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Diabetes insipidus:A. usually involves reduction of renal V2 receptors for antidiuretic hormone (ADH).B. is associated with increased water channels (aquaporins)in the collecting ducts.C. results in excretion of large quantities of hypertonicurine.D. occurs mainly in children.E. results in excretion of large quantities of hypotonicurine.A.B.C.D.E.excretion of ammonium.excretion of bicarbonate.excretion of sodium.excretion of titratable acids.distal H+ secretion.16. Using the following blood values, determine the acid–basedisorder:Pco2HCO3−Na+K+Cl−7.28 26 mm Hg 14 mEq/L 136 mEq/L 5.0 mEq/L 100 mEq/LpHA.B.C.D.E.Metabolic alkalosisMetabolic acidosis from acid gainMetabolic acidosis from diarrheaRespiratory acidosisRespiratory alkalosis17.
When compared to normal values, in acute uncompensated respiratory alkalosis, plasma:A.B.C.D.E.pH and PCO2 are high, and HCO3− is normal.pH and PCO2 are low, and HCO3− is high.pH is high, and PCO2 and HCO3− are low.pH is high, PCO2 is low, and HCO3− is normal.pH is high, PCO2 is low, and HCO3− is high.18. The major titratable acids (TA) excreted in the urine arein the form of:A.B.C.D.E.H2PO4−.lactic acid.K2PO4.H2CO3.keto acid.Section6GASTROINTESTINALPHYSIOLOGYWhat makes the gastrointestinal (GI) tract interesting? It is an area of ourbodies that we are acutely aware of multiple times a day. We have hungerpangs and feel “full”; we become thirsty, hear intestinal sounds, expel gasses,and feel the urge to defecate. Also, GI-related distress and pathology is high onthe list of personal and medical complaints.
Homeostasis in the wholeorganism relies on the input of nutrients (gas exchange, food, and water), sothe GI tract is a critical component of the supply side.Chapter 21Overview of the Gastrointestinal TractChapter 22Motility through the Gastrointestinal TractChapter 23Gastrointestinal SecretionsChapter 24Hepatobiliary FunctionChapter 25Digestion and AbsorptionReview Questions241This page intentionally left blank243CHAPTER21Overview of theGastrointestinal TractSTRUCTURE AND OVERALL FUNCTION OF THEGASTROINTESTINAL TRACTThe GI tract can be thought of as one long tube with aninput (mouth) and output (anus), with specialized areas,and direct input from associated organs (liver, pancreas, andgallbladder) (Fig.
21.1). Along the tract there are sphinctersthat separate major sections of the tract, allowing the regulation of flow of food into the stomach and chyme (foodand digestive juices) out of the stomach and through theintestines.A brief tour down the tract:■■■■Mouth: Mechanical digestion occurs through mastication (chewing), and chemical digestion begins on thepredominant dietary carbohydrate, starch, and to asmall degree on lipids.
The salivary glands secrete saliva,which helps buffer, digest, and lubricate the food so itcan be swallowed.Esophagus: As soon as swallowing occurs, the bolus offood enters the esophagus through the relaxation of theupper esophageal sphincter.
The esophagus is a tube thatallows passage of the bolus of food through the chest andinto the stomach. At the end of the esophagus the loweresophageal sphincter (LES) relaxes to allow the bolusinto the stomach.Stomach: This is a pouch in the tract that is specializedfor the storage of food, secretion of digestive enzymesand hydrochloric acid (HCl), and mixing of the foodand digestive juices to make chyme.
The pyloric sphincter, at the distal end of the stomach, regulates the entryof chyme into the small intestine.Small intestine: The small intestine is made of three sections, the duodenum (∼1 foot long), the jejunum (∼8feet long), and the ileum (∼10 feet long). Secretionsfrom the liver, gallbladder, and pancreas enter the smallintestine via the common bile duct and through thesphincter of Oddi, into the duodenum. A key aspect ofthe small intestine is the presence of finger-like projections in the intestinal mucosa (the villus brush border),which increases the surface area for absorption. Throughthe tract, there is only one layer of intestinal epithelial■cells, or enterocytes, between the lumen of the tract andthe systemic blood supply.
While this effectively keepsthe lumenal contents in the small intestine, when absorption occurs, there is rapid and efficient movement ofnutrients into the blood. However, the enterocytes arevulnerable to the harsh environment of the gut lumenand need to be protected. This protection is conferredby intestinal buffers and mucus. Despite the protection,intestinal cells will slough off and be replaced by newcells every 3 to 4 days.The majority of digestion occurs in the jejunum bypancreatic enzymatic action. When nutrients are digestedto the constitutive elements (e.g., monosaccharides,monoglycerides, amino acids, etc.), absorption canoccur all along the small intestine, although most nutrients are absorbed by the mid- jejunum.
The terminalpart of the ileum is also the site of vitamin B12 absorption and recycling of bile. Chyme that remains in thetract leaves the small intestine through the ileocecalsphincter, into the large intestine (colon).Large intestine: The large intestine does not have a brushborder, and the primary functions are dehydration ofchyme to produce feces, and then storage of the fecesuntil defecation. The large intestine consists ofthe cecum, appendix, colon, rectum, and anal canal. Atthe end of the rectum, the internal and external analsphincters regulate expulsion of feces.Another important aspect of the tract is that its musculatureis smooth muscle, except for the mouth, upper esophagus,and external anal sphincter, which have skeletal muscle.
Skeletal muscle allows voluntary control of both input (chewing,swallowing) and output (defecating). The rest of the tract haslongitudinal and circular bands of smooth muscle that allowfor propulsion and mixing of the chyme. Figure 21.2 illustrates these muscle layers, as well as the oblique layer that isunique to the stomach. The oblique layer aids in the mixingcomponent that is important in stomach motility.Other distinct aspects of the GI tract include a large blood andlymphatic system, secretion of substances from a variety ofglands, and a nervous system that is unique to the GI tract(Fig. 21.3).244Gastrointestinal PhysiologyPharynxPharyngeal muscles propelfood into esophagusLiverSecretion of bile (important for lipiddigestion), storage of nutrients,production of cellular fuels, plasmaproteins, clotting factors, anddetoxification and phagocytosisPancreasSecretion of buffers anddigestive enzymes byexocrine cells;secretion of hormonesby endocrine cellsto regulate digestionGallbladderStorage andconcentration of bileLarge intestineDehydration and compactionof indigestible materialsfor elimination; resorption ofwater and electrolytes; hostdefenseOral cavity, teeth, tongueMechanical breakdown, mixingwith salivary secretionsSalivary glandsSecretion of lubicating fluidcontaining enzymes that initiatedigestionEsophagusTransport of foodinto the stomachStomachChemical breakdownof food by acid andenzymes; mechanicalbreakdown via muscularcontractionsSmall intestineEnzymatic digestion and absorptionof water, organic substrates,vitamins, and ions; host defenseFigure 21.1 Overview of Gastrointestinal Tract The GI tract begins at the mouth and is made upof discrete areas that aid the digestion and absorption of nutrients.
Each area of the tract contributes to theefficient processing of the nutrients. This includes the GI-associated organs (liver, pancreas), which provideimportant secretions including bile, enzymes, and buffers.Blood SupplyThe GI tract has a large blood supply to perform the myriadof functions necessary for digestion, absorption, and propulsion. Oxygen, nutrients, and hormones are brought by theblood to aid in the many secretory, absorptive, and propulsivefunctions. In addition, efficient absorption of nutrientsthrough the intestinal cells requires a high blood flow, whichensures that there is always a gradient for nutrient entry intothe bloodstream.Blood supply to the GI tract is from the splanchnic bed (superior and inferior mesenteric arteries), with the capillary systemDuring active digestion, blood flow to the GI tractincreases to ∼3 times normal to facilitate the metabolicand digestive needs.
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