Digestive System

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Digestive System

• http//www.hhmi.org/biointeractive/media/fate_of_f
  at-lg.mov

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The Digestive System

• Gastrointestinal (GI) tract or alimentary canal
• Continuous tube from mouth to anus
• Mouth, Pharynx, Esophagus, Stomach, Small
  intestine, and Large intestine
• Length in cadaver 30ft., shorter in living due to
  muscle contraction
• Accessory digestive organs
• teeth, tongue, salivary glands, liver,
  gallbladder, and pancreas

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Overview of GI tract Functions

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

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Processes of Digestive System

• Ingestion is taking food into the mouth (eating).
• Defecation is emptying of the rectum, eliminating
  indigestible substances from the GI tract.

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Digestion

• Digestion breaks food into small molecules to
  facilitate absorption
• Polysaccharide Monosaccharide
• Proteins - Amino acids
• Fats - fatty acids and glycerol
• By hydrolysis
• Aided by enzymes increases rate at body temp

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Chemical Digestion

• Secretion is the release, by cells within the
  walls of the GI tract and accessory organs, of
  water, acid, buffers, and enzymes into the lumen
  of the tract (total of 7 liters).

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Mechanical Digestion

• Mixing and propulsion result from the alternating
  contraction (peristalsis) and relaxation of the
  smooth muscles within the walls of the GI tract.
• Aids chemical digestion

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• Absorption is the passage of end products of
  digestion from the GI tract into blood or lymph
  for distribution to cells.
• Assimilation occurs when food molecules become
  part of body tissues/metabolic processes

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LAYERS OF THE GI TRACT
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Histology of Digestive Tract

• Epithelial layer is stratified squamous in mouth,
  esophagus, and anal canal protection
• Stomach and intestines are lined by simple
  columnar epithelium secretion and absorption

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Digestion in the Mouth

• Mechanical digestion (mastication or chewing)
• Chemical digestion
• Salivary amylase
• begins starch digestion at pH of 6.5 or 7.0
• swallowed too quickly for all starches to break
  down Continues to digest for another hour
  Stomach acid inactivates
• Substrate starch
• Product - maltose

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Composition and Functions of Saliva

• 99.5 water and 0.5 solutes
• Bicarbonate ions buffer acidic foods (pH
  6.35-6.85)
• Chemical digestion of starch begins with enzyme
  (salivary amylase)
• Mucus lubricates food
• Enzyme (lysozyme) ---helps destroy bacteria

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Structure and Function of the Tongue

• Muscles move tongue to assist in chewing and
  swallowing
• Papillae are the bumps---taste buds
• Lingual glands secrete mucus and serous fluid
  with enzyme lipase

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Dentition

• Primary or baby teeth
• 20 teeth that start erupting at 6 months
• 1 new pair of teeth per month
• Permanent teeth
• 32 teeth that erupt between 6 and 12 years of age
• incisors for biting, canines for tearing,
  premolars molars for crushing and grinding food
• Often not enough room for third molars (wisdom
  teeth)

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Esophagus

• Collapsible muscular tube, passageway
• 10 inches long
• Anterior to vertebrae
• Posterior to trachea
• Posterior to the heart
• Pierces the diaphragm at hiatus
• hiatal hernia

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Disorders

• Achalasia lower esophageal sphincter fails to
  relax, meal stuck in esophagus
• Heartburn lower esophageal sphincter fails to
  close after food enters stomach

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Stomach--Mechanical Digestion

• peristaltic movements called mixing waves.
• Pyloric sphincter remains almost, but not
  completely, closed
• Gastric emptying about 3mL of chyme is forced
  out
• Most chyme is forced back into body of stomach to
  continue mixing

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Stomach--Chemical Digestion

• Salivary amylase continues to work
• Secretion of HCl inactivates amylase, activates
  lingual lipase and pepsinogen, kills microbes,
  and stimulates secretion of hormones that promote
  bile and pancreatic juice
• Protein digestion
• Fat digestion

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Absorption in the Stomach

• Epithelial cells are impermeable to most
  materials
• Water
• Electrolytes
• Short-chain fatty acids
• 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

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Anatomy of Stomach
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Stomach Lining
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Disorders

• Peptic ulcer Disease
• H. pylori bacterium
• Pain relievers
• Hypersecretion of HCl

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Composition and Functions of Pancreatic Juice

• 1 1/2 Quarts/day at pH of 7.1 to 8.2
• Contains water, enzymes sodium bicarbonate
• Digestive enzymes
• pancreatic amylase and pancreatic lipase
• Pancreatic nucleases
• Pancreatic trypsin (protease)

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Histology of the Pancreas

• Acini
• Exocrine
• 99 of gland
• Islets of Langerhans
• Endocrine
• 1 of gland

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Bile Production

• One quart of bile/day is produced by the liver
• yellow-green in color pH 7.6 to 8.6
• Components
• Water, cholesterol, bile salts, bile pigments
  (bilirubin) from hemoglobin molecule
• Functions
• Emulsification
• Produced by hepatocytes in liver, stored in
  gallbladder

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Anatomy of the Small Intestine

• Major events of digestion and absorption occur in
  small intestine
• Large surface area
• 3 parts
• duodenum---10 inches
• jejunum---8 feet
• ileum---12 feet
• Small diameter

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Histology of Small Intestine
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Cells of Intestinal Glands
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Chemical Digestion in Small Intestine

• Chyme entering small intestine contains partially
  digested carbohydrates, proteins, and lipids
• The completion of the digestion is a collective
  effort of pancreatic juice, bile, and intestinal
  juice in the small intestine
• Occurs in duodenum

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Review Digestion of Carbohydrates

• Mouth---salivary amylase
• 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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Review Digestion of Proteins

• Stomach
• HCl denatures or unfolds proteins
• pepsin turns proteins into small polypeptides
• Pancreas
• Pancreatic trypsin--split peptide bonds between
  amino acids
• brush border enzymes-----aminopeptidase or
  dipeptidase

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Review Digestion of Lipids

• Mouth----lingual lipase
• Most lipid digestion, in an adult, occurs in the
  small intestine.
• emulsification by bile
• pancreatic lipase---splits triglycerides into
  fatty acids monoglycerides

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Digestion of Nucleic Acids

• Nucleic acids are broken down into nucleotides
  for absorption.
• Pancreatic juice contains 2 nucleases
• Nucleotides produced are further digested by
  brush border enzymes

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Absorption in Small IntestineJejunum and ileum
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Where will the absorbed nutrients go?
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Anatomy of Large Intestine5 feet long by 2½
inches in diameter
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Disorders

• Appendicitis inflammation of appendix
• Due to blockage or bacterial infection
• Can rupture and spread to abdomen

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Histology of Large Intestine

• Mucosa
• smooth tube -----no villi or plica
• Micro-villi of absorptive cells are present
• simple columnar cells absorb water goblet cells
  secrete mucus

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Digestion in Large Intestine

• Mechanical digestion includes peristalsis
• Chemical digestion - bacteria
• undigested carbohydrates into carbon dioxide
  methane gas
• undigested proteins into simpler substances
  (odor)
• turn bilirubin into simpler substances that
  produce color
• Bacteria produce vitamin K and B in colon

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Absorption Feces Formation

• Some electrolytes (Na and Cl-) and vitamins
• After 3 to 10 hours, Feces are semisolid
• Feces dead epithelial cells, undigested food
  such as cellulose, bacteria (live dead)
• 90 of H2O has been removed from chyme in small
  intestine, but large intestine absorbs enough
  water to make it important in maintaining water
  balance

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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

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Timing

• After food is chewed and swallowed 5-10 seconds
  to pass down esophagus to stomach
• Partial digestion in stomach 2-6 hours
• Final digestion and absorption in small intestine
  5-6 hours
• In 12-24 hours undigested material passes through
  large intestine