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Digestive System. Chapter 23. Overview. Alimentary canal or gastrointestinal ( GI ) tract Accessory digestive organs Food moved by peristalsis Regulated by sphincters Processing time varies. Digestive Processes. Ingestion : act of eating Propulsion : moving food through GI tract

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Digestive System

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  • Alimentary canal or gastrointestinal (GI)tract
  • Accessory digestive organs
  • Food moved by peristalsis
  • Regulated by sphincters
  • Processing time varies
digestive processes
Digestive Processes
  • Ingestion: act of eating
  • Propulsion: moving food through GI tract
    • Deglutition and peristalsis
  • Digestion: break down of food
    • Mechanical uses chewing, mixing, and churning
    • Chemical uses enzymes
  • Absorption: nutrients, vitamins, minerals, and H2O into blood
  • Defecation: elimination of undigested materials
  • Serous membrane of the abdominal cavity
    • Review: what are components?
    • Mesenteries fuse parietal to visceral
      • Routes for vascular, lymph, and nervous supply to viscera
      • Holds organs in place and stores fat
  • Retroperitoneal organs lie outside peritoneum
    • Includes pancreas, kidneys, parts of large intestine
  • Intraperitoneal organs stay w/i cavity
    • Includes stomach, small and parts of large intestine, liver
  • Blood supply
    • Celiac trunk and mesenteric arteries to digestive viscera
    • Hepatic portal to liver for storage and processing


basic histology
Basic Histology
  • 4 tunics line entire GI tract
    • Mucosa from mouth to anus
      • Epithelium is simple columnar w/goblet cells
      • Lamina propria is areolar CT a nd MALT (chpt 20 review)
      • Muscularis mucosae is smooth muscle
    • Submucosa
      • Areolar CT, blood, lymph, and nerves
    • Muscularisexterna
      • Smooth muscle in circular (inner) and longitudinal (outer) layers
    • Serosa (visceral peritoneum)
      • Areolar CT w/ simple squamous layer
  • Predominant tissue type facilitates processing role
  • Fig 23.6
oral cavity mouth
Oral Cavity: Mouth
  • Stratified squamous epithelium
  • Lips (orbicularis oris) and cheeks (buccinators)
    • Mastication and forming words/pronunciation
  • Palate
    • Hard (palatine process of maxilla and palatine bones)
    • Soft (skeletal muscle) with hanging uvula
  • Tongue (skeletal muscle)
    • Food manipulation (bolus), taste, and speech production
    • Tastebuds
      • Filiform papillae: not tastebuds, roughen tongue surface
      • Fungiform papillae: mushroom shaped w/ tastebuds on top
      • Circumvallate papillae: circular w/tastebuds on sides
oral cavity salivary glands
Oral Cavity: Salivary Glands
  • Parotid, submandibular, sublingual, and buccal
    • Serous and mucus cells produce saliva components
  • Saliva
    • Lysozymes, IgA, and defensins cleanse and protects mouth from microorganisms
    • Mucin and water (97 – 99%) to moisten and dissolve food
    • Salivary amylase breaks down carbs
  • Control
    • Intrinsic continuously secrete to keep mouth moist
    • Extrinsic in response to food; sight, smell & thought too
    • SNS inhibits causing dry mouth, xerostomia
      • Halitosis and caries from food particles accumulating and bacteria flourishing
oral cavity teeth
Oral Cavity: Teeth
  • Chew, tear and grind food to initiate breakdown
  • Deciduous, or milk teeth, form 1° dentition (20)
  • Permanent teeth from permanent dentition (32*)
    • Types
      • Incisors cut and nip (8, 2 pairs top and bottom)
      • Canines tear and pierce (4, 1 pair top and bottom)
      • Premolars grinding and crushing (8, 2 pairs top and bottom)
      • Molars see premolars (12, 3*pairs top and bottom)
    • Dental formula
      • (__I x __C x __M / __I x __C x __M) x 2 =
  • Impacted teeth when remain in jaw
oral cavity teeth cont
Oral Cavity: Teeth (cont.)
  • Crown exposed above gingiva
    • Covered in enamel, hardest material in body
      • No mitotic activity to fix cracks and decay
      • Overlays bone-like dentin
  • Root anchored in jaw by periodontal ligament
    • Gomphosis in alveolar margin
    • Number varies by tooth type and location
    • Covered in cementum, calcified CT
  • Pulp cavity become root canal w/ apical foramen
    • Damage to nerve and blood supply facilitates infection
    • Root canal therapy to sterilize before filling
tooth and gum disease
Tooth and Gum Disease
  • Caries or cavities when enamel and dentin decay
    • Dental plaque adheres and acidity increases
      • Ca2+ salts removed, organics dissolved by bacteria
      • Brushing and flossing daily prevent
    • Calculus, or tarter causes gingiva separation
      • Gingivitis, an early indication, causes bleeding and swelling
  • Periodontal disease when calculus untreated
    • Immune system attacks bacteria and periodontal ligaments
    • Osteoclast activity increased
  • Research links w/increase risk of heart attack and stroke
  • Oropharynx laryngopharnx
    • Food and fluids to esophagus
    • Air to trachea
  • Lined w/ stratified squamous and goblet cells
  • Skeletal muscle layers for swallowing
    • Inner longitudinal
    • Outer circular pharyngeal constrictors
  • Laryngopharynx esophageal sphincter  esophageal hiatus  cardiac orifice  gastroesophageal sphincter
    • Heartburn when GE sphincter not tightly closed/leaks
  • Histology
    • Mucosa w/ strat. squ.  sim. colum.
      • Folds w/o food that flatten w/ food
    • Submucosa w/ mucus glands to aid passage
    • Muscularisexterna w/ skeletal  skeletal & smooth  smooth
    • Fibrous CT fibrous adventitia instead of serosa
mouth to esophagus digestion
Mouth to Esophagus Digestion
  • Mouth ingests
    • Mastication uses teeth and tongue to start mechanical digestion
      • Food and structure mov’t influences
    • Salivary amylase starts chemical digestion of carbs
  • Tongue forms bolus for deglutition
    • Tongue pushes to oropharynx in buccal phase (voluntary)
      • Esophageal sphincter constricts
    • Pharyngeal-esophageal phase (involuntary)
      • Uvula and epiglottis protection triggered
      • Esophageal sphincter relaxes
  • Peristalsis moves bolus down esophagus
    • Gastroesophagealspincter relaxes then constricts
    • Prevents chyme escape
  • Chemical breakdown of


  • Regions
    • Cardia
    • Fundus
    • Body
    • Pyloric region (antrum, canal, and sphincter)
  • Mesenteries (greater and lesseromentums) at respective curvatures
  • Rugaeseen when empty, allow distension
stomach microscopic anatomy
Stomach: Microscopic Anatomy
  • Muscularisexternaw/ 3rd oblique layer to break food and push to SI
  • Mucosa
    • Simple columnar epithelia w/ goblet cells
      • Insoluble alkaline mucus layer w/ bicarbonate fluid below
      • Protects stomach from acid and enzyme digestion
    • Invaginates into gastric pits leading to gastric glands that produce gastric juices, mucus, and gastrin
      • Pits are primarily mucus cells
      • Glands vary w/ region
gastric glands
Gastric Glands
  • Mucus neck cells produce acidic mucus
    • Function not understood
  • Parietal cells secrete H+ and Cl- ions  HCl
    • Acidity (pH ~ 2) denatures proteins, kill bacteria, and digest cellulose
    • Intrinsic factor production: B12 absorption for RBC production
  • Chief cells produce pepsinogen  pepsin
      • HCl initiates, but pepsin maintains (positive feedback)
  • Enteroendocrine cells release chemical messengers
    • Histamine, gastrin, serotonin, endorphins, cholecystokinin (CCK), and somatostatin
protecting the stomach
Protecting the Stomach
  • Insoluble mucus layer on surface
  • Bicarbonate ions (HCO3-)
  • Tight junctions b/w epithelial cells
  • H+ and Cl- ions secreted separately
  • Epithelial replaced ~ 3 – 6 days
  • Gastric ulcers form when mechanisms fail
    • Chronic can rupture stomach
    • Helicobacter pylori usually responsible
controlling gastric secretion
Controlling Gastric Secretion
  • Cephalic (reflex) phase before food entry
    • Smell, taste, thought, or sight excites
    • Depression or lack of appetite suppress
  • Gastric phase once food enters stomach
    • Stretch receptors signal ACh to increase gastric juices
    • Peptides, caffeine, and high pH stimulate gastrin (HCl) release
    • Low pH, emotions, and SNS inhibit gastrin
  • Intestinal phase once food enters duodenum (SI)
    • W/ low pH and food stimulates intestinal gastrin release
    • Distension and prolonged low pH inhibit CN X, activate SNS to tighten pyloric sphincter, and release inhibiting hormones
  • Fig 23.17
controlling mobility and emptying
Controlling Mobility and Emptying
  • Peristalsis pushes to pyloric for mixing before retropulsion pushes back
    • Chyme squirts through pyloric valve w/ each cycle
  • Duodenum monitoring tightly coupled
    • Inhibiting hormones from intestinal phase (previous)
    • Carb rich chyme moves quickly, but fat rich chyme slower for digestion
  • Vomiting or emesis
  • Fig 23.30
small intestine si
Small Intestine (SI)
  • Digestion finishes and nutrient absorption occurs
  • 3 subdivision w/ no clear external distinctions
    • Duodenum (retroperitoneal)
      • Receives chyme from pyloric sphincter
      • Hepatopancreaticampulla where bile and pancreatic duct join (sphincterofOddi controls)
    • Jejunum (mesentery support)
    • Ileum (mesentery support)
      • Ileocecal valve at LI junction
small intestine microscopic anatomy
Small Intestine: Microscopic Anatomy
  • Mucosa and submucosa form plicaecirculares
    • Simple columnar mucosa folds into villi
      • Lacteal, artery, and vein w/i
      • Intestinal crypts b/w
        • Intestinal juice, Panethcells, and new cells
    • PM of mucosa cells form microvilli (brush border)
      • Enzymes finalize carb and protein digestion
    • Increase SA for absorption
  • Submucosa variations throughout SI region
    • Intestinal crypts decrease
    • Goblet cell number increases
    • Peyer’s patches (MALT) increase
    • Brunner’s glands (HCO3- mucus) in duodenum only
  • Muscularisexterna
    • Duodenum w/ adventitia (retroperitoneal)
  • Right, left, caudate, and quadratelobes
    • Falciformligament is the mesentery
      • Separates R & L and suspends liver from diaphragm
    • Ligamentteres is remnant of umbilical vein
  • Lesseromentum connects curvature to liver
    • Hepatic artery, vein, and portal vein
  • Hepaticducts hepatic duct commonbileduct
  • Hepatitis and cirrhosis
liver microscopic anatomy
Liver: Microscopic Anatomy
  • Hexagonal liver lobules are functional units
    • Hepatocytes arranged around a central vein
  • Portal triads at each corner
      • Bile duct, hepatic artery, and hepatic portal vein
  • Liver sinusoids (capillaries) b/w
    • Kupffer cells
  • Stores and concentrates liver produced bile
    • Hepatic duct  cystic duct  storage
    • Cystic duct  common bile duct  sphincter of Oddirelease
  • Bile
    • Alkaline solution to emulsify lipids
    • Facilitate fat and cholesterol absorption
    • Bile salts recycled in illeum back to liver (hepatic portal vein)
    • Bilirubin, chief pigment, absorbed in liver and released in bile
  • Gallstones from cholesterol crystallization
  • Jaundice when bile pigments accumulate in skin from blockage
  • Retroperitoneal organ
  • Acinar cells produce digestive enzymes that breakdown all foodstuffs
    • Pancreatic duct joins common bile to drain to duodenum (where specifically?)
  • Islets of Lagerhansproduce insulin and glucagon
pancreatic juice
Pancreatic Juice
  • Water based solution of enzymes and HCO3-
    • Neutralize chyme (pH ~ 2 vs pH ~ 8)
    • Optimal pH for enzymes
  • Enzymes hydrolyze monomers
    • Inactive proteases
    • Active amylases, lipases, and nucleases
accessory secretion regulation
Accessory Secretion Regulation
  • Neural (CN X) and hormonal (CCK & secretin) control
  • CCK released w/ fatty chyme presence
      • Gallbladder contract and sphincter of Oddi relax
        • Vagus nerve weakly
      • Bile and pancreatic juice (enzyme rich) secreted
  • Secretin released w/ HCl presence
    • HCO3- rich pancreatic juice release
  • Bile salt presence stimulates more bile release
    • Positive feedback
large intestine
Large Intestine
  • Absorb water & vitamins and eliminate feces
  • Tone of teniae coli create pouch-like haustra
  • Divisions
    • Cecum
    • Appendix (MALT)
      • Appendicitis when blocked and bacterial growth
    • Ascending and descending colon are retroperitoneal
    • Transverse and sigmoid colon w/ mesocolons
    • Rectum w/ rectal valves to separate feces and gas
    • Anal canal w/ internal and external sphincters
  • Imbalances
    • Diarrheavsconstipation
large intestine microscopic anatomy
Large Intestine: Microscopic Anatomy
  • Mucosa
    • Simple columnar except anal canal (strat. squa.)
    • No folds, villi, or digestive enzyme cells
    • Thicker w/ deeper crypts and more goblet cells
  • Bacterial flora
    • Enter via anus or survive SI
    • Functions
      • Breakdown indigestible carbs (cellulose)
      • Release gases from breakdown
      • Synthesize vitamins B and K
    • Generally peaceful existence
      • Irritable bowl syndrome (disease) when not
nutrient digestion
Nutrient Digestion
  • Catabolic process of hydrolysis
  • Carbohydrates
    • Amylases (salivary and pancreatic)
    • Brush border enzymes (dextrinase, lactase, maltase, sucrase)
    • From villi to liver in hepatic portal vein
  • Proteins
    • Pepsin in stomach (pepsinogen + HCl)
    • Brush border peptidases (amino-, carboxy-, di)
    • Pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase)
  • Lipids
    • Bile sales emulsifies fats to expose triglycerides
    • Pancreatic lipase forms glycerol and FA’s
  • Nucleic acids
    • Brush border enzymes (nuclesidases and phosphotases)
    • Pancreatic nucleases (deoxy- and ribonucleases)
  • Fig 23.32
nutrient absorption
Nutrient Absorption
  • Carbohydrates
    • Glu and gal: active transport (w/ Na+) to epithelium then facilitated diffusion to capillaries
    • Fru: only facilitated diffusion
  • Proteins
    • See glucose and galactose above
    • Inefficient in newborns = food allergies as endocytized whole proteins ‘seen’ as antigens; allow IgA from breast milk into blood
  • Lipids
    • Monomers bind w/ bile salts = micelles to reach epithelium for diffusion
    • Chylomicrons in epi. when reassembled  water soluble  lacteal absorb
  • Nucleic Acids
    • See glucose and galactose above
nutrient absorption cont
Nutrient Absorption (cont.)
  • Vitamins
    • Fat soluble see lipids previous; need to eat w/
    • Water soluble by diffusion, active, or passive transport
  • Electrolytes
    • Anions move w/ Na+ gradient (glucose & glactose)
    • Iron and Ca2+ regulated by body needs
    • K+ related to water volume/absorption
  • Water
    • 95% in SI, rest in LI
    • Bidirectional, but net osmosis w/ [gradient] from active solute absorption