23 The Digestive System: Part C

Pancreas Location Mostly retroperitoneal, deep to greater curvature of stomach Head encircled by duodenum; tail abuts spleen © 2013 Pearson Education, Inc.

Pancreas Endocrine function Exocrine 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.

Small duct Acinar cell Basement membrane Zymogen granules Rough 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.

Acinar cells Pancreatic duct Figure 23.26b Structure of the enzyme-producing tissue of the pancreas. Acinar cells Pancreatic duct © 2013 Pearson Education, Inc.

Watery alkaline solution (pH 8) neutralizes chyme 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.

Protease activation in duodenum Pancreatic Juice Protease activation in duodenum Trypsinogen activated to trypsin by brush border enzyme enteropeptidase Procarboxypeptidase and chymotrypsinogen activated by trypsin © 2013 Pearson Education, Inc.

Figure 23.27 Activation of pancreatic proteases in the small intestine. Stomach Pancreas Epithelial cells Membrane-bound enteropeptidase Trypsinogen (inactive) Chymotrypsinogen Procarboxypeptidase Trypsin Chymotrypsin Carboxypeptidase © 2013 Pearson Education, Inc.

Regulation of Bile Secretion Bile secretion stimulated by Bile salts in enterohepatic circulation Secretin from intestinal cells exposed to HCl and fatty chyme Hepatopancreatic sphincter closed unless digestion active  bile stored in gallbladder Released to small intestine ~ only with contraction © 2013 Pearson Education, Inc.

Regulation of Bile Secretion Gallbladder contraction stimulated by Cholecystokinin (CCK) from intestinal cells exposed to acidic, fatty chyme Vagal stimulation (minor stimulus) CCK also causes Secretion of pancreatic juice Hepatopancreatic sphincter to relax © 2013 Pearson Education, Inc.

Regulation of Pancreatic Secretion CCK induces 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) © 2013 Pearson Education, Inc.

Chyme enter -ing duodenum causes duodenal Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice. Slide 1 Chyme enter -ing duodenum causes duodenal enteroendocrine cells to release cholecystokinin (CCK) and secretin. 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. CCK (red dots) and secretin (yellow dots) enter the bloodstream. 2 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3− -rich pancreatic juice. 3 During cephalic and gastric phases, vagal Nerve stimu- lates gallbladder to contract weakly. 6 CCK secretion Secretin secretion © 2013 Pearson Education, Inc.

Chyme enter -ing duodenum causes duodenal Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice. Slide 2 Chyme enter -ing duodenum causes duodenal enteroendocrine cells to release cholecystokinin (CCK) and secretin. 1 © 2013 Pearson Education, Inc.

Chyme enter -ing duodenum causes duodenal Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice. Slide 3 Chyme enter -ing duodenum causes duodenal enteroendocrine cells to release cholecystokinin (CCK) and secretin. 1 CCK (red dots) and secretin (yellow dots) enter the bloodstream. 2 CCK secretion Secretin secretion © 2013 Pearson Education, Inc.

Chyme enter -ing duodenum causes duodenal Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice. Slide 4 Chyme enter -ing duodenum causes duodenal enteroendocrine cells to release cholecystokinin (CCK) and secretin. 1 CCK (red dots) and secretin (yellow dots) enter the bloodstream. 2 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3− -rich pancreatic juice. 3 CCK secretion Secretin secretion © 2013 Pearson Education, Inc.

Chyme enter -ing duodenum causes duodenal Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice. Slide 5 Chyme enter -ing duodenum causes duodenal enteroendocrine cells to release cholecystokinin (CCK) and secretin. 1 4 Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate Liver to produce bile more rapidly. CCK (red dots) and secretin (yellow dots) enter the bloodstream. 2 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3− -rich pancreatic juice. 3 CCK secretion Secretin secretion © 2013 Pearson Education, Inc.

Chyme enter -ing duodenum causes duodenal Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice. Slide 6 Chyme enter -ing duodenum causes duodenal enteroendocrine cells to release cholecystokinin (CCK) and secretin. 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. CCK (red dots) and secretin (yellow dots) enter the bloodstream. 2 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3− -rich pancreatic juice. 3 CCK secretion Secretin secretion © 2013 Pearson Education, Inc.

Chyme enter -ing duodenum causes duodenal Figure 23.28 Mechanisms promoting secretion and release of bile and pancreatic juice. Slide 7 Chyme enter -ing duodenum causes duodenal enteroendocrine cells to release cholecystokinin (CCK) and secretin. 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. CCK (red dots) and secretin (yellow dots) enter the bloodstream. 2 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes secretion of HCO3− -rich pancreatic juice. 3 During cephalic and gastric phases, vagal Nerve stimu- lates gallbladder to contract weakly. 6 CCK secretion Secretin secretion © 2013 Pearson Education, Inc.

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.

Requirements for Digestion and Absorption in the Small Intestine Slow delivery of acidic, hypertonic chyme Delivery of bile, enzymes, and bicarbonate ions from liver and pancreas Mixing © 2013 Pearson Education, Inc.

Motility of the Small Intestine Segmentation Most common motion of small intestine Initiated by intrinsic pacemaker cells Mixes/moves contents toward ileocecal valve Intensity altered by long & short reflexes; hormones Parasympathetic  ; sympathetic  Wanes in late intestinal (fasting) phase © 2013 Pearson Education, Inc.

Mucus granules Microvilli forming the brush border Absorptive cell 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 From duodenum  ileum ~ 2 hours © 2013 Pearson Education, Inc.

Figure 23.3a Peristalsis and segmentation. From mouth Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, moving food along the tract distally. © 2013 Pearson Education, Inc.

Motility of the Small Intestine Local enteric neurons coordinate intestinal motility Cholinergic sensory neurons may activate myenteric plexus Causes contraction of circular muscle proximally and of longitudinal muscle distally Forces chyme along tract © 2013 Pearson Education, Inc.

Motility of the Small Intestine Ileocecal sphincter relaxes, admits chyme into large intestine when Gastroileal reflex enhances force of segmentation in ileum Gastrin increases motility of ileum Ileocecal valve flaps close when chyme exerts backward pressure Prevents regurgitation into ileum © 2013 Pearson Education, Inc.

Large Intestine Unique features Teniae coli Haustra Three bands of longitudinal smooth muscle in muscularis Haustra Pocketlike sacs caused by tone of teniae coli Epiploic appendages Fat-filled pouches of visceral peritoneum © 2013 Pearson Education, Inc.

Large Intestine Regions Cecum Appendix Colon Rectum Anal canal © 2013 Pearson Education, Inc.

External anal sphincter Figure 23.29a Gross anatomy of the large intestine. Left colic (splenic) flexure Transverse mesocolon Right colic (hepatic) flexure Epiploic appendages Transverse colon Superior mesenteric artery Descending colon Haustrum Ascending colon IIeum Cut edge of mesentery IIeocecal valve Tenia coli Sigmoid colon Cecum Appendix Rectum Anal canal External anal sphincter © 2013 Pearson Education, Inc.

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.

Retroperitoneal except for transverse and sigmoid regions Colon Retroperitoneal except for transverse and sigmoid regions Ascending colon (right side – to level of right kidney)  right colic (hepatic) flexure  Transverse colon  left colic (splenic) flexure  Descending colon (left side)  Sigmoid colon in pelvis  rectum © 2013 Pearson Education, Inc.

Greater omentum Transverse colon Transverse mesocolon Descending colon 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.

Liver Lesser omentum Pancreas Stomach Duodenum Transverse mesocolon 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 Sphincters Three rectal valves stop feces from being passed with gas (flatus) 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.

Anal columns Pectinate line Anal sinuses Anus 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.

Large Intestine: Microscopic Anatomy Thicker mucosa of simple columnar epithelium except in anal canal (stratified squamous to withstand abrasion) No circular folds, villi, digestive secretions Abundant deep crypts with goblet cells Superficial venous plexuses of anal canal form hemorrhoids if inflamed © 2013 Pearson Education, Inc.

Enter from small intestine or anus 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.

Viruses and protozoans 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 antibody-mediated 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.

Motility of the Large Intestine Most contractions of colon Haustral contractions Slow segmenting movements Haustra sequentially contract in response to distension © 2013 Pearson Education, Inc.

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

Mass movements force feces toward rectum Defecation Mass movements force feces toward rectum Distension initiates spinal defecation reflex Parasympathetic signals Stimulate contraction of sigmoid colon and rectum Relax internal anal sphincter Conscious control allows relaxation of external anal sphincter © 2013 Pearson Education, Inc.

Muscles of rectum contract to expel feces Defecation Muscles of rectum contract to expel feces Assisted by Valsalva's maneuver Closing of glottis, contraction of diaphragm and abdominal wall muscles  increased intra-abdominal pressure Levator ani muscle contracts  anal canal lifted superiorly  feces leave body © 2013 Pearson Education, Inc.

stimulating stretch receptors there. The receptors transmit Figure 23.31 Defecation reflex. Slide 1 Impulses from cerebral cortex (conscious control) Sensory nerve fibers Feces move into and distend the rectum, stimulating stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons. 1 Voluntary motor nerve to external anal sphincter Sigmoid colon Stretch receptors in wall 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. 2 Rectum External anal sphincter (skeletal muscle) Involuntary motor nerve (parasympathetic division) Internal anal sphincter (smooth muscle) If it is convenient to defecate, voluntary motor neurons are inhibited, allowing the external anal sphincter to relax so feces may pass. 3 © 2013 Pearson Education, Inc.

stimulating stretch receptors there. The receptors transmit Figure 23.31 Defecation reflex. Slide 2 Sensory nerve fibers Feces move into and distend the rectum, stimulating stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons. 1 Sigmoid colon Stretch receptors in wall © 2013 Pearson Education, Inc. 47

stimulating stretch receptors there. The receptors transmit Figure 23.31 Defecation reflex. Slide 3 Sensory nerve fibers Feces move into and distend the rectum, stimulating stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons. 1 Sigmoid colon Stretch receptors in wall 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. 2 Rectum Involuntary motor nerve (parasympathetic division) Internal anal sphincter (smooth muscle) © 2013 Pearson Education, Inc. 48

stimulating stretch receptors there. The receptors transmit Figure 23.31 Defecation reflex. Slide 4 Impulses from cerebral cortex (conscious control) Sensory nerve fibers Feces move into and distend the rectum, stimulating stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons. 1 Voluntary motor nerve to external anal sphincter Sigmoid colon Stretch receptors in wall 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. 2 Rectum External anal sphincter (skeletal muscle) Involuntary motor nerve (parasympathetic division) Internal anal sphincter (smooth muscle) If it is convenient to defecate, voluntary motor neurons are inhibited, allowing the external anal sphincter to relax so feces may pass. 3 © 2013 Pearson Education, Inc. 49

Digestion Digestion Enzymes Hydrolysis 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.

Digestion of Carbohydrates Only monosaccharides can be absorbed Monosaccharides absorbed as ingested Glucose, fructose, galactose Digestive enzymes Salivary amylase, pancreatic amylase, and brush border enzymes (dextrinase, glucoamylase, lactase, maltase, and sucrase) Break down disaccharides sucrose, lactose, maltose; polysaccharides glycogen and starch © 2013 Pearson Education, Inc.

Digestion of Carbohydrates Starch digestion Salivary amylase (saliva)  oligosaccharides at pH 6.75 – 7.00 Pancreatic amylase (small intestine)  breaks down any that escaped salivary amylase  oligosaccharides Brush border enzymes (dextrinase, glucoamylase, lactase, maltase, sucrase)  oligosaccharides  monosaccharides © 2013 Pearson Education, Inc.

Starch and disaccharides Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (1 of 4) Foodstuff Enzyme(s) and source Site of action Path of absorption Starch and disaccharides Glucose and galactose are absorbed via cotransport with sodium ions. Fructose passes via facilitated diffusion. All monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Salivary amylase Mouth Pancreatic amylase Small intestine Oligosaccharides and disaccharides Carbohydrate digestion Brush border enzymes in small intestine (dextrinase, gluco- amylase, lactase, maltase, and sucrase) Small intestine Lactose Maltose Sucrose Galactose Glucose Fructose © 2013 Pearson Education, Inc.

Digestion of Proteins Source is dietary, digestive enzymes, mucosal cells; digested to amino acid monomers Begins with pepsin in stomach at pH 1.5 – 2.5 Inactive in high pH of duodenum Pancreatic proteases Trypsin, chymotrypsin, and carboxypeptidase Brush border enzymes Aminopeptidases, carboxypeptidases, and dipeptidases © 2013 Pearson Education, Inc.

Amino acids of protein fragments Figure 23.33 Protein digestion and absorption in the small intestine. Slide 1 Lumen of intestine Amino acids of protein fragments Brush border enzymes Pancreatic proteases Apical membrane (microvilli) Na+ Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxy- peptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase) of mucosal cells. 1 Absorptive epithelial cell Na+ The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side. 2 Amino acid carrier The amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts. 3 Capillary © 2013 Pearson Education, Inc.

Amino acids of protein fragments Figure 23.33 Protein digestion and absorption in the small intestine. Slide 2 Lumen of intestine Amino acids of protein fragments Brush border enzymes Pancreatic proteases Apical membrane (microvilli) Na+ Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxy- peptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase) of mucosal cells. 1 Absorptive epithelial cell Na+ Capillary © 2013 Pearson Education, Inc. 56

Amino acids of protein fragments Figure 23.33 Protein digestion and absorption in the small intestine. Slide 3 Lumen of intestine Amino acids of protein fragments Brush border enzymes Pancreatic proteases Apical membrane (microvilli) Na+ Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxy- peptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase) of mucosal cells. 1 Absorptive epithelial cell Na+ The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side. 2 Amino acid carrier Capillary © 2013 Pearson Education, Inc. 57

Amino acids of protein fragments Figure 23.33 Protein digestion and absorption in the small intestine. Slide 4 Lumen of intestine Amino acids of protein fragments Brush border enzymes Pancreatic proteases Apical membrane (microvilli) Na+ Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxy- peptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase) of mucosal cells. 1 Absorptive epithelial cell Na+ The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side. 2 Amino acid carrier The amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts. 3 Capillary © 2013 Pearson Education, Inc. 58

Figure 23. 32 Flowchart of digestion and absorption of foodstuffs 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 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. Pepsin (stomach glands) in presence of HCl Stomach Large polypeptides Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) Small intestine Protein digestion Small polypeptides, small peptides Brush border enzymes (aminopeptidase, carboxypeptidase, and dipeptidase) Small intestine Amino acids (some dipeptides and tripeptides) © 2013 Pearson Education, Inc.

Pre-treatment—emulsification by bile salts Enzymes—pancreatic lipases Digestion of Lipids Pre-treatment—emulsification by bile salts Does not break bonds Enzymes—pancreatic lipases  Fatty acids and monoglycerides © 2013 Pearson Education, Inc.

with bile salts to form micelles which Figure 23.34 Emulsification, digestion, and absorption of fats. Slide 1 Fat globule Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets). 1 Bile salts 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. 2 Fat droplets coated with bile salts Micelles made up of fatty acids, monoglycerides, and bile salts 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. 3 Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals and are carried away from the intestine in lymph. 4 Epithelial cells of small intestine Lacteal © 2013 Pearson Education, Inc.

Figure 23.34 Emulsification, digestion, and absorption of fats. Slide 1 Fat globule Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets). 1 Bile salts Fat droplets coated with bile salts © 2013 Pearson Education, Inc. 62

with bile salts to form micelles which Figure 23.34 Emulsification, digestion, and absorption of fats. Slide 3 Fat globule Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets). 1 Bile salts 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. 2 Fat droplets coated with bile salts Micelles made up of fatty acids, monoglycerides, and bile salts © 2013 Pearson Education, Inc. 63

with bile salts to form micelles which Figure 23.34 Emulsification, digestion, and absorption of fats. Slide 4 Fat globule Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets). 1 Bile salts 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. 2 Fat droplets coated with bile salts 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 Lacteal © 2013 Pearson Education, Inc. 64

with bile salts to form micelles which Figure 23.34 Emulsification, digestion, and absorption of fats. Slide 5 Fat globule Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets). 1 Bile salts 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. 2 Fat droplets coated with bile salts 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. Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals and are carried away from the intestine in lymph. 4 Epithelial cells of small intestine Lacteal © 2013 Pearson Education, Inc. 65

Figure 23. 32 Flowchart of digestion and absorption of foodstuffs 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 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. Lingual lipase Mouth Gastric lipase Stomach Emulsification by the detergent action of bile salts ducted in from the liver Small intestine Fat digestion Pancreatic lipases Small intestine Monoglycerides (or diglycerides with gastric lipase) and fatty acids © 2013 Pearson Education, Inc.

Digestion of Nucleic Acids Enzymes Pancreatic ribonuclease and deoxyribonuclease  nucleotide monomers Brush border enzyme nucleosidases and phosphatases  free bases, pentose sugars, phosphate ions © 2013 Pearson Education, Inc.

Figure 23. 32 Flowchart of digestion and absorption of foodstuffs Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (4 of 4) Foodstuff Enzyme(s) and source Site of action Path of absorption Nucleic acids Units enter intestinal cells by active transport via membrane carriers. Units are absorbed into capillary blood in the villi and transported to the liver via the hepatic portal vein. Pancreatic ribo- nuclease and deoxyribonuclease Small intestine Nucleic acid digestion Brush border enzymes (nucleosidases and phosphatases) Small intestine Pentose sugars, N-containing bases, phosphate ions © 2013 Pearson Education, Inc.

~ All food; 80% electrolytes; most water absorbed in small intestine 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.

Absorption of Carbohydrates Glucose and galactose Secondary active transport (cotransport) with Na+  epithelial cells Move out of epithelial cells by facilitated diffusion  capillary beds in villi Fructose Facilitated diffusion to enter and exit cells © 2013 Pearson Education, Inc.

Absorption of Carbohydrates Glucose and galactose Secondary active transport (cotransport) with Na+  epithelial cells Move out of epithelial cells by facilitated diffusion  capillary beds in villi Fructose Facilitated diffusion to enter and exit cells © 2013 Pearson Education, Inc. 71

Amino acids transported by several types of carriers Absorption of Protein Amino acids transported by several types of carriers Most coupled to active transport of Na+ Dipeptides and tripeptides actively absorbed by H+-dependent cotransport; digested to amino acids within epithelial cells Enter capillary blood by diffusion © 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 Cluster with bile salts and lecithin to form micelles Released by micelles to diffuse into epithelial cells Combined with lecithin, phospholipids, cholesterol, & coated with proteins to form chylomicrons 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 Absorption of short chain fatty acids Diffuse into portal blood for distribution © 2013 Pearson Education, Inc.

Absorption of Nucleic Acids Active transport across epithelium  bloodstream © 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.

Absorption of Electrolytes Most ions actively along length of small intestine Iron and calcium are absorbed in duodenum Na+ coupled with active absorption of glucose and amino acids Cl– transported actively K+ diffuses in response to osmotic gradients; lost if poor water absorption Usually amount in intestine is amount absorbed © 2013 Pearson Education, Inc.

Absorption of Electrolytes Iron and calcium absorption related to need Ionic iron stored in mucosal cells with ferritin When needed, transported in blood by transferrin Ca2+ absorption regulated by vitamin D and parathyroid hormone (PTH) © 2013 Pearson Education, Inc.

9 L water, most from GI tract secretions, enter small intestine Absorption of Water 9 L water, most from GI tract secretions, enter small intestine 95% absorbed in the small intestine by osmosis Most of rest absorbed in large intestine Net osmosis occurs if concentration gradient established by active transport of solutes Water uptake coupled with solute uptake © 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.

Developmental Aspects Oral membrane  mouth opening Cloacal membrane  anus By week 5 alimentary canal continuous tube from mouth to anus Shortly after, accessory organs bud from mucosa © 2013 Pearson Education, Inc.

Figure 23.36 Embryonic development of the digestive system. Lung bud Stomodeum Brain Foregut Oral membrane Liver Stomach Heart Site of liver development Bile duct Yolk sac Midgut Spinal cord Gall- bladder Dorsal pancreatic bud Cloacal membrane Hindgut Cystic duct Duodenum Body stalk Ventral pancreatic bud Proctodeum Endoderm © 2013 Pearson Education, Inc.

Homeostatic Imbalance Cleft palate and cleft lip Tracheoesophageal fistula Opening between esophagus and trachea Cystic fibrosis Genetic disease  thick mucus  can block pancreatic duct © 2013 Pearson Education, Inc.

Developmental Aspects Fetal nutrition via placenta, but GI tract stimulated to mature by amniotic fluid swallowed in utero Newborn's rooting reflex helps infant find nipple; sucking reflex aids in swallowing Newborns double birth weight in six months; adult diet by 2 years Cholecystitis, ulcers – problems of middle age © 2013 Pearson Education, Inc.

Developmental Aspects During old age GI tract activity declines, less digestive juice, absorption less efficient, peristalsis slows  less frequent bowel movements Taste/smell less acute; periodontal disease often develops Diverticulosis, fecal incontinence, and cancer of GI tract fairly common © 2013 Pearson Education, Inc.

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.