Digestive System

1
Digestive System
2
Overview of GI tract Functions

• Mouth---bite, chew, swallow
• Pharynx and esophagus----transport
• Stomach----mechanical disruption absorption of
  water alcohol
• Small intestine--chemical mechanical digestion
  absorption
• Large intestine----absorb electrolytes vitamins
  (B and K)
• Rectum and anus---defecation

3
Layers of the GI Tract

• 1. Mucosal layer
• 2. Submucosal layer
• 3. Muscularis layer
• 4. Serosa layer

4
Mucosa

• Epithelium
• stratified squamous(in mouth,esophagus anus)
  tough
• simple columnar in the rest
• secretes enzymes and absorbs nutrients
• specialized cells (goblet) secrete mucous onto
  cell surfaces
• enteroendocrine cells---secrete hormones
  controlling organ function
• Lamina propria
• thin layer of loose connective tissue
• contains BV and lymphatic tissue
• Muscularis mucosae---thin layer of smooth muscle
• causes folds to form in mucosal layer
• increases local movements increasing absorption
  with exposure to new nutrients

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Submucosa

• Loose connective tissue
• containing BV, glands and lymphatic tissue
• Meissners plexus---
• Parasympathetic
• innervation
• vasoconstriction
• local movement

6
Muscularis

• Skeletal muscle voluntary control
• in mouth, pharynx , upper esophagus and anus
• control over swallowing and defecation
• Smooth muscle involuntary control
• inner circular fibers outer longitudinal fibers
• mixes, crushes propels food along by peristalsis

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Serosa

• An example of a serous membrane
• Covers all organs and walls of cavities not open
  to the outside of the body
• Secretes slippery fluid
• Consists of connective tissue covered with simple
  squamous epithelium

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Peritoneum

• Peritoneum
• visceral layer covers organs
• parietal layer lines the walls of body cavity
• Peritoneal cavity
• potential space containing a bit of serous fluid

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Parts of the Peritoneum

• Mesentery
• Mesocolon
• Lesser omentum
• Greater omentum
• Falciform ligament

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PHYSIOLOGY OF DIGESTION

• Takes place through GI tract i.e. mouth to anus
• Accessory organs (pancreas, liver, gall bladder)
  perform important functions

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Digestion in the Mouth

• Mechanical digestion (mastication or chewing)
• Food breaks into pieces
• mixes with saliva so it forms a bolus
• Chemical digestion
• Salivary amylase
• begins starch digestion at pH of 6.5 or 7.0 found
  in mouth
• when bolus enzyme hit the pH 2.5 gastric
  juices, hydrolysis ceases
• lingual lipase
• secreted by glands in tongue
• begins breakdown of triglycerides into fatty
  acids and glycerol

12
Composition and Functions of Saliva

• Wet food for easier swallowing
• Dissolves food for tasting
• Bicarbonate ions buffer acidic foods
• bulemia---vomiting hurts the enamel on your teeth
• Chemical digestion of starch begins with enzyme
  (salivary amylase)
• Enzyme (lysozyme) ---helps destroy bacteria
• Protects mouth from infection with its rinsing
  action.

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Salivation

• Increase salivation
• sight, smell, sounds, memory of food, tongue
  stimulation (rock in mouth)
• cerebral cortex signals the salivatory nuclei in
  brainstem
• Salivation stops under sympathetic stimulus
• dry mouth when you are afraid

14
Pharynx

• Deglutition or swallowing is facilitated by
  saliva and mucus
• starts when bolus is pushed into the oropharynx
• sensory nerves send signals to deglutition center
  in brainstem
• soft palate is lifted to close nasopharynx
• larynx is lifted as epiglottis is bent to cover
  glottis

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Physiology of the Deglutition(Swallowing)

• Buccal phase Voluntary, tongue pushes bolus to
  back of oral cavity. Lifting of larynx and
  bending of epiglottis is simultaneous with tongue
  pushing bolus into oropharynx
• Once food is in oropharynx, next phase starts

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Deglutition Pharyngeal phase

• Involuntary (Peristalsis) phase
• breathing stops airways are closed
• soft palate uvula are lifted to close off
  nasopharynx
• vocal cords close
• epiglottis is bent over airway as larynx is
  lifted
• Peristalsis begins, bolus moves toward esophagus

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Deglutition Esophageal phase

• Upper sphincter relaxes when larynx is lifted
• Peristalsis pushes food down
• Travel time is 4-8 seconds for solids and 1 sec
  for liquids
• Lower sphincter relaxes as food approaches

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Gastroesophageal Reflex Disease

• If lower sphincter fails to open
• distension of esophagus feels like chest pain or
  heart attack
• If lower esophageal sphincter fails to close
• stomach acids enter esophagus cause heartburn
• for a weak sphincter---don't eat a large meal and
  lay down in front of TV
• smoking and alcohol make the sphincter relax
  worsening the situation
• Control the symptoms by avoiding
• coffee, chocolate, tomatoes, fatty foods, onions
  mint
• take Tagamet HB or Pepcid AC 60 minutes before
  eating
• neutralize existing stomach acids with Tums

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Anatomy of Stomach

• Size when empty
• - large sausage
• stretches due to rugae
• Parts of stomach
• cardia
• fundus---air in x-ray
• body
• pylorus---starts to narrow
• as approaches pyloric sphincter

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Mucosa Gastric Glands

• Hydrochloric acid and Intrinsic factor(Parietal
  cell)
• Gastric lipase and pepsinogen (Chief cell)
• Gastrin hormone (g cell)
• Mucus(mucous neck cells)

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Muscularis

• Three layers of smooth muscle--outer
  longitudinal, circular inner oblique
• Permits greater churning mixing of food with
  gastric juice

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Physiology--Mechanical Digestion

• Gentle mixing waves (in fundus)
• mixes bolus with 2 quarts/day of gastric juice to
  turn it into chyme (a thin liquid)
• Allows salivary amylase to continue to work for
  one hour
• More vigorous waves
• travel from body of stomach to pyloric region
• Intense waves near the pylorus
• open it and squirt out 1-2 teaspoons full with
  each wave

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Physiology--Chemical Digestion

• Protein digestion begins
• HCl denatures (unfolds) protein molecules
• HCl transforms pepsinogen into pepsin that breaks
  peptides bonds between certain amino acids
  (protein--- peptone)
• Fat digestion continues
• gastric lipase splits the triglycerides in milk
  fat
• most effective at pH 5 to 6 (infant stomach)
• HCl kills microbes in food
• Mucous cells protect stomach walls from being
  digested with 1-3mm thick layer of mucous

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Regulation of Gastric Secretion

• Cephalic phase
• Gastric phase
• Intestinal phase

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Cephalic Phase Stomach Getting Ready

• Cerebral cortex sight, smell, taste thought
• stimulate parasympathetic nervous system
• Vagus nerve
• increases stomach muscle and glandular activity,
  gastric secretion begins

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Gastric Phase Stomach Working

• Nervous control keeps stomach active
• stretch receptors chemoreceptors provide
  information
• vigorous peristalsis and glandular secretions
  continue
• chyme is released into the duodenum
• Endocrine influences over stomach activity
• distention and presence of caffeine or protein
  cause G cells to secrete gastrin into bloodstream
• gastrin hormone increases stomach glandular
  secretion, churning and sphincter relaxation

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Intestinal Phase Stomach Emptying

• Stretch receptors in duodenum (after chyme enters
  it) slow stomach activity increase intestinal
  activity
• Distension, fatty acids or sugar (in Duodenum)
  signals medulla
• sympathetic nerves slow stomach activity
• Hormonal influences
• secretin hormone decreases stomach secretions
• cholecystokinin(CCK) decreases stomach emptying,
  makes gall bladder release bile.
• Takes 2-4 hrs to completely empty the stomach

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Regulation of Gastric Emptying

• Release of chyme is regulated by neural and
  hormonal reflexes
• Distention stomach contents increase secretion
  of gastrin hormone initiates emptying
• Enterogastric reflex regulates amount released
  into intestines
• Secretin and Cholecystokinin inhibit gastric
  emptying

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Absorption of Nutrients by the Stomach

• Some Water and electrolytes
• Some drugs (especially aspirin) alcohol
• Gastric mucosal cells contain alcohol
  dehydrogenase that converts some alcohol to
  acetaldehyde-----more of this enzyme found in
  males than females
• Females have less total body fluid that same size
  male so end up with higher blood alcohol levels
  with same intake of alcohol

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Vomiting (emesis)

• Forceful expulsion of contents of stomach
  duodenum through the mouth
• Cause
• irritation or over distension of stomach
• unpleasant sights, general anesthesia, dizziness
  certain drugs
• Sensory input from medulla cause stomach
  contraction complete sphincter relaxation
• Contents of stomach squeezed between abdominal
  muscles and diaphragm and forced through open
  mouth
• Serious because loss of acidic gastric juice can
  lead to alkalosis

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Pancreatic Juice

• Contains water, enzymes sodium bicarbonate
• Digestive enzymes
• pancreatic amylase, pancreatic lipase, proteases
• trypsinogen---activated by enterokinase (a brush
  border enzyme)
• chymotrypsinogen----activated by trypsin
• carboxypeptidase---activated by trypsin
• trypsin inhibitor---combines with any trypsin
  produced inside pancreas
• Ribonuclease deoxyribonuclease to digest
  nucleic acids

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Regulation of Pancreatic Secretions

• Secretin
• acidity in intestine causes increased sodium
  bicarbonate release thru secretin
• GIP
• fatty acids sugar causes increased insulin
  release thru GIP
• CCK
• fats and proteins cause increased digestive
  enzyme release thru CCK

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Anatomy of the Liver and Gallbladder

• Liver
• below diaphragm
• right lobe larger
• gallbladder on right lobe
• size causes right kidney to be lower than left
• Gallbladder
• Stores bile

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Bile Production

• One quart of bile/day is secreted by the liver
• yellow-green in color pH 7.6 to 8.6
• Components
• water cholesterol
• bile salts Na K salts of bile acids
• bile pigments (bilirubin) from hemoglobin
  molecule
• Gall bladder concentrates bile by absorbing water
• Bile emulsifies fat

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Liver Functions-- Metabolism

• Turn proteins and triglycerides into glucose
• Turn excess glucose into glycogen store in the
  liver and turn glycogen back into glucose as
  needed
• Synthesize cholesterol
• Synthesize lipoproteins----HDL and LDL(used to
  transport fatty acids in bloodstream)
• Deamination removes NH2 (amine group) from
  amino acids so can use what is left as energy
  source
• Converts resulting toxic ammonia (NH3) into urea
  for excretion by the kidney
• Convert one amino acid into another

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Other Liver Functions

• Detoxifies the blood by removing or altering
  drugs hormones(thyroid estrogen)
• Removes the waste product--bilirubin
• Releases bile salts help digestion by
  emulsification
• Stores fat soluble vitamins-----A, B12, D, E, K
• Stores iron and copper
• Phagocytizes worn out blood cells bacteria
• Activates vitamin D (the skin can also do this
  with 1 hr of sunlight a week)

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Summary of Digestive Hormones

• Gastrin
• stomach, gastric ileocecal sphincters
• Gastric inhibitory peptide--GIP
• stomach pancreas
• Secretin
• pancreas, liver stomach
• Cholecystokinin--CCK
• pancreas, gallbladder stomach

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Anatomy of the Small Intestine

• 20 feet long----1 inch in diameter
• Large surface area for majority of absorption
• 3 parts
• duodenum---10 inches
• jejunum---8 feet
• ileum---12 feet
• ends at ileocecal valve

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Histology of the Small Intestine

• Structures that increase surface area
• Villi
• Core is lamina propria of mucosal layer
• Contains vascular capillaries and
  lacteals(lymphatic capillaries)
• microvilli
• cell surface feature known as brush border
• Functions of Microvilli Absorption and digestion
• Digestive enzymes found at cell surface on
  microvilli, known as brush border enzymes.

40
Roles of Intestinal Juice Brush-Border Enzymes

• Submucosal layer has duodenal (Brunners) glands
• secretes alkaline mucus
• Mucosal layer contains intestinal glands
• secretes intestinal juice
• brush border enzymes
• paneth cells secrete lysozyme kills bacteria

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Mechanical Digestion in the Small Intestine

• Weak peristalsis in comparison to the
  stomach---chyme remains for 3 to 5 hours
• Segmentation---local mixing of chyme with
  intestinal juices---sloshing back forth

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Chemical Digestion in Small Intestine

• Groups enzymes by region where they are found
• Need to trace breakdown of nutrients
• carbohydrates
• proteins
• lipids

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Digestion of Carbohydrates

• Mouth---salivary amylase
• Esophagus stomach---nothing happens
• Duodenum----pancreatic amylase
• Brush border enzymes (maltase, sucrase lactase)
  act on disaccharides
• produces monosaccharides--fructose, glucose
  galactose
• lactose intolerance (no enzyme bacteria ferment
  sugar)--gas diarrhea

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Lactose Intolerance

• Mucosal cells of small intestine fail to produce
  lactase
• essential for digestion of lactose sugar in milk
• undigested lactose retains fluid in the feces
• bacterial fermentation produces gases
• Symptoms
• diarrhea, gas, bloating abdominal cramps
• Dietary supplements are helpful

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Digestion of Proteins

• Stomach
• HCl denatures or unfolds proteins
• pepsin turns proteins into peptones
• Small Intestine
• Pancreatic enzymes--changes peptones into
  peptides
• brush border enzymes-----split peptides into
  amino acids

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Digestion of Lipids

• Mouth----lingual lipase
• Small intestine
• emulsification by bile
• pancreatic lipase---splits fat into fatty acids
  monoglyceride/glycerol
• no enzymes in brush border

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Digestion of Nucleic Acids

• Pancreatic juice contains 2 nucleases
• ribonuclease which digests RNA
• deoxyribonuclease which digests DNA
• Nucleotides produced are further digested by
  brush border enzymes (nucleosidease and
  phosphatase)
• pentose, phosphate nitrogenous bases
• Absorbed by active transport

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Regulation of Secretion Motility

• Enteric reflexes that respond to presence of
  chyme
• increase intestinal motility and secretion
• segmentation depends on distention which sends
  impulses to the enteric plexus CNS
• Sympathetic impulses decrease motility

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Absorption in Small Intestine
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Where will the absorbed nutrients go?
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Absorption of Monosaccharides

• Absorption into epithelial cell
• Mostly by facilitated diffusion, some active
  transport

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Absorption of Amino Acids Dipeptides

• Absorption into epithelial cell
• Mostly by active transport

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Absorption of Lipids

• Small fatty acids enter cells then blood by
  simple diffusion
• Larger lipids exist only within micelles (bile
  salts coating)
• Lipids enter cells by simple diffusion leaving
  bile salts behind in gut
• Bile salts reabsorbed into blood reformed into
  bile in the liver

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Absorption of Lipids (2)

• Inside epithelial cells fats are rebuilt and
  coated with protein to form chylomicrons
• Chylomicrons leave intestinal cells by exocytosis
  into a lacteal

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Absorption of Vitamins

• Fat-soluble vitamins
• travel in micelles are absorbed by simple
  diffusion
• Water-soluble vitamins
• absorbed by diffusion

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Absorption of Water

• 9 liters of fluid dumped into GI tract each day
• Small intestine reabsorbs 8 liters
• Large intestine reabsorbs 90 of that last liter
• Absorption is by osmosis through cell walls into
  vascular capillaries inside villi

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Anatomy of Large Intestine

• 5 feet long by 2½ inches in diameter
• Ascending descending colon are retroperitoneal
• Cecum appendix
• Rectum last 8 inches of GI tract anterior to
  the sacrum coccyx
• Anal canal last 1 inch of GI tract
• internal sphincter----smooth muscle involuntary
  
• external sphincter----skeletal muscle voluntary
  control

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Appendicitis

• Inflammation of the appendix due to blockage of
  the lumen by chyme, foreign body, carcinoma,
  stenosis, or kinking
• Symptoms
• high fever, elevated WBC count, neutrophil count
  above 75
• referred pain, anorexia, nausea and vomiting
• pain localizes in right lower quadrant
• Infection may progress to gangrene and
  perforation within 24 to 36 hours

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Digestion in Large Intestine

• Mechanical Peristaltic waves cause haustral
  churning----relaxed pouches are filled from below
  by muscular contractions
• Chemical No enzymes are secreted, only mucus
• Bacteria ferment
• undigested carbohydrates into carbon dioxide
  methane gas
• undigested proteins into simpler substances
  (indoles)----odor
• turn bilirubin into simpler substances that
  produce color
• Bacteria produce vitamin K and B in colon

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Absorption Feces Formation in the Large
Intestine

• Some electrolytes---Na and Cl-
• After 3 to 10 hours, 90 of H2O has been removed
  from chyme
• Feces is semisolid by the time it reaches
  transverse colon
• Feces dead epithelial cells, undigested food
  such as cellulose, bacteria (live dead)

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Defecation

• Gastrocolic reflex moves feces into rectum
• Stretch receptors signal sacral spinal cord
• Parasympathetic nerves contract muscles of rectum
  relax internal anal sphincter
• External sphincter is voluntarily controlled

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Defecation Problems

• Diarrhea chyme passes too quickly through
  intestine
• H20 not reabsorbed
• Constipation--decreased intestinal motility
• too much water is reabsorbed
• remedy fiber, exercise and water

63
Dietary Fiber

• Insoluble fiber
• woody parts of plants (wheat bran, vegie skins)
• speeds up transit time reduces colon cancer
• Soluble fiber
• gel-like consistency beans, oats, citrus white
  parts, apples
• lowers blood cholesterol by preventing
  reabsorption of bile salts so liver has to use
  cholesterol to make more

64
Aging and the Digestive System

• Changes that occur
• decreased secretory mechanisms
• decreased motility
• loss of strength tone of muscular tissue
• changes in neurosensory feedback
• diminished response to pain internal stimuli
• Symptoms
• sores, loss of taste, peridontal disease,
  difficulty swallowing, hernia, gastritis, ulcers,
  malabsorption, jaundice, cirrhosis, pancreatitis,
  hemorrhoids and constipation
• Cancer of the colon or rectum is common