Transcript Pancreas
Pancreas • Location • Behind the stomach • Head is encircled by the duodenum; tail abuts the spleen Copyright © 2010 Pearson Education, Inc. Pancreas • Endocrine function • Pancreatic islets secrete insulin and glucagon • Exocrine function • Acini (clusters of secretory cells) secrete pancreatic juice • Zymogen granules of secretory cells manufacture digestive enzymes Copyright © 2010 Pearson Education, Inc. Small duct Acinar cells Basement membrane Zymogen granules Rough endoplasmic reticulum (a) Copyright © 2010 Pearson Education, Inc. Figure 23.26a Pancreatic Juice • Approximately 1200-1500 ml produced daily • Watery alkaline solution (pH 8) neutralizes chyme • Electrolytes (primarily HCO3–) • Enzymes • Amylase, lipases, nucleases are secreted in active form but require ions or bile for optimal activity • Proteases secreted in inactive form prevents auto-digestion of pancreas Copyright © 2010 Pearson Education, Inc. Pancreatic Juice • Protease activation in duodenum • Trypsinogen is activated to trypsin by brush border enzyme enteropeptidase • Procarboxypeptidase and chymotrypsinogen are activated by trypsin Copyright © 2010 Pearson Education, Inc. Stomach Pancreas Epithelial cells Membrane-bound enteropeptidase Trypsinogen Trypsin (inactive) Chymotrypsin Chymotrypsinogen (inactive) Carboxypeptidase Procarboxypeptidase (inactive) Copyright © 2010 Pearson Education, Inc. Figure 23.27 Regulation of Bile Secretion • Gallbladder contraction is stimulated mainly by • Cholecystokinin (CCK) from intestinal cells exposed to proteins and fat in chyme • CKK also causes the hepatopancreatic sphincter to relax Copyright © 2010 Pearson Education, Inc. Digestion in the Small Intestine • Chyme from stomach contains • Partially digested carbohydrates and proteins • Undigested fats Copyright © 2010 Pearson Education, Inc. Requirements for Digestion and Absorption in the Small Intestine • Slow delivery of chyme • Chyme is hyperosmotic • Low Ph must be buffered in the duodenum • Delivery of bile, enzymes, and bicarbonate from the liver and pancreas • Mixing Copyright © 2010 Pearson Education, Inc. Motility of the Small Intestine • Segmentation • Initiated by intrinsic pacemaker cells • Mixes and moves contents slowly and steadily toward the ileocecal valve • Wanes in the late intestinal (fasting) phase Copyright © 2010 Pearson Education, Inc. Motility of the Small Intestine • Peristalsis • Initiated by motilin secreted by duodenal mucosa in the late intestinal phase • Each wave starts distal to the previous • Meal remnants, bacteria, and debris are moved to the large intestine • As food enters the stomach again, peristalsis is replaced by segmentation Copyright © 2010 Pearson Education, Inc. From mouth (a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally. Copyright © 2010 Pearson Education, Inc. Figure 23.3a Large Intestine • Regions • Cecum (pouch with attached vermiform appendix) • Colon • Ascending • Transverse • Descending • Sigmoid • Rectum • Anal canal Copyright © 2010 Pearson Education, Inc. Left colic (splenic) flexure Transverse mesocolon Epiploic appendages Right colic (hepatic) flexure Transverse colon Superior mesenteric artery Haustrum Descending colon Ascending colon IIeum Cut edge of mesentery Teniae coli IIeocecal valve Cecum Vermiform appendix Sigmoid colon Rectum Anal canal (a) Copyright © 2010 Pearson Education, Inc. External anal sphincter Figure 23.29a 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 Copyright © 2010 Pearson Education, Inc. Rectal valve Rectum Hemorrhoidal veins Levator ani muscle Anal canal External anal sphincter Internal anal sphincter Anal columns Pectinate line Anal sinuses Anus (b) Copyright © 2010 Pearson Education, Inc. Figure 23.29b Bacterial Flora • Enter from the small intestine or anus • Colonize the colon • Ferment indigestible carbohydrates • Release irritating acids and gases • Synthesize B complex vitamins and vitamin K Copyright © 2010 Pearson Education, Inc. 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 Copyright © 2010 Pearson Education, Inc. Motility of the Large Intestine • Haustral contractions • Slow segmenting movements • Haustra sequentially contract in response to distension • Occur mostly in the transverse and descending colon Copyright © 2010 Pearson Education, Inc. 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) Copyright © 2010 Pearson Education, Inc. Defecation • Mass movements force feces into rectum • Distension of rectal wall initiates defecation reflex • Stimulate contraction of the sigmoid colon and rectum • Relax the internal anal sphincter • Conscious control allows relaxation of external anal sphincter Copyright © 2010 Pearson Education, Inc. Chemical Digestion • Catabolic process (breaks down large molecules into smaller ones (monomers)) • Hydrolysis – enzymatic breakdown of any type of food molecule Copyright © 2010 Pearson Education, Inc. Chemical Digestion of Carbohydrates Carbohydrates Monosaccharides • Digestive enzymes • Salivary amylase, pancreatic amylase, and brush border enzymes (dextrinase, glucoamylase, lactase, maltase, and sucrase) Copyright © 2010 Pearson Education, Inc. Chemical Digestion of Proteins Proteins Amino Acids • Enzymes: pepsin in the stomach • Pancreatic proteases • Trypsin, chymotrypsin, and carboxypeptidase • Brush border enzymes • Aminopeptidases, carboxypeptidases, and dipeptidases Copyright © 2010 Pearson Education, Inc. Chemical Digestion of Lipids Lipids Fatty Acids & Monoglycerides • Pre-treatment—emulsification by bile salts • Enzymes—pancreatic lipase • Absorption of glycerol and short chain fatty acids Copyright © 2010 Pearson Education, Inc. Vitamin Absorption • In small intestine • Fat-soluble vitamins (A, D, E, and K) are carried by micelles (fatty clusters) and then diffuse into absorptive cells • Water-soluble vitamins (vitamin C and B vitamins) are absorbed by diffusion or by passive or active transporters. • Vitamin B12 binds with intrinsic factor, and is absorbed by endocytosis Copyright © 2010 Pearson Education, Inc. Vitamin Absorption • In large intestine • Vitamin K and B vitamins from bacterial metabolism are absorbed Copyright © 2010 Pearson Education, Inc. Malabsorption of Nutrients • Causes • Anything that interferes with delivery of bile or pancreatic juice • Damaged intestinal mucosa (e.g., bacterial infection) Copyright © 2010 Pearson Education, Inc. Malabsorption of Nutrients • Gluten-sensitive enteropathy (celiac disease) • Gluten is found in wheat, rye, and barley products • Gluten damages the intestinal villi and brush border • Symptoms include: bloating, diarrhea, pain, & malnutrition • Treated by eliminating gluten from the diet (all grains but rice and corn) Copyright © 2010 Pearson Education, Inc. Developmental Aspects • During old age • GI tract activity declines, absorption is less efficient, and peristalsis is slowed • Diverticulosis, fecal incontinence, constipation, and cancer of the GI tract Copyright © 2010 Pearson Education, Inc. Cancer • Oral cancers are detected during routine dental exams • Stomach and colon cancers rarely have early signs or symptoms • Metastasized colon cancers frequently cause secondary liver cancer • Prevention • Regular dental and medical examination Copyright © 2010 Pearson Education, Inc.