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
– 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
Peritoneum
• 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
http://www.vivo.colostate.edu/hboo
ks/pathphys/misc_topics/peritoneu
m.html
• Blood supply
– Celiac trunk and mesenteric arteries to digestive viscera
– Hepatic portal to liver for storage and processing
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
– Muscularis externa
• 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
• 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
• 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
• Chew, tear and grind food to initiate breakdown
• Deciduous, or milk teeth, form 1° dentition (20)
• Permanent teeth from permanent dentition (32*)
– Types
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•
•
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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.)
• 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
• 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
Pharynx
• 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
Esophagus
• 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
– Muscularis externa w/ skeletal  skeletal & smooth
 smooth
– Fibrous CT fibrous adventitia instead of serosa
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
– Gastroesophageal spincter relaxes then constricts
– Prevents chyme escape
Stomach
• Chemical breakdown of
proteins
• Regions
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–
–
–
Cardia
Fundus
Body
Pyloric region (antrum, canal, and sphincter)
• Mesenteries (greater and lesser omentums) at
respective curvatures
• Rugae seen when empty, allow distension
Stomach: Microscopic Anatomy
• Muscularis externa w/ 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
• 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
•
•
•
•
•
•
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
• 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
• 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)
• Digestion finishes and nutrient absorption
occurs
• 3 subdivision w/ no clear external distinctions
– Duodenum (retroperitoneal)
• Receives chyme from pyloric sphincter
• Hepatopancreatic ampulla where bile and pancreatic
duct join (sphincterof Oddi controls)
– Jejunum (mesentery support)
– Ileum (mesentery support)
• Ileocecal valve at LI junction
Small Intestine: Microscopic Anatomy
• Mucosa and submucosa form plicae circulares
– Simple columnar mucosa folds into villi
• Lacteal, artery, and vein w/i
• Intestinal crypts b/w
– Intestinal juice, Paneth cells, 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
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–
–
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Intestinal crypts decrease
Goblet cell number increases
Peyer’s patches (MALT) increase
Brunner’s glands (HCO3- mucus) in duodenum only
• Muscularis externa
– Duodenum w/ adventitia (retroperitoneal)
Liver
• Right, left, caudate, and quadrate lobes
– Falciform ligament is the mesentery
• Separates R & L and suspends liver from diaphragm
– Ligament teres is remnant of umbilical vein
• Lesser omentum connects curvature to liver
– Hepatic artery, vein, and portal vein
• Hepatic ducts  hepatic duct  common bile
duct
• Hepatitis and cirrhosis
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
Gallbladder
• Stores and concentrates liver produced bile
– Hepatic duct  cystic duct  storage
– Cystic duct  common bile duct  sphincter of Oddi
release
• 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
Pancreas
• Retroperitoneal organ
• Acinar cells produce digestive enzymes that
breakdown all foodstuffs
– Pancreatic duct joins common bile to drain to
duodenum (where specifically?)
• Islets of Lagerhans produce insulin and
glucagon
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
• 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
• Absorb water & vitamins and eliminate feces
• Tone of teniae coli create pouch-like haustra
• Divisions
– Cecum
– Appendix (MALT)
• Appendicitis when blocked and bacterial growth
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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
– Diarrhea vs constipation
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
• 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
• 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.)
• 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