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The primary secretion is produced at the blindends of the ducts, in the acini (Fig. 23.3). The glands are highlyvascularized, and the parasympathetic nerves stimulate activesecretion by increasing blood flow to the acini. Isotonic ultrafiltrate from the plasma diffuses through the acinar cells intothe lumen, where this “primary secretion” is mixed with theadditional products of the serous and mucous cells (α-amylaseand mucins, respectively).
The secretion drains down the striated ducts, and the electrolyte content is modified dependingon the rate of flow through the ducts. Thus, as the rate of flowincreases, there is less time for Na+ and Cl− reabsorption, andconcentration of these electrolytes in saliva is higher (seeFig. 23.3B).
Under conditions of reduced vascular volume(dehydration, hemorrhage), Na+ and water reabsorption bythe ductal cells is increased by antidiuretic hormone (ADH)and the mineralocorticoid aldosterone (see Chapter 19). TheADH increases water reabsorption (Na+ follows), while aldosterone increases Na+ reabsorption (water follows). Thus,when plasma volume is low, saliva production is decreased toconserve fluid. As indicated, regardless of flow rate, the salivais always hypotonic to plasma.When the saliva enters the mouth, it mixes with secretionsfrom Von Ebner’s glands (and other minor glands), addinglipase to the fluid. In addition, R proteins found in the salivabind to the essential vitamin B12 in the stomach, protectingit from degradation.
In the duodenum, pancreatic proteasescleave the R proteins and B12 binds to intrinsic factor (secretedby the gastric parietal cells), which protects it until it isabsorbed in the terminal ileum. If any part of the pathway(especially intrinsic factor) is lost, B12 cannot be absorbed,and because B12 is critical for maturation of red blood cells,pernicious anemia develops.Neural Control of SalivaSalivary flow is dependent on parasympathetic nerves (glossopharyngeal and facial). They innervate the blood vessels of272Gastrointestinal PhysiologyMouth:Saliva, lingual lipase,salivary ␣-amylase,R-proteinsLiver and gallbladder:Bile and buffers (electrolytes)Esophagus: Electrolytes and mucusStomach:HCl, intrinsic factor,pepsinogens, gastric lipase,mucus, gastrin, somatostatin,histaminePancreas:Exocrine glands: buffers and enzymes(trypsin, chymotrypsin, procarboxypeptidase,lipase, colipase, proelastase, ␣-amylase),monitor peptide (trypsin inhibitor;stimulates CCK)Endocrine glands:insulin, glucagon, somatostatinSmall intestine:Buffers (mucus and electrolytes),enterokinase, brush border disaccharidases,brush border peptidases, secretin, gastrin,CCK, VIP, GIP, motilin, 5HT, somatostatinFigure 23.1 Major Secretions into the GI Tract In order to digest and absorb ingested nutrients,liters of secretions are added daily to the GI tract from associated glands, organs, and tissues.
The generalsources of digestive enzymes, acid, buffers, and bile are shown in the figure above. CCK, cholecystokinin;5HT, serotonin; GIP, gastric inhibitory peptide; VIP, vasoactive intestinal peptide.Table 23.1Salivary Gland SecretionsTable 23.2Regulation of Salivary FlowGlandSecretionCranial NerveIncrease Salivary FlowReduce Salivary FlowParotidWatery fluid, withoutmucins (serous), ∼25%of totalGlossopharyngeal (IX)SubmaxillaryMixed serous/mucousfluid, ∼70% of totalFacial (VII)SublingualMucous fluid, 5%of totalFacial (VII)• Parasympathetics (CN VII& IX), ACh, VIP• CNS (in cephalic,“sensory” phase)• Nausea• Esophageal distension• Chewy, flavorful foods• Dry, acidic foods• Meats, sweets, and bitterfoods• Sympathetics, NE• Hormones (ADH, aldosterone)conserve water and salt whenvolume depleted• Sleep• Dehydration (activateshormones)• Drugs, chemotherapy• Aging (decreases ANS tone;glands can atrophy)salivary glands, and when stimulated, increase blood flow,increasing ultrafiltrate secretion into the acini.
If the parasympathetic nerves are severed, salivation will significantlydecrease, and the glands will atrophy. The sympathetic nervesalso innervate blood vessels around the glands as well as themyoepithelial cells in the acini. Sympathetic stimulation (forexample, during stress) reduces blood flow but contracts themyoepithelial cells, releasing preformed mucous saliva. AfterACh, acetylcholine; ADH, antidiuretic hormone; ANS, autonomicnervous system; CN, cranial nerve; CNS, central nervous system;NE, norepinephrine; VIP, vasoactive intestinal peptide.that, flow will decrease (resulting in dry mouth). The salivatory nucleus in the brainstem transduces a sensory, centralnervous system (CNS) effect, for example, when visual orolfactory input stimulates salivation (Table 23.2).Gastrointestinal Secretions273Transversefacial arteryAccessoryparotid glandParotid ductBuccinator muscleMasseter muscleTongueSublingual plica withopenings of minorsublingual ductsParotid glandBranches of facial nerveSublingual caruncleMajor sublingual ductSublingual glandSubmandibular ganglionSubmandibular ductLingual nerveExternal jugular veinSublingual arteryMylohyoid muscleSternocleidomastoid muscleSubmandibular glandInternal jugular veinHyoid boneParotid glandExternal carotid arterySubmandibular glandSublingual glandFigure 23.2 Salivary Gland Structure There are three paired salivary glands: the parotid, the submandibular, and the sublingual.
The glands are specialized to secrete mainly serous (parotid), mucous(sublingual), or mixed (submandibular) saliva (see representative histology). About 1.5 liters of saliva issecreted daily, with the greatest secretion while eating.EsophagusGlands in the esophageal mucosa produce serous and mucoussecretions, which lubricate the bolus of food and help propelit to the stomach. This is especially important if food getsstuck and secondary peristalsis occurs.
The increased salivation and esophageal secretion aid movement of the food intothe stomach.GASTRIC PIT SECRETIONS AND REGULATIONOF HCL SECRETIONThe Gastric PitGastric secretions facilitate and regulate digestion, lubricatethe bolus of food, and protect the gastric mucosa. The mixingGastrointestinal PhysiologyEndpiecesStriated and excretory ducts160Na+“Primary Secretion”• Contains amylase• [Na+, K+, Cl–, HCO3–]similar to plasmaConcentration (mEq/L)140K+Saliva160140120100120Cl–Na+80HCO3–604020Cl– HCO3–PlasmaNa+HCO3–K+Cl–K+0Modification ofionic content100806040Concentration (mEq/L)2742003.04.01.02.0Flow of saliva (mL/min)SalivaFigure 23.3 Salivary Acinar Cells During salivary secretion, blood flow to the acini is increased by parasympathetic stimulation, and ultrafiltratefrom plasma (mostly serous fluid) enters the acini.
Filtrate from the cells enters the lumen of the acinar cells, mixing with secreted mucus and αamylase, creating the primary secretion. This secretion is modified as it passes through the ducts into the mouth. Lingual lipase (secreted from theVon Ebner’s glands of the tongue) is added to the saliva in the mouth.
The graph illustrates the effect of increasing salivary secretion on compositionof saliva. The changes are due to the reduced modification of saliva through the ducts as flow increases.of gastric juices with food produces chyme.
The gastric pit ismade of a variety of cells (Fig. 23.4):■■■■■■■■Superficial epithelial cells or surface mucous cells areformed from undifferentiated cells in the pit, thenmigrate to the surface. They secrete mucus and bicarbonate (HCO3−), which protect the gastric epitheliumfrom the corrosive acid in the lumen, and are replacedevery few days.Mucous neck cells protect the gastric mucosa by secreting mucus.Stem/regenerative cells differentiate to replace othercells in the pit.Parietal cells secrete HCl and intrinsic factor.Chief cells secrete pepsinogens and gastric lipase.Endocrine cells secrete somatostatin, and mast cellssecrete histamine.Because of the harsh physical environment, cells lining thestomach (and intestines) are continually replaced.
Thisrequires substantial energy expenditure, as well as a constantsupply of proteins and lipids to make new cells.■■■■Intrinsic factor: Essential gastric secretion required forB12 absorption in the terminal ileum.Pepsinogens: The inactive form of pepsins, which areproteases that are activated in the acid environment.Thus, protein digestion begins in the stomach.Gastrin: A gut hormone secreted from G-cells located inthe antrum of the stomach and the duodenum. Gastrinstimulates HCl secretion and gastric motility (mixing),as well as lower GI motility (mass movements).Lipase (gastric lipase): Continues process of lipiddigestion.Mucus: Thick mucus is secreted with HCO3−.
The HCO3−remains trapped in the mucus layer at the epithelial cellsurface, effectively buffering the cells from the acid environment in the lumen (Fig. 23.5).Other factors: Somatostatin (SS) inhibits HCl secretion;histamine stimulates HCl secretion.In general, factors that stimulate gastric activity (e.g.,vagus, gastrin) elicit secretion from chief, parietal, andmucus cells.Production and Regulation of Gastric HClSecretions from the Pit CellsThe gastric secretions primarily serve to digest the food;however, they also contribute to the regulation of acid secretion, motility, and protecting Vitamin B12.
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