 Microvilli of the small intestine. Mucus granules Microvilli forming the brush border Absorptive cell © 2013 Pearson Education, Inc. Motility of the Small Intestine • Peristalsis – Initiated by rise in hormone motilin in late intestinal phase; every 90–120 minutes – Each wave starts distal to previous • Migrating motor complex – Meal remnants, bacteria, and debris moved to large intestine © 2013 Pearson Education, Inc. Figure 23.3a Peristalsis and segmentation. From mouth © 2013 Pearson Education, Inc. Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, moving food along the tract distally. Large Intestine • Regions – Cecum – Appendix – Colon – Rectum – Anal canal © 2013 Pearson Education, Inc. Figure 23.29a Gross anatomy of the large intestine. Left colic (splenic) flexure Right colic (hepatic) flexure Transverse mesocolon Transverse colon Epiploic appendages Superior mesenteric artery Descending colon Haustrum Ascending colon IIeum Cut edge of mesentery IIeocecal valve Tenia coli Sigmoid colon Cecum Appendix Rectum Anal canal © 2013 Pearson Education, Inc. External anal sphincter Subdivisions of the Large Intestine • Cecum – first part of large intestine • Appendix – masses of lymphoid tissue – Part of MALT of immune system – Bacterial storehouse recolonizes gut when necessary – Twisted enteric bacteria accumulate and multiply © 2013 Pearson Education, Inc. Colon • • • • Ascending colon (right side Transverse colon Descending colon (left side) Sigmoid colon in pelvis rectum © 2013 Pearson Education, Inc. Figure 23.30c Mesenteries of the abdominal digestive organs. Greater omentum Transverse colon Transverse mesocolon Descending colon Jejunum Mesentery Sigmoid mesocolon Sigmoid colon Ileum © 2013 Pearson Education, Inc. Figure 23.30d Mesenteries of the abdominal digestive organs. Liver Lesser omentum Pancreas Stomach Duodenum Transverse mesocolon Transverse colon Mesentery Greater omentum Jejunum Ileum Visceral peritoneum Parietal peritoneum Urinary bladder Rectum © 2013 Pearson Education, Inc. Rectum and Anus • Rectum • Anal canal – Last segment of large intestine – Opens to body exterior at anus • Sphincters – Internal anal sphincter—smooth muscle – External anal sphincter—skeletal muscle © 2013 Pearson Education, Inc. Figure 23.29b Gross anatomy of the large intestine. Rectal valve Rectum Hemorrhoidal veins Levator ani muscle Anal canal External anal sphincter Internal anal sphincter Anal columns Pectinate line Anal sinuses Anus © 2013 Pearson Education, Inc. Bacterial Flora • Enter from small intestine or anus – Colonize colon – Synthesize B complex vitamins and vitamin K – Metabolize some host-derived molecules (mucin, heparin, hyaluronic acid) – Ferment indigestible carbohydrates – Release irritating acids and gases (~500 ml/day) © 2013 Pearson Education, Inc. Intestinal Flora • Viruses and protozoans • Bacteria prevented from breaching mucosal barrier – Epithelial cells recruit dendritic cells to mucosa sample microbial antigens present to T cells of MALT IgA antibodymediated response restricts microbes © 2013 Pearson Education, Inc. Digestive Processes in the Large Intestine • Residue remains in large intestine 12–24 hours • No food breakdown except by enteric bacteria • Vitamins (made by bacterial flora), water, and electrolytes (especially Na+ and Cl–) reclaimed • Major functions - propulsion of feces to anus; defecation • Colon not essential for life © 2013 Pearson Education, Inc. Homeostatic Imbalance • Low fiber diet narrowed colon strong contractions increased pressure on walls diverticula (herniations of mucosa) • Diverticulosis commonly in sigmoid colon – Affects ½ people > 70 years • Diverticulitis – Inflamed diverticula; may rupture and leak into peritoneal cavity; may be life threatening © 2013 Pearson Education, Inc. Homeostatic Imbalance • Irritable bowel syndrome – Functional GI disorder – Recurring abdominal pain, stool changes, bloating, flatulence, nausea, depression – Stress common precipitating factor • Stress management important in treatment © 2013 Pearson Education, Inc. Figure 23.31 Defecation reflex. Slide 1 Impulses from cerebral cortex (conscious control) Sensory nerve fibers Voluntary motor nerve to external anal sphincter Sigmoid colon External anal sphincter (skeletal muscle) Rectum Stretch receptors in wall 2 A spinal reflex is initiated in which parasympathetic motor (efferent) fibers stimulate contraction of the rectum and sigmoid colon, and relaxation of the internal anal sphincter. Involuntary motor nerve (parasympathetic division) Internal anal sphincter (smooth muscle) 3 If it is convenient to defecate, voluntary motor neurons are inhibited, allowing the external anal sphincter to relax so feces may pass. © 2013 Pearson Education, Inc. 1 Feces move into and distend the rectum, stimulating stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons. Digestion • Digestion – Catabolic; macromolecules monomers small enough for absorption • Enzymes – Intrinsic and accessory gland enzymes break down food • Hydrolysis – Water is added to break bonds © 2013 Pearson Education, Inc. Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (2 of 4) Foodstuff Enzyme(s) and source Site of action Path of absorption Proteins Pepsin (stomach glands) in presence of HCl Stomach Small intestine Small polypeptides, small peptides Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) Small intestine Amino acids (some dipeptides and tripeptides) Brush border enzymes (aminopeptidase, carboxypeptidase, and dipeptidase) Large polypeptides Protein digestion © 2013 Pearson Education, Inc. • Amino acids are absorbed via cotransport with sodium ions. • Some dipeptides and tripeptides are absorbed via cotransport with H+ and hydrolyzed to amino acids within the cells. • Infrequently, transcytosis of small peptides occurs. • Amino acids leave the epithelial cells by facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Digestion of Lipids • Pre-treatment—emulsification by bile salts – Does not break bonds • Enzymes—pancreatic lipases – Fatty acids and monoglycerides © 2013 Pearson Education, Inc. Figure 23.34 Emulsification, digestion, and absorption of fats. Fat globule 1 Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets). Bile salts Fat droplets coated with bile salts 2 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa. Micelles made up of fatty acids, monoglycerides, and bile salts 3 Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other fatty substances and proteins to form chylomicrons. Epithelial cells of small intestine © 2013 Pearson Education, Inc. 4 Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals and are carried away from the intestine in lymph. Lacteal Slide 1 Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (3 of 4) Foodstuff Enzyme(s) and source Site of action Path of absorption Unemulsified triglycerides Fat digestion Monoglycerides (or diglycerides with gastric lipase) and fatty acids © 2013 Pearson Education, Inc. Lingual lipase Mouth Gastric lipase Stomach Emulsification by the detergent action of bile salts ducted in from the liver Small intestine Pancreatic lipases Small intestine • Fatty acids and monoglycerides enter the intestinal cells via diffusion. • Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells. The resulting chylomicrons are extruded by exocytosis. • The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct. • Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein. Absorption • ~ All food; 80% electrolytes; most water absorbed in small intestine – Most prior to ileum • Ileum reclaims bile salts • Most absorbed by active transport blood – Exception - lipids © 2013 Pearson Education, Inc. Homeostatic Imbalance • Whole proteins not usually absorbed • Can be taken up by endocytosis/exocytosis – Most common in newborns food allergies • Usually disappear with mucosa maturation – Allows IgA antibodies in breast milk to reach infant's bloodstream passive immunity © 2013 Pearson Education, Inc. Absorption of Lipids • Absorption of monoglycerides and fatty acids – Enter lacteals; transported to systemic circulation – Hydrolyzed to free fatty acids and glycerol by lipoprotein lipase of capillary endothelium • Cells can use for energy or stored fat © 2013 Pearson Education, Inc. Absorption of Vitamins • In small intestine – Fat-soluble vitamins (A, D, E, and K) carried by micelles; diffuse into absorptive cells – Water-soluble vitamins (vitamin C and B vitamins) absorbed by diffusion or by passive or active transporters. – Vitamin B12 (large, charged molecule) binds with intrinsic factor, and is absorbed by endocytosis © 2013 Pearson Education, Inc. Absorption of Vitamins • In large intestine – Vitamin K and B vitamins from bacterial metabolism are absorbed © 2013 Pearson Education, Inc. Malabsorption of Nutrients • Causes – Anything that interferes with delivery of bile or pancreatic juice – Damaged intestinal mucosa (e.g., bacterial infection; some antibiotics) © 2013 Pearson Education, Inc. Malabsorption of Nutrients • Gluten-sensitive enteropathy (celiac disease) – Immune reaction to gluten – Gluten causes immune cell damage to intestinal villi and brush border – Treated by eliminating gluten from diet (all grains but rice and corn) © 2013 Pearson Education, Inc. Cancer • Stomach and colon cancers rarely have early signs or symptoms • Metastasized colon cancers frequently cause secondary liver cancer • Prevention – Regular dental and medical examination © 2013 Pearson Education, Inc.