1625915643-5d53d156c9525bd62bd0d3434ecdc231 (843955), страница 68
Текст из файла (страница 68)
CCK, cholecystokinin.249250Gastrointestinal PhysiologySweat is hypo-osomotic. Its concentration of salts(mainly sodium and chloride) is less than found inplasma. Thus, sweating depletes the extracellular fluid (ECF) ofmore fluid than salt, which increases the plasma osmolarity.That stimulates the thirst response and reduces fluid lossesby the kidney and colon, helping to maintain blood pressure(see Fig. 21.5).response is affected by body mass and will provide nutrientsfor energy expenditure.Saliva1500 mL/dayIngest2000 mL/dayH2OFLUID SHIFTS AND pH THROUGH THE TRACTAlthough only ∼2 liters of fluid is ingested daily, the fluid fluxin and out of the GI tract is actually about 9 liters.
This isnecessary for digestion of the chyme; however, the secretionsneed to be reabsorbed back into the bloodstream with theadded nutrients (Fig. 21.7). With the addition of secretions,the pH of the chyme changes. In the stomach, gastric acidsecretion brings the pH to ∼2, allowing for indiscriminantdigestion of food. To protect the small intestine from the corrosive effects of the acid, buffers (pH ∼8) are secreted into theduodenum from the liver, pancreas, and gallbladder, as wellas from intestinal crypt cells. This raises the pH of the chymein the first few centimeters of the duodenum to ∼5. The addition of the buffers through the tract brings the pH of thechyme up to ∼7.4 by the early jejunum.
This is optimal for thepancreatic enzyme action.While you are conscious of the salivary secretions that occurin anticipation of, and during, feeding, the other fluid fluxestend to go unnoticed, although at times intestinal soundscalled borborygmi may be heard—these are the result of fluidand gas fluxes in the lumen.THE ENTERIC NERVOUS SYSTEMThe GI tract is unique in having an intrinsic nervous system,made up of the myenteric and submucosal plexuses (seeFig. 21.3). This enteric nervous system (ENS) is able to function independently, using input from mechanoreceptors, chemoreceptors, and osmoreceptors located in the luminalepithelium of the tract.
The ENS receives input from thecentral and autonomic nervous system (CNS and ANS) andhormones, which help fine-tune and regulate the ENS.Without autonomic innervation, the ENS would still function, but in a less coordinated manner.Located between the circular and longitudinal muscle layers,the myenteric (Auerbach’s) plexus regulates contraction andrelaxation of the musculature, causing motility and mixing ofthe luminal contents. The submucosal plexus (Meissner’sBile500 mL/dayGastricsecretions2000 mL/dayPancreaticjuices1500 mL/daySmallintestineabsorbs8500 mL/dayIntestinalsecretions1500 mL/dayColonabsorbs400 mL/day~100 mL/dayH2O excretedFigure 21.7 Fluid Shifts through the GI Tract While we ingest∼2 liters (L) of fluid in food and drink each day, the GI tract adds ∼8 L ofsecretions to facilitate digestion and absorption of the nutrients.
Thesesecretions, including buffers, acid, and enzymes, are absorbed back intothe blood (primarily by absorption in the small intestine) so that only ∼200milliliters (mL) of fluid is excreted in feces each day.Overview of the Gastrointestinal Tract■plexus) is located between the circular muscle and submucosaand regulates local fluid secretions.THE GI TRACT AS AN ENDOCRINE ORGANEndocrine cells in the gastric and intestinal epithelia synthesize and release a variety of hormones into the bloodstream.The hormones act on other areas of the gastrointestinalsystem, such as the liver and pancreas, in addition to thestomach and intestines. These hormones regulate GI function,as well as hunger and satiety and insulin secretion.
In addition, some hormones act on adjacent cells in a paracrinemanner. An example of this is the action of somatostatin(produced in gastric pit cells) on adjacent parietal cells. GIhormones will be covered in Chapter 23.■INTEGRATED REGULATION OF GI FUNCTIONProper GI function results from the integrated responses fromcentral and local factors including:■■■■■Central nervous system (CNS): Sensory input to theCNS provides initial stimulus for salivary and gastricacid secretion and is integral to many reflexes. Simplysmelling or seeing food will initiate a central response.The CNS acts thorough the sympathetic and parasympathetic nervous systems.Enteric nervous system (ENS): The myenteric and submucosal nerve plexuses are unique to the GI tract.
Thesenerve nets receive input from the PNS and SNS, as wellas hormones, peptides, and lumenal receptors that sensethe chemical composition of the chyme.Parasympathetic nervous system (PNS): In general, thePNS promotes secretion and motility in the GI tract, andmany of the actions are through the vagus nerve.Sympathetic nervous system (SNS): The SNS slowssecretion and motility in the GI tract. Think of whathappens in “fight or flight” reactions, in which you needmore blood flow to skeletal muscle, and require lessperfusion of the intestines.Lumenal receptors: The GI tract has many differentreceptors that react to the chyme in the lumen of thetract and act locally on the ENS to regulate motility andsecretions through the general scheme:Myenteric plexusSubmucosal plexusRegulate local muscle/sphincter toneStimulate endocrine cellsto secrete hormonesMechanoreceptors sense stretch of the smoothmuscle, and the generated signal is transducedthrough the myenteric plexus, stimulatingcontractions.■ Chemoreceptors sense the chemical composition ofthe chyme and regulate motility and secretion ofbuffers to control luminal pH during the influx ofacidic chyme into the duodenum.■ Osmoreceptors sense the osmolarity of the chyme inthe small intestine.
This is important, because thereis only a one-cell barrier between the chyme in thelumen of the small intestine and the capillaries, andhypertonic chyme can exert an osmotic force, pullingfluid out of the cells (and ultimately the plasma).Thus, the osmoreceptors control the amount ofchyme entering the small intestine, as well as theamount of secretions necessary to buffer the chyme.Hormones: As stated above, the GI tract produces avariety of endocrine, paracrine, and autocrine factors,which increase the efficiency of the tract (both for motility and secretions).Lastly, while all of these areas are important in the coordinated function of the GI tract, it bears noting that the parasympathetic nerves, primarily via the vagus, are responsiblefor multiple effects early in the digestive process.
Some ofthe actions they stimulate or participate in include thefollowing:■■■■■■■Chemo-, Osmo-, Mechanoreceptors251■■Initiating salivation upon seeing, smelling, tasting foodduring the cephalic phase of salivation (through facialand glossopharyngeal nerves).Initiating acid production when food is in the mouthduring the cephalic and gastric phases of gastric acidsecretion (vagus).Stimulating pancreatic enzyme secretion during thecephalic and gastric phases of pancreatic secretion(vagus).Stimulating pancreatic enzyme and buffer secretionduring the intestinal phase of pancreatic secretion(vagus).Stimulating primary (swallowing center of medulla andvagus) and secondary esophageal peristalsis (vagus, withenteric nerves).Initially relaxing the sphincter of Oddi during cephalicand gastric phases (vagus).Causing receptive relaxation of stomach and duodenum(vagus), to accommodate entry of food/chyme.Stimulating the synthesis of bile by the liver (vagus).Stimulating intestinal motility (ileal motility and colonicmass movements) (vagus to upper colon, pelvic nerveson lower colon; with gastrin and cholecystokinin[CCK]).Thus, if vagal innervation of these areas is impaired, the digestive and propulsive processes will be dysregulated.This page intentionally left blank253CHAPTER22Motility through theGastrointestinal TractELECTRICAL POTENTIALSMotility in the gastrointestinal (GI) tract involves bothmixing and propulsive movements, and is determined bythe mechanical activity of the intestinal smooth muscle.
Themovements serve to mix, transport, and eliminate matter. Toaccomplish the muscle contraction, electrical potentials mustbe generated.The electrical activity in the GI tract is unique. Unlike othertissues, there are undulations in the resting membrane potential known as slow waves, which are generated by the interstitial cells of Cajal (ICC). The ICC are located throughoutthe GI tract between the longitudinal and circular musclelayers, and the undulations in their resting potential are probably caused by the small changes in membrane charge resulting from Na+/K+ ATPase activity. The ICC act as pacemakers,determining the number of waves that occur per minute alongdifferent segments of the GI tract.
Because of gap junctionsbetween cells, the slow waves (also called the basic electricalrhythm [BER]) can be propagated over relatively long segments of the tract.Under resting conditions, the slow waves undulate between−70 and −80 millivolts (mV) and do not cause contractions.However, if the slow waves are depolarized (made less negative) by nerve activity or circulating hormones, the amplitudeof the waves may increase, and when peaks of the slow wavecross the threshold of −40 mV, the cells will generate one ormore action (or spike) potentials.Neurotransmitters such as acetylcholine (ACh) and substanceP (tachykinin) released from parasympathetic nerves terminating on the myenteric plexus depolarize the slow waves,generating action potentials and causing contractions(Figs. 22.1 and 22.2).
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
