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It is considered dangerous and is performed less frequently today.Gastric bypass, a common surgery, involves keeping only asmall part of the fundus and attaching it to the early jejunum.This leaves little space for food (about a tablespoon-full). Inthis procedure, the main body of the stomach is “stapled” offand the duodenum is reattached to the jejunum, allowing thedigestive juices (primarily from the pancreas and liver) to mixwith the food. The overall effect is a reduction in food intake,and weight loss.Bariatric surgery is not without risk and can be associated withvomiting, diarrhea, reflux, leaking from the surgical sites, andinfection.
It is usually only recommended in morbidly obesepatients.Gastric stapling (vertical banded gastroplasty)Gastric bypass (Roux-en-Y)EsophagusStomach pouchBand257StomachpouchOversewnstaple linesEnd-to-side typeanastomosis betweenthe gastric pouch andthe Roux-en-y limbBypassedportion ofthe stomachDuodenumLaparoscopic adjustable gastric bandingJejunumAdjustable bandStomachSkinSubcutaneousport (reservoir)Rectus abdominis muscle258Gastrointestinal PhysiologyTable 22.2Main Reflexes through the GI TractReflexActionMediated BySwallowingContraction of pharynx and upperesophagus, inhibition of respirationTouch receptors in pharynx to swallowingcenter of the medulla; back to pharynx andupper esophagus through cranial nerves, andto the rest of the esophagus through vagalnervesReflexive relaxation of stomach andduodenumRelaxes the fundus and body of stomachwhen food, water, or gas is present.
Alsooccurs in duodenum when bolus enters.Vagal fiber release of VIPVomitingExpulsion of upper intestinal and gastriccontents by reverse peristalsisIrritation of pharynx, esophagus, stomach, orintestines, via vagal and sympathetic afferentsto vomiting center; or stimulation ofchemoreceptor trigger zone (in medulla neararea postrema) to vomiting centerPeristalsisContraction behind bolus, relaxation aheadof bolusMechanoreceptor and hormone action onenteric nervesGastrocolicMass movements in colon after mealPNS and hormones (CCK, gastrin)GastroilealIncreased segmentation in ileum inresponse to gastric emptyingPNS and hormones (CCK, gastrin)IleogastricChyme in the ileum decreases gastricemptyingEnteric and autonomic nervesEnteroenteric (aka Intestinointestinal)If one area of SI is overdistended (e.g., bybacterial infection) the rest of the SI willrelax, causing cessation of motilityEnteric and autonomic nervesColonocolonicDistension of one area of colon will relaxother areasEnteric and sympathetic nervesRectosphincteric (aka DefecationReflex)Feces entering the rectum will causeperistalsis and relax the internal analsphincterBoth local (enteric nerves) and PNSThese reflexes do not act in an all-or-nothing manner; many act at the same time to create efficient movement of the chyme.VIP, vasoactive intestinal peptide; PNS, parasympathetic nervous system; CCK, cholecystokinin; SI, small intestine.Table 22.3Effects of Nerves and Hormones onGastric EmptyingEffectorAction on PyloricSphincterSympathetic nervesConstrictionParasympathetic vagal nervesExcitatory via ACh motor neuronInhibitory via VIP motor neuronConstrictionRelaxationHormones—gastrin, GIP, CCK, secretinConstrictionACh, acetylcholine; VIP, vasoactive intestinal peptide; GIP, glucoseinsulinotropic peptide; CCK, cholecystokinin.contraction progress into the antrum, the antral cycle occurs,with relaxation of the sphincter and ejection of some chymeinto the duodenal bulb (see Fig.
22.5B, 3). This cycle (ejectioninto duodenum) is under tight control, and as chymeenters the antrum and duodenum, the release of hormonesdecreases gastric emptying by constricting the sphincter (seeTable 22.3). Thus, relaxation via vagal inhibitory signals allowsthe antral cycle to occur, while the other factors prevent toomuch acidic chyme from entering the duodenum at once.Small IntestineWhen chyme enters the duodenal bulb, receptive relaxationalso occurs via the vagus nerve. In the small intestine, there areMotility through the Gastrointestinal TractThoracic esophagusNeural20mm HgLES, a complex of myogenic,neural, and hormonal factors,acts to maintain pressure of12–20 mm Hg in distal3–5 cm of esophagusNormal LES toneis physiologicbarrier to reflux046Distancefrom nostril(cm)3242cm38343638LES40424446Decreased LES toneor shortened LESsegment allowsacid reflux20Myogenic505220mm HgStomach480460⫺20mm Hg42cm38AcidHormonalFigure 22.4 Lower Esophageal Sphincter The resting tone of the lower esophageal sphincter (LES)is usually very high, preventing reflux of stomach contents.
When food is swallowed, esophageal peristalsisis initiated by the vagus nerve and is propagated by the enteric nerves. As the bolus of food reaches theLES, local nitric oxide (NO) and vasoactive intestinal peptide (VIP) are released, and the sphincter tonerelaxes, allowing the bolus to enter the stomach. If LES tone is decreased when at rest, acid reflux canoccur.259260Gastrointestinal PhysiologyA. Factors affecting gastric emptyingDuodenalchemoreceptorsGastrointestinalhormonesAcidSecretinFatsCholecystokininGastricinhibitorypeptide (GIP)Aminoacids/peptidesDecreasedgastricemptyingGastrinDuodenal stimuli elicit hormonal inhibition of gastric emptyingB. Sequence of gastric motilityAABBC1.
Stomach is filling. A mild peristaltic2. Wave (A) fading out as pylorus fails to 3. Pylorus opens as wave (B) approaches it.wave (A) has started in antrum and isopen. A stronger wave (B) is originatingDuodenal bulb is filled, and some contentspassing toward pylorus. Gastric contentsat incisure and is again squeezingpass into second portion of duodenum.are churned and largely pushed back intogastric contents in both directions.Wave (C) starting just above incisure.body of stomach.HoursCD1112 1210938476 54.
Pylorus again closed. Wave (C) fails to5. Peristaltic waves are now originating 6. 3 to 4 hours later, stomach is almost empty.Small peristaltic wave empties duodenalevacuate contents. Wave (D) startshigher on body of stomach. Gastricbulb with some reflux into stomach.higher on body of stomach. Duodenalcontents are evacuated intermittently.bulb may contract or may remain filledContents of duodenal bulb area pushed Reverse and antegrade peristalsis presentin duodenum.as peristaltic wave originating just beyondpassively into second portion as moreit empties second portion.gastric contents emerge.Figure 22.5 Gastric Motility The presence of food in the stomach stimulates gastric motility throughrelease of vasoactive intestinal peptide (VIP) stimulated by vagal nerves. The stomach contractions forcethe gastric contents toward the pylorus, where most of the contents are pushed back into the body of thestomach (1).
As spurts of chyme enter the duodenum (3), duodenal hormones are released into the blood,circulate back to the stomach, and decrease gastric emptying (A). Thus, there is a coordinated effectof local and autonomic nerves, and hormones, controlling gastric motility: The entire sequence is depictedin B, images 1–6.Motility through the Gastrointestinal Tract261VillousmovementsRhythmic segmentationIntraluminalpressuremarkedlyelevatedIntraluminalpressureslightly elevatedPeristaltic waveReverse peristalsisPeristaltic rush(2 to 25 cm per second)Head of column arrivesat ileocecal valve 3 to 5hours after ingestionFigure 22.6 Peristalsis and Segmentation Motility in the small intestine is primarily under localcontrol of the myenteric plexus and consists of both peristalsis and segmentation.
Segmentation formspockets of chyme and serves to mix and propel the chyme, although normally peristalsis creates aboralmovement (away from mouth). Peristaltic rushes can occur when the intestines are irritated, moving chymerapidly through the intestines.two types of propulsion, peristalsis and segmentation.
Peristalsis follows the “law of the intestines,” according to which contraction takes place behind the bolus of chyme and relaxationahead of the chyme (Fig. 22.6). This process is accomplished bythe simultaneous stimulation of excitatory and inhibitorymotor neurons (see Fig. 22.2). In this manner, the chyme ispropelled aborally (away from the mouth).
In the small intestines, there can also be peristaltic rushes, which quickly propelthe chyme over long segments. These occur when there is irritation or bacteria in the segments, and the movement facilitatesremoval of the irritant. Because this quickly sweeps the chymethrough the intestines, much less absorption occurs, and thiscan result in diarrhea (see Fig. 22.6). Reverse peristalsis canalso occur (seen in vomiting), rapidly moving intestinal andstomach contents toward the mouth.Segmentation is the formation of moving pockets of chyme,by close constriction of circular muscle (see Fig. 22.6). Unlikeperistalsis, which contracts behind the bolus, segmentationcontracts in the middle of the bolus, spreading the chymeproximally and distally. These constrictions occur rhythmically and are the most frequent type of contractions in thesmall intestine.
Because the simultaneous constriction movesin waves, the “pockets” slowly move aborally, mixing contentsbackward and forward. Although much mixing occurs, thereis also net aboral movement. Segmentation and peristalsis262Gastrointestinal PhysiologyR. and L. hepatic ductsR. and L. hepatic arteriesCommon hepatic ductCysticarteryCystic ductProper hepatic arteryCommon bile ductLiverR.
gastric arteryDuGallbladderodenumGastroduodenal arteryStomachCut edge ofanterior layerof lesseromentumColonPancreasFigure 22.7 Structure of the Gallbladder The gallbladder is the storage site for bile and is locatedadjacent to the liver. It can store between 25 and 50 milliliters (mL) of bile, which is released by vagal stimulation and cholecystokinin. When food and chyme are in the stomach, the vagus stimulates initial contraction.Subsequently, when chyme enters the duodenum, cholecystokinin is released, causing strong contractionsand emptying of the gallbladder. Bile is concentrated (salts and water removed) in the gallbladder, allowingmore storage.occur in adjacent segments of the small intestine throughoutthe digestion and absorption of the chyme.
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