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Copyright 2010 Pearson Education, Inc. Pancreas Location Behind the stomach Head is encircled by the duodenum; tail abuts the spleen

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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

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Copyright 2010 Pearson Education, Inc. Figure 23.26a Small duct Acinar cells Basement membrane Zymogen granules Rough endoplasmic reticulum (a)

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Copyright 2010 Pearson Education, Inc. Pancreatic Juice Approximately 1200-1500 ml produced daily Watery alkaline solution (pH 8) neutralizes chyme Electrolytes (primarily HCO 3 ) 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

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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

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Copyright 2010 Pearson Education, Inc. Figure 23.27 Stomach Pancreas Epithelial cells Trypsinogen (inactive) Chymotrypsinogen (inactive) Procarboxypeptidase (inactive) Trypsin Chymotrypsin Carboxypeptidase Membrane-bound enteropeptidase

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Copyright 2010 Pearson Education, Inc. 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

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Copyright 2010 Pearson Education, Inc. Digestion in the Small Intestine Chyme from stomach contains Partially digested carbohydrates and proteins Undigested fats

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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

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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

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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

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Copyright 2010 Pearson Education, Inc. Figure 23.3a From mouth (a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally.

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Copyright 2010 Pearson Education, Inc. Large Intestine Regions Cecum (pouch with attached vermiform appendix) Colon Ascending Transverse Descending Sigmoid Rectum Anal canal

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Copyright 2010 Pearson Education, Inc. Figure 23.29a Left colic (splenic) flexure Transverse mesocolon Epiploic appendages Descending colon Teniae coli Sigmoid colon Cut edge of mesentery External anal sphincter Rectum Anal canal (a) Right colic (hepatic) flexure Transverse colon Superior mesenteric artery Haustrum Ascending colon IIeum IIeocecal valve Vermiform appendix Cecum

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Copyright 2010 Pearson Education, Inc. 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 sphinctersmooth muscle External anal sphincterskeletal muscle

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Copyright 2010 Pearson Education, Inc. Figure 23.29b (b) Rectal valve Rectum Anal canal Levator ani muscle Anus Anal sinuses Anal columns Internal anal sphincter External anal sphincter Hemorrhoidal veins Pectinate line

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Copyright 2010 Pearson Education, Inc. 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

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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

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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

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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)

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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

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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

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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)

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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

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Copyright 2010 Pearson Education, Inc. Chemical Digestion of Lipids Lipids Fatty Acids & Monoglycerides Pre-treatmentemulsification by bile salts Enzymespancreatic lipase Absorption of glycerol and short chain fatty acids

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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 B 12 binds with intrinsic factor, and is absorbed by endocytosis

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Copyright 2010 Pearson Education, Inc. Vitamin Absorption In large intestine Vitamin K and B vitamins from bacterial metabolism are absorbed

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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)

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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)

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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

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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