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This rerouting of blood from other tissues(especially skeletal muscle) to the gut can significantly alter thedistribution of cardiac output. The proportions of cardiacoutput perfusing various tissues are described in Section 3(Chapter 8) for the normal resting state. These proportions arealtered postprandially, favoring blood flow to the GI tract.Overview of the Gastrointestinal Tract245Longitudinal muscleCircular muscleCardiac incisuraFundus of stomachMiddle muscle layer of stomach (circular muscle)Inner muscle layer of stomach forming slingWindow cut in middle muscle layerOuter muscle layer of stomach(longitudinal muscle)Figure 21.2 Musculature of the Stomach The stomach has longitudinal, circular, and obliquemuscle layers.
The oblique layer helps mix and grind the chyme.extending into all of the villi. Blood flows out of the tractthrough the portal veins, directly to the liver (Fig. 21.4). Thisis the “first pass” effect, whereby absorbed substances arerouted directly to the liver (without entering into the generalcirculation), where much processing occurs.Glandular SecretionsGeneral Functions of the GI Tract■■To efficiently move and digest the chyme, a tremendousamount of substances are secreted into the lumen of the tract.Glands produce liters of saliva, mucus, buffers, and enzymesto facilitate the process.LymphaticsLymph lacteals (lymphatic capillaries) are found throughoutthe intestines and extend into the villi of the small intestine,allowing absorption of lipids.
The lymphatic system is alsoextensive in the liver, where it absorbs fluids and proteins andtransports them to the systemic venous blood. Lymphatictransport of proteins contributes substantially to plasmaoncotic pressure and provides binding proteins for circulatingsubstances including hormones, calcium, and iron.■■Digestion: Both mechanical and chemical digestionoccur in the tract. Mechanical digestion refers to thephysical breakdown of the food by initial chewing andgrinding in the stomach.
Chemical digestion is throughspecific enzymatic action and gastric acid (HCl).Endocrine: A great number of hormones are producedby GI-associated cells (from stomach, intestines, pancreas, and liver). Some are hormones, entering thegeneral circulation and affecting other parts of the GItract as well as other organs. There are also hormonesproduced that act on adjacent GI tissues in a paracrinemanner. In fact, the first substance identified as a “circulating hormone” was the GI hormone secretin.Expulsion: After absorption of nutrients through thetract, the fecal waste matter (fiber, undigested material,bacteria, fat from sloughed cells) is expelled throughdefecation.Protection: We constantly ingest bacteria and antigens,and the GI tract keeps this under control through degradation by gastric HCl, as well as immune activity (IgAand Peyer’s patches).246Gastrointestinal PhysiologySubserous plexusLongitudinalintramuscularplexusMyenteric plexus (cross section;hematoxylin-eosin, ⫻200)Myenteric(Auerbach’s)plexusCircularintramuscularplexusSubmucosal(Meissner’s)plexusMyenteric plexus (parallelsection; methylene blue, ⫻200)PeriglandularplexusSubmucosal plexus (longitudinalsection; hematoxylin-eosin, ⫻200)LumenMucosa and mucosal glandsMuscularis mucosaeBrunner’s glandsSubmucosaCircular muscleIntermuscular stromaLongitudinal muscleSubserous connective tissueVisceral peritoneumFigure 21.3 The Enteric Nervous System The myenteric plexus (nerve net) is between the longitudinal and circular muscle layers of the GI tract, and stimulation of these nerves affects muscle contraction.The submucosal plexus is between the circular muscle and submucosa, and stimulation can affect GI secretions.
The enteric nerves can function without extrinsic nerves, through local mechanisms.■■Motility: The chyme needs to be propelled through thetract to allow the digestion, absorption, and expulsionto occur. There are several different forms of propulsionthrough the tract, and if an area loses the ability to movechyme, serious pathology can occur.Absorption: The ultimate goal for the elaborate GIsystem is getting bulk nutrients into the body.
The intestines absorb most of the nutrients that are digested, andthe majority of absorption occurs in the first half of the■■small intestine (through the jejunum). This is accomplished because of the large surface area provided by thevilli on the enterocytes of the small intestine.Secretion: The secretion of mucus, buffers, hormones,and enzymes facilitates lubrication, digestion, motility,and absorption through the tract.Storage: Both the stomach and large intestine act asstorage sites. The stomach accommodates food as it ismixed with the gastric acid and enzymes promotingOverview of the Gastrointestinal Tract247Esophageal veinsBlood from superiormesenteric veinBlood from splenic, gastric,and inferior mesenteric veinsMixture of above twoRightgastricveinCaval tributariesLeft gastric veinPortalveinShortgastricveinsPosterior,anteriorsuperiorpancreaticoduodenalveinsLeftgastroomental(gastroepiploic)veinSuperior mesenteric veinSplenic veinPosterior,anteriorinferiorpancreaticoduodenalveinsRight gastroomental (gastroepiploic) veinInferiormesentericveinMiddlecolic veinLeft colicveinRightcolic veinSigmoid andrectosigmoidveinsIleocolicveinLeft and rightsuperiorrectal veinsAnterior,posteriorcecalveinsAppendicularveinMiddle rectal veins3Levator ani muscleInferior rectal veinsFigure 21.4 The Portal System The portal vessels carry venous blood directly from the intestines tothe liver through the portal vein.breakdown of food into chyme that is small enough topass through the pyloric sphincter into the duodenum.This storage is facilitated by receptive relaxation, mediated by the vagus nerve (see Chapter 22).
If the relaxation did not occur, it would dramatically limit thequantity of food that could be ingested during a mealand enter the stomach. On the opposite end, the largeintestine dehydrates the chyme to produce feces, and thetype of motility there allows retention of the feces untilexpulsion.THIRST AND HUNGER: BEHAVIORALRESPONSES TO MAINTAIN THEMILIEU INTERIEURThirstTo maintain a homeostatic internal environment, we havebehavioral responses to ensure adequate ingestion of fluid andnutrients: thirst and hunger. Thirst is a response to cell andvascular dehydration causing elevated plasma osmolarity.
The248Gastrointestinal PhysiologyDecreasedblood volumeIncreased plasmaosmolalityDry mouth,pharynxWater uptakeby GI tractOsmoreceptorsTHIRSTBaroreceptors(arterial)(renal)ReninAngiotensin IIFigure 21.5 Thirst The thirst response is initiated by either a small increase in plasma osmolarity (1%to 2%) or a significant decrease in blood volume (∼10%). The mechanism is depicted in this illustration.(Modified from Widmaier E, Raff H, Strang K: Vander, Sherman, and Luciano’s Human Physiology, 9th ed., NewYork, Mc-Graw Hill, 2003.)loss of fluid can be a result of urine and fecal fluid loss, aswell as insensible loss of water from sweating and breathing(Fig.
21.5). The feeling of thirst can be stimulated when hypothalamic osmoreceptors sense elevated plasma osmolarity, orarterial baroreceptors sense low blood pressure (often associated with reduced extracellular fluid volume). Both the baroreceptors and osmoreceptors regulate the thirst response(mediated through the hypothalamus) and the release ofantidiuretic hormone (ADH, also known as vasopressin).ADH has effects on renal retention of water (discussed inSection 5) and vascular smooth muscle contraction (discussedin Section 3).
Thirst is controlled at both central and locallevels. Dry mouth can be relieved temporarily by rinsing themouth with fluid; however, if the overall cause is dehydration,the thirst response will return rapidly.Appetite and HungerAlthough the mechanisms are not as clearly understood asthose involved in thirst, hunger is a powerful behavioralresponse to ensure nutrient intake (Fig.
21.6). The main siteof control, as with thirst, resides in the hypothalamus, whichhas both the hunger and satiety centers. The basic control ofthe centers is through essential nutrients such as glucose andfatty acids. Stimulation of appetite can occur through lowblood glucose, as well as through sensory input (e.g., smelling, seeing food). The GI tract also plays a role, releasinghormones that stimulate (ghrelin, galanin, orexin) anddecrease (cholecystokinin, peptide YY) hunger at the level ofthe hypothalamus.
A key factor in satiety is the hormoneleptin, which can be released from adipose tissues in responseto insulin. Leptin receptors are present on the hypothalamus(as well as other tissues) and leptin stimulates the satietycenter. In addition to such specific factors, the hungerThe control of plasma osmolarity is so important that asmall (2%) change (e.g., from 300 to 306 mosm/L) caninduce thirst and ADH secretion. In contrast, a 12% to 15%reduction in extracellular fluid (ECF) volume is needed to stimulate the same ADH response if osmolarity is unchanged.
Thus,when thirst is stimulated by dehydration, sufficient fluid hasbeen lost to raise plasma osmolarity and stimulate ADH.Overview of the Gastrointestinal TractVentromedial hypothalamic area(inhibitory: “satiety center”)Lateral hypothalamic area(facilitative: “appetite center”)Smell of foodSightof foodThalamusCalcarinefissureIIITouch(sucking Memory and/or fantasyreflex)Tasteof foodChordatympaniGeniculatebodies(schematic)VVIIVIIIDorsal nucleusof vagusNucleus ofsolitary tractIXXHearing (sounds offood preparation,call to dinner, Decreased levelsof CCK, GLP-1,bell, etc.)and leptinDepletion of bodynutrient storesMyentericandsubmucousplexusesMode ofinducing hungercontractionsunknownKEYSympathetic efferentsParasympathetic efferentsAfferents (and CNS connections)Hunger contractions in stomachIndefinite pathsVagus Thoracicnerve sympatheticganglionicchainDorsal rootganglionT6T7T8ThoracicsplanchnicnervesT9T10CeliacganglionFigure 21.6 Appetite and Hunger The feelings of hunger and satiety are not completely understood;however, general pathways are outlined above.
The hypothalamus (see Chapter 26) plays important rolesin both hunger (lateral hypothalamic area) and satiety (ventromedial hypothalamic area) and responds toendocrine and nerve input. Multiple hormonal systems facilitate the feeling of satiety: cholecystokin, peptideYY and GLP-1 (glucagon-like peptide-1) are released in response to ingested food, decreasing hunger. Inaddition, the hormone leptin is released in response to insulin and acts at the satiety center of the hypothalamus. In contrast, hunger “pains” are elicited through vagal stimulation of the enteric nerves, and therelease of other GI hormones, including ghrelin and orexin, stimulate the appetite center of the hypothalamus.
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