t he d igestive s ystem chapter 19. g astrointestinal (gi) t ract tube that includes: mouth,...
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THE DIGESTIVE SYSTEM
Chapter 19
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GASTROINTESTINAL (GI) TRACT
Tube that includes: mouth, Pharynx, Esophagus, Stomach, Small intestine, Large intestine
Accessory organs: teeth, tongue, salivary glands, liver, gallbladder, and pancreas
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FIGURE 19.1
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OVERVIEW- OPERATIONS
Ingestion: eatingSecretion: release of water,
enzymes & buffersMixing & propulsion:
movement along GI tractDigestion: mechanical and
chemical breakdown of foodsAbsorption: getting it into the
bodyDefecation: dumping waste
products = defecation
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WALL LAYERS- EVERYWHERE
4 layers Mucosa- epithelium, connective layer,
glands, muscularis mucosae Submucosa- connective tissue, blood
vessels, lymphatic vessels, enteric nervous system
Muscularis- circular layer, longitudinal layer In mouth, pharynx & upper esophagus –skeletal
muscle Also in external anal sphincter
Serosa or Visceral peritoneum
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FIGURE 19.2
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FIGURE 19.3A
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FIGURE 19.3B
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MOUTH
Formed by cheeks, hard & soft palate & tongue
Soft palate at back includes a “hangy down” part = uvulaDuring swallowing uvula prevents
entry into nasal cavityTongue- muscular accessory organ
maneuvers food for chewingAdjusts shape for speech & swallowing
Lingual tonsils at base of tongue
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SALIVARY GLANDS
3 pairs of salivary glandsDucts empty into oral cavity
Parotid- inferior & anterior to ears
Submandibular- in floor of mouth, medial & inferior to
mandibleSublingual
Beneath tongue and superior to submandibular
Saliva contains 99.5% water, salivary amylase, mucus and other solutesDissolves food & starts digestion of
starches
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FIGURE 19.4
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TEETH
Accessory organs in bony sockets of mandible & maxilla
3 external regions: Crown- above gumsRoot- 1 or more parts embedded in
socketNeck – between crown and root near gum
line3 layers of material
Enamel- covers crownDentin- majority of interior of toothPulp cavity - nerve, blood vessel &
lymphatics
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FIGURE 19.5
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DIGESTION IN THE MOUTH
Mechanical breakdown- chewing
Mixed with saliva by tongueSalivary amylase chemically breaks down polysaccharides (starch)maltose and larger fragmentsContinues in the stomach until
acidifiedRounds up food into a soft bolus for swallowing
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PHARYNX & ESOPHAGUS
On swallowing:Bolus of food oropharynxLaryngopharynx esophagus
Muscular contractions in pharynx help
Upper esophageal sphincter (UES)Skeletal muscle –controls entry to
esophagusLower esophageal sphincter (LES)
Smooth muscle- regulates entry to stomach
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FIGURE 19.6A,B
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SWALLOWING
Voluntary: bolus forced into oropharynx
Triggers oropharyngeal stageInvoluntary & breathing interruptedSoft palate move up-close
nasopharynxEpiglottis seals off larynxBolus moves into esophagus through
UESEsophageal stage peristalsis
moves it toward stomach
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FIGURE 19.6C
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STOMACH
J- shaped enlargement of tractServes as mixing chamber and
holding reservoirVery elastic & muscular4 regions
Cardia- surrounds upper openingFundus- superior & to left of cardiaBody – large central portionPylorus- lower part leading to pyloric
sphincter & duodenum
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FIGURE 19.7
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STOMACH WALL
Mucosa:Folds called rugaeEpithelium- simple columnar mucousForm gastric glands lining gastric pits
Secretory cells: mucous neck cellsChief cells inactive enzyme
pepsinogeParietal cells HCl & intrinsic factorCollectively = gastric juice
Muscularis- 3 Layers: longitudinal, circular & oblique
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FIGURE 19.8
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FIGURE 19.9
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DIGESTION & ABSORPTION
Food entry stretch & rise in pHNerve impulses secretion & mixing
waves Food mixed with juice ChymeSmall amount pushed through pyloric
sphincter= gastric emptying- Carb. foods fastest,
lipids next & proteins slowestEntry in duodenum feedback inhibition
of stomach activityPepsin digests protein peptidesLittle absorption- water, ions & some
drugs
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PANCREAS
Behind stomach- Produces pancreatic juice in acinar cellsto duodenum via pancreatic duct
NaHCO3 solution (pH 7.1-8.2)– 1000ml/dayNeutralize stomach acid and dilutes chyme
Panceas- digestive enzymesProteases: chymotrypsinogen, trypsinogen,
et. al.Activated by entreokinase from intestineStarch digesting- pancreatic amylasePancreatic lipaseNucleotidases – RNAase & DNAase
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LIVER & GALL BLADDER
Largest organ after the skinOn right below diaphragmFunctional unit is lobule-
Hepatocytes around central veinOpen capillaries = sinusoids
Bile canaliculi ducts hepatic duct
Gall bladder =Pear-shaped organ on front (stores bile)
cystic duct common bile duct
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BILE
Bicarbonate, bile salts & waste. – 1000 ml/day
Important for emulsifying fats Increases surface area for digestion
Pigment is bilirubin- from broken-down heme during RBC recycling
Digested to strecobilin- brown colorBile salts reabsorbed at end of small
intestine- ileumrecycle to liver in portal circulation
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FIGURE 19.10
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FIGURE 19.11A
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FIGURE 19.11B
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LIVER FUNCTION
Maintains blood glucoseStores as glycogen Uses absorbed sugars & Converts amino
acids glucoseLipid metabolism
Produces cholesterol & triglycerides, makes bile
Makes lipoproteins for lipid transportExcretion of bilirubinProcesses drugs and other chemicalsStore fat soluble vitaminsMake active vitamin D
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SMALL INTESTINE
3 parts: duodenum, jejunum, ileum Where most of the digestion occurs Essentially all of the nutrient absorption Ends in ileocecal sphincter
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FIGURE 19.12A
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FIGURE 19.12B
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WALL STRUCTURE
Same 4 layersEpithelial- simple columnar
Absorptive cells with microvilliGoblet cells- secrete mucus
Intestinal glands- intestinal juice & hormonesSecretin, cholecystokinin (CCK),
Glucose-dependent-insulinotrophic peptide (GIP)
Lymphatic tissue- defense
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WALL STRUCTURE (CONT.)
Duodenal glands- alkaline mucus Helps neutralize stomach acid
Circular folds- increase surface area Villi- finger like projections of mucosa
Increase surface area for absorption Include lacteals for lipid absorption
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FIGURE 19.13
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MOTILITY & SECRETIONS Secretions: alkaline, some enzymes
Peptidases-breaks small peptides Disaccharidases attached to wall Water and salt to balance osmolality ~2000 ml/day
Segmentation activity- for mixing Peristalsis for movement after most
absorption completed- slow waves
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DIGESTION & ABSORPTION
Chyme enters with partially digested carbohydrates & proteins
Bile + pancreatic juice + intestinal juice completes the job
Absorption is of monosaccharides; amino acids; phosphate sugar & bases of DNA & RNA; fatty acids & monoglycerides
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CARBOHYDRATE DIGESTION
Amylases: Starch & dextrin maltose
Disaccharidases at surface: Maltose: maltose glucose Sucrase: sucrose glucose & fructose Lactase: lactose glucose & galactose
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PROTEIN & FAT DIGESTION Trypsin, chymotrypsin, elastase,
carboxypeptidase & pepsinProteins small peptides
Peptidases at surface:Peptides amino acids & di- & tri-
peptides Lipase:
glycerides fatty acids & monoglycerides
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ABSORPTION
By diffusion, facilitated diffusion, osmosis & active transport
Carbohydrates monosaccharidesVia portal system to liver
Proteins (jejunum & ileum) amino acidsVia portal system to liver
Lipids reformed to triglyceridesPackaged in chlyomicrons with proteinVia lacteals lymphatics
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ABSORPTION (CONT.)
Water & salt Primarily osmotic movement along with other
nutrients Vitamins:
Fat soluble absorbed with fat Water soluble with simple diffusion B12 combines with intrinsic factor & absorbed by
active transport in ileum
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FIGURE 19.14A
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FIGURE 19.14B
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LARGE INTESTINE
Cecum, colon, rectum, anal canalIleocecal canal large intestine
Below is cecum with appendixColon- ascending, transverse,
descending & sigmoid rectum anal canalStandard 4 layers with mucus
secretionFew folds , little specialization for
absorptionMuscularis: circular + bands of
longitudinal muscle
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FIGURE 19.15A
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FIGURE 19.15B
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FIGURE 19.16
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DIGESTION & ABSORPTION
Slow emptying of ileumSlow peristalsisMass peristalsis with food in stomachMoves from middle of colon
rectumBacterial digestion
Produce some B-vitamins & Vit. KProduce gases= flatusColon absorbs salt & water
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DEFECATION REFLEX
Stretch of rectum wall neural reflex contraction of longitudinal muscleCombined pressure + parasympathetic
activity relaxing of internal anal sphincter
External anal sphincter is voluntaryContraction of diaphragm & abdominal
wall muscles aid defecation
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CONTROL
Rule: activate forward and inhibit behindthree phases: Cephalic, gastric, intestinalCephalic- smell, sight, thought of food
Neural signals stimulates salivary glands & gastric glands
Gastric- stretching, pH of stomachGastrin activates stomach & LES & relaxes
pyloric sphincterNeural signals + gastrin signal satiety
(fullness)
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CONTROL (CONT.)
Intestinal- responses to food entering duodenumneural & endocrine
CCK stimulated by AA & fatsPancreatic enzyme releaseGall bladder contractionContraction of pyloric sphincter
Acid stimulates secretinStimulates HCO3
- ions in pancreatic juiceInhibits gastrin action in stomach
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AGING
Decreased secretion, motility, strength of responses
loss of taste, periodontal disease, hiatal hernia, gastritis & peptic ulcer disease
Increased incidence of gall bladder problems, cirrhosis of liver, pancreatitis, constipation, hemorrhoids & diverticulitis