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B12/IF dimers are recognized by receptors on the enterocytes in the terminal ileum, and thecomplex is internalized. The B12 is bound to transcobalamin II (TCII) fortransport through the blood to the liver for storage, or bone marrow forred blood cell maturation.(Reprinted with permission from Hansen J: Netter’s Atlas of HumanPhysiology, Philadelphia, Elsevier, 2002.)to the liver and spleen for storage.
Typically, ∼1 milligram(mg) of iron is absorbed daily, which balances the ∼1 mg lossof iron (from intracellular ferritin stores) that occurs whenenterocytes slough off each day. If iron utilization increases(such as in growing children and menstruating women), ironintake must increase, or anemia will result.VITAMIN ABSORPTIONFat soluble vitamins A, D, E, and K are transported in micellesto the enterocytes, although vitamin A and D are less hydrophobic, and are also able to access the enterocytes independently. Table 25.2 details the sites and mechanism of vitaminabsorption in the small intestine.Vitamin B12 AbsorptionVitamin B12 (cobalamin) is an essential vitamin and is protected from digestion until it is absorbed in the terminalileum.
The process of protecting B12 begins in the mouth:■■■In the saliva, vitamin B12 is bound by R-proteins.Intrinsic factor is secreted by gastric parietal cells. Itdoes not bind B12 until it reaches the duodenum, wherethe R-proteins are cleaved by trypsin.The B12-intrinsic factor complex dimerizes and continues down the small intestine to the terminal ileum,where the complex binds to a transport protein andenters the cell. The B12 binds to transcobalamin II inthe cytosol, exits the cell, and is transported to the bonemarrow (where it promotes red blood cell maturation)or to the liver for storage (Fig.
25.7).Surgical removal of the terminal ileum causes multipleproblems, including loss of both bile recycling andvitamin B12 absorption. In this circumstance, bile productionby the liver is upregulated, but production is not sufficient forabsorption of fat contained in a normal meal, and some steatorrhea may result. To treat vitamin B12 deficiency, injections areadministered every few months (infrequent injections are sufficient because excess B12 will be stored in the liver).300Gastrointestinal PhysiologyCLINICAL CORRELATEGluten Enteropathy and Celiac DiseaseCeliac sprue is an autoimmune “allergic” reaction to gluten proteins in wheat and results from the plasma cell production ofantibodies (IgA and IgG) against the intestinal epithelium.
Theα-gliadin (part of gluten proteins) is deaminated in the enterocytes, increasing the immunogenicity of gliadin. The antibodiescross-react with the intestinal tissue, producing inflammation;this results in flattening of the villous lining and crypt hyperplasia.The surface area for absorption is diminished, and brush borderenzyme activity is impaired. The loss of brush border enzymeswith villous atrophy will reduce final digestion of carbohydratesand protein and cause general malabsorption of micronutrientsand macronutrients. The presence of undigested nutrients lowerin the GI tract facilitates bacterial production of gasses, causingcramps, odiferous gas, and feces.
Diarrhea, weight loss, anemia,and vitamin deficiency can occur.Although it is suggested that 1 in 250 persons have some degreeof gluten enteropathy, most cases are considered “silent” sprue,with little or no GI symptoms. In addition to GI complications,the immune reaction to wheat can also cause respiratory (rhinitis,asthma) and skin (eczema, dermatitis herpetiformis, and/or hives)problems.
Primary treatment involves complete avoidance ofgluten in the diet.Atrophy andthinning ofbowel wallFlattening,thickening,and clubbingof intestinal villiGluten Enteropathy (Celiac Disease)Review Questions301Review QuestionsCHAPTER 21: OVERVIEW OF THEGASTROINTESTINAL TRACT1. All of the following are functions of the gastrointestinaltract EXCEPT:A.B.C.D.E.secretion of endocrine hormones.secretion of digestive enzymes.absorption of nutrients from chyme.regulation of systemic blood flow.storage of chyme.2. Motility and secretion through the gastrointestinal tract isregulated by all of the following factors EXCEPT:A.B.C.D.E.duodenal hormones.chemoreceptors.autonomic nerves.enteric nerves.growth hormone.3.
In the absence of extrinsic innervation to the gastrointestinal tract,A. propulsive movements would not occur in the smallintestine.B. propulsive movements would occur in the small intestine,but secretion into the small intestine would be abolished.C. secretion would be abolished in the entire GI tract.D. motility and secretion throughout the tract would be stimulated by local mechanoreceptors, chemoreceptors, andosmoreceptors, as well as hormones.E.
motility and secretion would be affected only in the largeintestine.4. Input from vagal efferent nerves to the gastrointestinaltract regulates the following actions EXCEPT:A.B.C.D.E.lower intestinal motility.primary esophageal peristalsis.the cephalic phase of gastric acid secretion.receptive relaxation in the stomach.stimulation of hepatic bile production.CHAPTER 22: MOTILITY THROUGH THEGASTROINTESTINAL TRACT5. Action potentials in the GI smooth muscle:A. occur only in the upper GI tract.B. are stimulated when the slow waves are depolarized above−40 mV.C. occur only in the lower GI tract.D.
result from influx of sodium ions into the smooth musclecells.E. are only stimulated by extrinsic nerves.6. Slow waves constituting the basal electrical rhythm:A. are hyperpolarized by stretch, ACh, and gastrin.B. occur at a consistent rate throughout the GI tract.C. are undulations in the resting membrane potential resulting from Na+/K+ ATPase activity.D. are depolarized by sympathetic nerve stimulation.E.
are absent in the colon.7. In the small intestine, peristalsis:A. occurs primarily in response to the chemical compositionin the chyme.B. proceeds in both forward and backward directions fromthe site of contraction.C. involves receptive relaxation of the muscle proximal(toward the mouth) to the bolus of chyme.D. involves contraction of the muscle proximal to the bolusof chyme.E. is dependent on parasympathetic innervation.8. Select the FALSE answer about the migrating myoelectriccomplex (MMC).A. The MMC is a series of contractions that “sweep” undigested fibers and bacteria into the colon.B.
The parasympathetic nervous system controls the MMCcontractions.C. In each cycle, the main contractions of the MMC occurduring phase III.D. The hormone motilin stimulates phase III contractions.E. The MMC occurs during fasting, and stops uponfeeding.CHAPTER 23: GASTROINTESTINALSECRETIONS9. Secretions entering the lumen of the stomach include allof the following substances EXCEPT:A.B.C.D.E.HCl.lipase.mucus.gastric inhibitory peptide.intrinsic factor.302Gastrointestinal Physiology10. Parietal cell gastric acid secretion is regulated:A.B.C.D.E.B.C.D.E.directly by secretin.directly by peptide YY.directly and indirectly by the vagus nerve.indirectly by somatostatin.indirectly by gastrin.16. Metabolism of lipids within the hepatocytes includes:11.
Select the TRUE statement about gastric acid secretion.A. Histamine suppresses acid secretion.B. Blocking the H+/K+ ATPase pump increases alkaline tide.C. Basolateral HCO3−/Cl− exchangers decrease intracellularCl− in parietal cells.D. Parasympathetic stimulation of acid secretion is limited tothe cephalic phase of acid secretion.E. Proton pump (H+/K+ ATPase) activity is the rate-limitingstep in acid secretion.12. Which of the following is NOT a mechanism by whichsodium ions enter the intestinal epithelial cells from thelumenal membrane?A.B.C.D.E.Diffusion down the sodium concentration gradientActive transport via Na+/K+ ATPaseCo-transport with certain amino acidsCo-transport with monosaccharidesIn exchange for H+CHAPTER 24: HEPATOBILIARY FUNCTION14.
The liver performs all of the following functionsEXCEPT:synthesis of cholesterol.vitamin production.β-oxidation of fatty acids.bile acid production.metabolism of proteins.gastrointestinalA. hydrolyze dietary fats.B. are essential for creating micelles.C. are efficiently absorbed into the portal blood all along thesmall intestine.D.
do not contribute to intestinal buffering of acidic chyme.E. production is not affected by loss of the terminal ileum.A. Inhibition of gastric somatostatin releaseB. An increase in Na+/K+ ATPase activity in the basolateralmembranes of the enterocytesC. Stimulation of gastric pepsinogen releaseD. pH of 3 in the duodenum and jejunumE. Secretion of cholecystokinin19. Selectdigestion.theTRUEstatementaboutcarbohydrateA. Digestion of carbohydrates begins in the intestines.B. Loss of the intestinal brush border has no effect on carbohydrate digestion.C. Disaccharidases are secreted into the duodenum from thecrypts of Lieberkühn.D.
Starch digestion is initiated by α-amylase.E. The majority of carbohydrate digestion occurs before theduodenum.20. When chyme is present in the stomach and duodenum,all of the following actions occur EXCEPT:15. Obstruction of blood flow through the liver (from cirrhosis or hepatitis) willon17. Bile salts:18. Which of the following would delay or diminish theabsorption of proteins?A. The release of pancreatic enzymes into the duodenum isprimarily stimulated by gastrin.B. The release of pancreatic enzymes into the duodenum isprimarily stimulated by the sympathetic nerves.C.
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