23.4 Digestive System The Pancreas & Pancreatic Regulation Regulation of Bile Secretions The Small Intestine The Large Intestine
Pancreas Location Mostly retroperitoneal Deep to greater curvature of stomach Head encircled by duodenum Tail abuts spleen
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
Small duct Acinar cell Basement membrane Zymogen granules Duct cell Rough endoplasmic reticulum One acinus Structure of the enzyme-producing tissue of the pancreas.
Acinar cells Pancreatic duct
Pancreatic Juice 1200 1500 ml/day Watery alkaline solution (ph 8) neutralizes chyme Electrolytes (primarily HCO 3 ) Enzymes Amylase, lipases, nucleases secreted in active form but require ions or bile for optimal activity Proteases secreted in inactive form = zymogens
Pancreatic Juice Protease activation in duodenum Trypsinogen activated to trypsin by brush border enzyme enteropeptidase (enterokinase) Procarboxypeptidase and chymotrypsinogen activated by trypsin
Activation of pancreatic proteases in the small intestine. Stomach Pancreas Epithelial cells Membrane-bound enteropeptidase Trypsinogen Trypsin (inactive) Chymotrypsinogen (inactive) Procarboxypeptidase (inactive) Chymotrypsin Carboxypeptidase
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
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
Regulation of Pancreatic Secretion CCK induces secretion of enzymerich 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)
1 Chyme enter -ing duodenum causes duodenal enteroendocrine cells to release cholecystokinin (CCK) and secretin. 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. CCK secretion Secretin secretion
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 HCO 3 -rich pancreatic juice. CCK secretion Secretin secretion
1 Chyme enter -ing duodenum causes duodenal enteroendocrine cells to release cholecystokinin (CCK) and secretin. 4 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 HCO 3 -rich pancreatic juice. CCK secretion Secretin secretion
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 HCO 3 -rich pancreatic juice. 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
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 HCO 3 -rich pancreatic juice. 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
Digestion in the Small Intestine Chyme from stomach contains Partially digested carbohydrates and proteins Undigested fats 3 6 (4 hrs!) hours in small intestine Most water absorbed ~ All nutrients absorbed Small intestine, like stomach, no role in ingestion or defecation
Requirements for Digestion and Absorption in the Small Intestine Slow delivery of acidic, hypertonic chyme to small intestines Delivery of bile, enzymes, and bicarbonate ions from liver and pancreas Mixing
Motility of the Small Intestine Segmentation Most common motion of small intestine Initiated by intrinsic pacemaker cells Mixes contents toward ileocecal valve Intensity altered by long & short reflexes; hormones Parasympathetic ; sympathetic Wanes in late intestinal (fasting) phase
Mucus granules Microvilli forming the brush border Absorptive cell
Motility of the Small Intestine Peristalsis Initiated by rise in hormone motilin in late intestinal phase Following segmentation / every 90 120 minutes / move chyme into ileum Each wave starts distal to previous Migrating motor complex Meal remnants, bacteria, and debris moved to large intestine From duodenum ileum ~ 2 hours
From mouth Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, moving food along the tract distally.
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
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 fecal matter exerts backward pressure Prevents regurgitation into ileum
Large Intestine Unique features Teniae coli 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
Large Intestine Regions Cecum Appendix Colon Ascending Transverse Descending Sigmoidal Rectum Anal canal
Right colic (hepatic) flexure Transverse colon Superior mesenteric artery Haustrum Ascending colon IIeum IIeocecal valve Cecum Left colic (splenic) flexure Transverse mesocolon Epiploic appendages Descending colon Cut edge of mesentery Tenia coli Sigmoid colon Appendix Rectum Anal canal 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
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
Greater omentum Transverse colon Transverse mesocolon Descending colon Jejunum Mesentery Sigmoid mesocolon Sigmoid colon Ileum 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 Mesenteries of the abdominal digestive organs.
Rectum and Anus Rectum 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
Rectal valve Hemorrhoidal veins Rectum Levator ani muscle Anal canal External anal sphincter Internal anal sphincter Anal columns Pectinate line Anus Anal sinuses
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
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)
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 / cytotoxic response Plasma cells / humoral response to produce antibodies (type IgA ) // antibody-mediated response to restricts microbes
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 Reabsorb water Propulsion of feces to anus Defecation Colon not essential for life
Motility of the Large Intestine Most contractions of colon = Haustral contractions Slow segmenting movements Haustra sequentially contract in response to distension
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)
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
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