DIGESTIVE SYSTEM

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DIGESTIVE SYSTEM
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Digestive System

• muscular tube
• digestive tract
• GI tract
• gastrointestinal tract
• alimentary canal
• runs from oral cavity? pharynx, esophagus,
  stomach, small large intestines, rectum to anus
• includes accessory organs
• teeth
• tongue
• salivary glands
• liver
• gall bladder
• pancreas

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Functions

• provides fuel to keep cells running
• provides building blocks for growth repair
• removes residue
• protective function for bacteria
• largest immune organ

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Functions

• four integrated steps
• Ingestion
• intake of food
• Digestion
• mechanical
• physical manipulations
• teeth tear crush
• stomach mixes churns
• chemical breakdown
• physically manipulated materials broken into
  smaller fragments by enzymes acids
• Absorption
• uptake of nutrients
• movement of organic substances, ions, vitamins
  water into blood
• Defecation
• excretion of undigested residue
• removal of waste products

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Histological Organization

• located in peritoneal cavity
• lined by serous membrane
• visceral peritoneum covers organs
• parietal peritoneum lines inner body wall surface
  
• 4 layers
• mucosa
• submucosa
• muscularis externa
• serosa

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Layers of Wall

• Mucosa
• innermost epithelial lining
• consists of inner epithelium thin layer of
  smooth muscle-muscularis mucosae
• Epithelium-simple columnar in most of tract
• from oral cavity through esophagus lower anal
  canal-stratified
• Submucosa
• loose connective tissue containing blood lymph
  vessels with nerve plexus
• Muscularis externa
• 2 layers of smooth muscle arranged in inner,
  circular layer outer, longitudinal layer
• important in mechanical processing movement of
  materials along tract.
• lining of muscularis externa is thrown into
  folds?increases surface area
• Serosa
• inner layer of loose connective tissue
• adipose tissue outer epithelial layer

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Nervous System Control of Digestive Functions

• sympathetic stimulation inhibits gastrointestinal
  secretion, motor activity contraction of
  gastrointestinal sphincters blood vessels
• parasympathetic stimuli stimulate these

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Nervous Control of Digestive Functions

• digestive system has its own, local nervous
  system
• enteric nervous system
• can function independently of CNS
• regulates motility, secretion blood flow in
  tract
• has more neurons than spinal cord
• comprised of two nerve networks
• submucosal plexus in submucosa
• myenteric plexus between two layers of
  muscularis externa

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Motility in Digestive Tract

• Peristalsis
• progressive contraction of circular
  longitudinal muscles
• propels bolus along tract
• circular muscles contract behind bolus while
  circular muscle ahead relaxes

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Motility in Digestive Tract

• Segmentation
• contraction relaxation of non-adjacent segments
  of tract
• moves contents forwards backwards
• mixes churns bolus
• breaks into fragments
• mixes with intestinal secretions

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Ingestion

• eating
• begins food processing
• begins chemical mechanical digestion
• oral or buccal cavity
• responsible for analysis of material prior to
  swallowing
• mechanical processing via tongue teeth
• lubrication-mixing ingested material with saliva
  mucus
• limited digestion of carbohydrates lipids
• lined with stratified squamous epithelium
• roof-hard soft palate
• floor-tongue

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Tongue

• mechanical processing
• manipulation to assist chewing
• sensory analysis
• touch, taste, temperature
• secretion of lingual lipase
• begins lipid breakdown

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Salivary Glands

• make saliva
• controlled by ANS-parasympathetic nerves?salivary
  reflex
• Lubricating
• Moistening
• Parotid
• serous, watery secretion containing salivary
  amylase (starch?maltose)
• Submandibular
• secretion contains mucus amylase
• Sublinguals
• mucus secretion
• buffer lubricant

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Teeth

• aid in mechanical digestion by mastication or
  chewing
• breaks down connective tissues in plant fibers
  meat
• helps saturate materials with salivary secretions
  enzymes
• permits easier deglutition
• during mastication, salivary glands secrete
  saliva?soften food into a bolus (semi-solid lump)

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Swallowing-Deglutition

• involves over 22 muscles in mouth, pharynx
  esophagus
• controlled by swallowing center in medulla
  pons
• occurs in three phases
• Buccal
• Pharyngeal
• Esophageal

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Buccal Phase

• voluntary
• tongue pushes just formed bolus toward oropharynx
• bolus stimulates tactile receptors
• activates next phase

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Pharyngeal Phase

• tactile receptors send impulses to deglutition
  center in medulla
• impulses returning from center cause soft palate
  uvula to more upward closing off nasopharynx-
  prevents food from entering nasal cavity
• epiglottis covers glottis-opening to larynx
• bolus driven downward by constriction of
  upper?then middle? then lower pharyngeal
  constrictors
• as bolus slides into esophagus

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Esophageal Phase

• esophagus stretches?triggers peristalsis ?pushes
  bolus ahead of it
• peristalsis carries bolus from upper esophageal
  sphincter through esophagus to lower esophageal
  sphincter?stomach

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Stomach

• bolus passes through lower esophageal sphincter
  into stomach
• expanded section of digestive tube between
  esophagus small intestine

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Stomach Functions

• bulk storage
• stores ingested food in upper part
• digestion
• mechanical breakdown of ingested food-lower parts
• disrupts chemical bonds by acids enzymes
• produces intrinsic factor
• for vitamin B12 absorption

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Stomach Parts

• Cardia
• smallest region
• contains mucous glands
• protects from stomach acids enzymes
• Fundus
• makes contact with diaphragm
• Body
• largest region
• mixing tank
• contains gastric glands?acids enzymes
• Pyloris
• leads into duodenum of small intestine
• inside empty stomach? mucosa submucosa are
  thrown into folds called rugae
• distended when food is in stomach
• allow stomach to expand

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Stomach Wall

• covered with simple columnar glandular epithelium
• gastric mucosa is covered with numerous small
  holes
• openings of gastric pits
• two or three tubular glands open into bottom of
  each gastric pit
• gastric glands
• secrete mucus
• secrete acid enzymes

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Gastric Gland Cells

• Mucous Neck Cells
• Parietal
• Chief
• Enteroendocrine

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Parietal Cells

• secrete hydrochloric acid
• assists in break down of food
• not made in cytoplasm
• too strong
• would dissolve secretory vessel destroy cell
• H Cl- are made then secreted out of cell
  assembled
• maintains pH between 0.8 2.0
• kills microorganisms
• breaks down plant cell walls connective tissue
  in meats
• essential for pepsin
• converts pepsinogen to pepsin
• make intrinsic factor
• needed for absorption of vitamin B12

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Cells of Gastric Pit

• Chief Cells
• secrete pepsinogen
• inactive precursor of pepsin
• digests proteins
• Enteroendocrine Cells-G cells
• make gastrin

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Absorption in Stomach

• little absorption
• aspirin ethanol
• absorption does not occur because
• cells are covered by mucus blanket therefore
  never contact chyme directly
• cells do not have transport mechanisms needed to
  absorb materials
• gastric lining is impermeable to water
• digestion has not been completed
• digested food pieces too big

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Digestion in Stomach

• mechanical digestion-churns bolus mixes it
  with digestive juices
• chemical digestion?breaks bonds
• food digests in stomach for several hours
• preliminary digestion of proteins by pepsin
• not completed
• limited time substances are in stomach
• pepsin attacks only specific types of peptide
  bonds
• digestion of carbohydrates lipids by salivary
  amylase lingual lipase
• enzymes continue to digest until pH falls below
  4.5

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Chyme Formation

• bolus secretions? soupy mixture-chyme
• each peristaltic wave delivers a bit of chyme to
  small intestine through pyloric
  sphincter-gastric emptying
• chemical digestion in small intestine depends on
  activity of pancreas, liver gall
  bladder-accessory digestive organs

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Pancreas

• lies posterior to greater curvature of stomach
• exocrine acini cells secrete 200-1500 ml of
  juice/day
• secreted into small ducts-unite to form larger
  ducts- pancreatic accessory
• pancreatic duct joins common bile duct from
  liver gall bladder
• enters duodenum as hepatopancreatic ampulla
• passage of pancreatic juice bile through this
  into small intestine is controlled by sphincter
  of the hepatopancreatic ampulla or Sphincter of
  Oddi.

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Pancreatic Juice

• mixture of water, salt, enzymes, zymogens
  sodium bicarbonate
• secretions controlled by hormones of duodenum
• chyme?duodenum? secretin? pancreas? watery buffer
  pH 7.5-8.8? raises pH of chyme
• chyme?duodenum?CCK? pancreas? pancreatic enzymes

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Pancreatic Secretions

• pancratic amylase
• starch breakdown
• ribonuclease deoxyribonuclease
• nucleic acid breakdown
• pancreatic lipase
• lipid breakdown
• zymogens trypsinogen carboxypeptidase
• a brush border enzyme-enterokinase cleaves
  trypsinogen?trypsin
• trypsin then works on other inactive
  precursors?active ones

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Liver Gall Baldder

• accessory digestive organs
• liver-inferior to diaphragm
• gall bladder-in a depression on posterior surface
  of liver
• Liver-two principle lobes-a larger right a
  smaller left lobe
• connected by a mesentery fold-falciform ligament
• right lobe includes an inferior quadrate a
  posterior caudate lobe

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Liver Gall Bladder

• hepatocytes-major functioning cells of liver make
  800 -1000 mls of bile each day
• bile leaves liver via right left hepatic ducts
  which unite as common hepatic duct
• these join with the cystic duct from the gall
  bladder to form the common bile duct
• bile is stored modified in the gall bladder
• enters small intestine via cystic duct
• does not enter small intestine until gallbladder
  contracts
• principal stimulus for release- cholecystokinin
  CCK

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Emulsification

• bile contains water, bile salts, bile pigments,
  cholesterol, lecithin several ions
• bile salts are important in digestion of lipids
• lipids are not water soluble
• mechanical processing results in large drops
• bile salts breaks down large lipid globules into
  a suspension of smaller lipid globules
• process called emulsification
• increases surface area available for enzymatic
  attack
• digested lipids are absorbed in lacteals of the
  small intestine

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Liver Functions

• carbohydrate metabolism
• stabilizes blood glucose by glycogenolysis
  gluconeogenesis
• lipid metabolism
• removes lipids for storage or breaks down lipids
  when needed
• amino acid metabolism
• removes excess amino acids
• removes waste products
• amino acids?ammonia
• neutralizes ammonia by converting it to urea
• important in drug inactivation
• vitamin storage
• fat soluble vitamins-A, D, E K B12
• mineral storage
• stores iron bound to ferritin
• phagocytosis antigen presentation
• Kupffer cells engulf old RBCs, debris, etc,
  stimulates immune system
• synthesis of plasma proteins

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Small Intestine

• about 20 feet long
• 90 of nutrient absorption
• Duodenum
• next to stomach
• mixing bowl
• Jejunum
• bulk of chemical digestion absorption
• Ileum
• longest part
• ends at ileocecal valve
• sphincter controlling release of substances into
  large intestine

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Small Intestine Lining

• folded into transverse folds-plicae
• permanent
• increase surface area for absorption
• covered by simple columnar epithelium
• microvilli project from cells of epithelium
  forming brush border
• increase surface area more
• allow chyme to contact more of small intestine
  wall
• increased contact means more efficient food
  absorption

Brush Border
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Small Intestine Epithelium

• absorptive cells
• digest absorb nutrients in chyme
• goblet cells
• make mucus

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Intestinal Villi

• mucosa is thrown into folds forming intestinal
  villi
• Core contains lymph vessel- lacteal
• absorbs products of fat digestion
• at base-entrance to intestinal glands- crypts of
  Lieberkuhn
• secrete 1-2 liters of intestinal juice each day
• produce brush border enzymes
• Paneth cells
• secrete lysozyme
• Enteroendocrine cells
• S cells
• make secretin
• CCK cells
• make cholecystokine
• K cells
• make GIP-glucose-dependent-insulinotropic peptide

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Duodenum

• contains duodenal or Brunners glands?mucous
• primary function of duodenum
• receive chyme neutralize acids

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Ileum

• contains Peyers patches
• aggregates of lymphoid nodules
• protection from bacteria

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Movement in Small Intestine

• as chyme enters duodenum?peristaltic contractions
  (migrating motility complexes) move it toward
  jejunum
• segmentation-contraction relaxation of
  non-adjacent segments of the tract mixes churns
  the material breaking it into fragments mixing
  it with intestinal secretions

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Chemical Digestion Absorption

• begins in mouth
• salivary glands?salivary amylase
• polysaccharides broken into di- tri-
  saccharides
• salivary amylase pancreatic amylase continue to
  break down in stomach
• a brush border enzyme-a dextrinase clips off one
  glucose at a time making di tri saccharides
• monosaccharides are made by brush border enzymes
  specific for specific disacchardies
• maltase splits maltose
• sucrase splits sucrose
• absorbed by facilitated transport

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Chemical Digestion Absorption

• fat digestion begins in mouth-lingual lipase
• continues in stomach
• fat enters small intestine as coarse emulsion
  created by lipase digestion mechanical mixing
• lipid droplets too big to be absorbed or further
  broken down
• bile salts (phospholipids) coat emulsion
  stabilizing fat droplets
• pancreatic lipase digests triglycerides in
  emulsion
• triglycerides?monoglycerides free fatty acids
• absorbed by simple diffusion into lacteals

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Chemical Digestion Absorption

• protein digestion begins in mouth
• mechanical processing
• chemical processing
• takes place in stomach with HCl
• pepsin continues digestion in stomach ?breaking
  peptide bonds
• other proteases peptidases found on brush
  border of intestinal villa cells-trypsin,
  elastin, chymotrypsin continue to breakdown
  protein bonds
• carboxypeptidases break off individual amino
  acids from ends of peptides
• absorbed via facilitated diffusion

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Large Intestine

• begins at end of ileum
• ends at anus
• attached to posterior abdominal wall by mesocolon
• ileocecal sphincter regulates passage of chyme
  from the small to the large intestine
• relaxes when food leaves stomach-gastroileal
  reflex
• cecum-first part
• hangs inferior to ileocecal valve
• attached-appendix
• functions reabsorbs water
• compacts intestinal contents into feces
• absorbs vitamins
• stores fecal materials

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Large Intestine-Colon

• colon-largest part
• ascending colon
• transverse colon
• descending colon
• sigmoid colon
• muscularis layer has inner circular layer
  longitudinal muscle layer concentrated into 3
  bands called teniae coli
• contraction of these pull wall into bulging
  pockets or haustra
• permits expansion elongation of colon

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Large Intestine-Rectum Anus

• Rectum
• short, terminal expandable part of large
  intestine
• Anus
• separates large intestine from outside
• closed by 2 sphincters
• internal smooth muscle sphincter
• not under voluntary control
• external skeletal muscle sphincter
• under voluntary control

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Digestion Absorption in Large Intestine

• takes 12 -24 hours to reduce residue of meal
  into feces
• no significant digestion of organic
  molecules-less than 10
• reabsorbs electrolytes
• reabsorbs water
• digestive system is one way body maintains water
  balance
• receives about 9 liters of water each day
• 7.5 L reabsorbed by small intestine
• 1500 ml of material enters colon each day only
  200 ml are excreted with feces

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Motility in Large Intestine

• movement begins when substances pass ileocecal
  sphincter
• hastral churning
• haustra remain relaxed begin to fill up?when
  distension reaches certain point?walls contract
  squeeze contents of one haustra into another
• mass peristalsis
• occur 3-4X/day
• called the gastrocoli reflexes

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Defecation

• final digestive process stage
• processed feces (undigested waste products)
  carried from sigmoid colon to rectum
• distends rectal wall?stimulates stretch
  receptors?sends nerve impulses to sacral spinal
  cord
• impulses return via parasympathetic fibers to
  colon, rectum anus
• causes contraction of longitudinal muscles
• causes rectum to shorten? increases pressure in
  it opens internal anal sphincter
• external anal sphincter must relax to complete
  the process
• feces are expelled

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Phases of Gastric Activity

• gastric activity occurs in three overlapping
  stages
• Cephalic
• Gastric
• Intestinal

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Cephalic Phase

• directed by CNS
• see, smell or anticipate food?cerebral cortex,
  hypothalamus and brain stem?facial,
  glossopharyngeal vagus nerves
• facial, glossopharyngeal nerves?salivary
  glands?saliva
• vagus nerve?gastric glands?gastric juice
  production
• lasts only a few minutes

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Gastric Phase

• begins with arrival of food in stomach
• stomach distends?stretch receptors
• increased pH undigested materials in
  stomach?chemoreceptors
• Stretch chemoreceptors?submucosal plexus ?
  parasympathetic enteric neureons?increase
  peristaltic waves stimulates secretion of
  gastric juice
• phase

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Intestinal Phase

• begins when chyme enters duodenum
• phase controls rate of gastric emptying
• important for small intestine to slow down
  gastric emptying to give time to neutralize acid
  to efficiently absorb incoming nutrients
• duodenum, jejunum ileum are able to process
  only small amounts of material at any one time
• stretching duodenum activates the enterogastric
  reflex
• stretch receptors?medulla?inhibit parasympathetic
  stimulate sympathetic fibers?inhibits gastric
  motility

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Intestinal Phase

• controlled by secretin cholecystokinin
• chyme containing amino acids fatty
  acids?Intestinal glands (CCK cells)?
  CCK?stsimulates secretion of pancreatic juice,
  causes gall bladder to contract, relaxes
  Sphincter of Oddi-slows gastric emptying
• immediate results?no more chyme released from
  stomach
• lowered pH (from chyme)? Intestinal glands (S
  cells)?secretin ?stimulates flow of pancreatic
  juices?bicarbonate buffers the acidic chyme
• secretin?inhibits secretion of gastric juice and
  enhances effects of CCK

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Other Gut Hormones

• tract makes at least 10 other gut hormones
• motilin, substance P, bombesin?affect motility
  of intestine
• VIP (vasoactive intestinal peptide)?stimulates
  secretion of ions waterinhibits acid
  production by stomach
• gastrin releasing peptide ? stimulates gastrin
  release
• somatostatin?inhibits gastrin release

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