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Digestion. Anatomy of digestive system (review!) 4 major processes of digestive system: Motility Secretion Digestion Absorption. Gastrointestinal System. Motility Secretion Digestion Absorption. Motility. 2 purposes: Forward movement of food Mechanical mixing

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Gastrointestinal System

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gastrointestinal system


  • Anatomy of digestive system (review!)
  • 4 major processes of digestive system:
  • Motility
  • Secretion
  • Digestion
  • Absorption

Gastrointestinal System



  • Secretion
  • Digestion
  • Absorption


2 purposes:

  • Forward movement of food
  • Mechanical mixing

GI smooth muscles contract spontaneously

  • Pacemaker cells, (Interstitial Cells of Cajal), connected by gap junctions, generate slow wave potentials
  • AP’s spread throughout longitudinal muscles (gap junctions)  wave of contraction
    • Like cardiac muscle, Ca2+ can regulate contraction strength

Different Patterns of Contraction

Tonic Contractions

Sustained contraction, usually in the stomach

Phasic Contractions

Peristaltic contractions

  • progressive waves moving along segments of longitudinal layer  forward propulsion
  • circular layer contracts proximal to bolus
    • Especially esophagus

Segmental contractions

  • alternate contraction & relaxation lead to mixing
  • A side effect of narcotics


  • Secretion
  • Digestion
  • Absorption


  • 9 L of fluid pass through the GIT (only 2 L from food & drink)  Secretion and Reabsorption important
    • Ions and water; similar to renal mechanisms
      • Saliva
    • Hydrochloric Acid (Parietal Cells)
    • Bicarbonate (enzyme necessary ?)
    • Enzymes (zymogens)
    • Mucus (Goblet cells)
    • Bile (bile salts – function?)
  • Net Fluid Balance in GI system

Regulation of GIT

  • ANS
    • Parasympathetic (R & R)
    • Sympathetic
  • Emotional (cephalic reflexes)
    • E.g., smell of food
  • ENS (Enteric Nervous System)
    • Self-contained (intrinsic)
  • GI peptides can have regulatory role as hormones or paracrines
    • E.g., Gastrin, CCK


  • Secretion
  • Digestion
  • Absorption

Digestion Overview

  • Mechanical breakdown and mixing aid enzymatic breakdown
    • Chewing
    • Tonic contractions, esp. stomach
  • Enzymatic breakdown converts macromolecules into absorbable units
  • Bile emulsifies fats
  • Optimal pH of enzymes indicates location of activity


  • Secretion
  • Digestion
  • Absorption

Absorption Overview

  • Most nutrient absorption takes place in ?
    • Fats absorbed into lacteals
    • Everything else absorbed into portal vein
  • Alcohol & aspirin across gastric epithelium
  • Additional: H2O, ions & some vitamins absorbed in ________________
  • Mechanisms analogous to renal absorption

CHO Digestion & Absorption

  • ~50% of calories in average American diet
    • Starch (polysaccharide) and sucrose (disaccharide)
    • Cellulose (roughage) not digestible
  • Enzymes: amylases, disaccharidases (maltase, sucrase, lactase)
  • Absorbed only as monosaccharides (glucose, fructose)
    • Small intestine

Protein Digestion and Absorption

  • Variable digestibility
  • 30-60% of protein not from diet
  • First digestion in Stomach by HCl
  • Proteases secreted as proenzymes
    • Pepsin(-ogen), trypsin, etc.
  • Absorption of single a.a. and di- and tripeptides
    • Specific receptors required for larger chains
      • May serve as allergens
      • DDAVP

Lipid (fat) Digestion

  • Mostly triglycerides in diet
    • Cholesterol, Fat-soluble vitamins, others.
  • Combination with bile salts creates an emulsion
    • Colipase and lipase allow formation of small micelles
  • Absorption of fat via diffusion across apical CM
  • Chylomicrons in the cell are absorbed into lacteals

Other Stuff

  • Water soluble vitamins—mediated transport
  • Fat soluble vitamins via absorption
  • Water, Ions and Minerals
    • Various locations and methods, e.g, diffusion, carrier proteins
  • Nucleic Acids

Colon Cancer

  • 2nd largest cause of cancer deaths
  • Cellulose (indigestible) = fiber, roughage
  • Significance of “roughage” in diet??

OlestraBiotechnology focus

  • Consumers want fat in their foods  search for low cal fat substitutes
  • 1996, Procter & Gamble market Olestra Olestra = Olean = sucrose polyester(sucrose molecule + 6-8 FA)
  • Cannot be digested, nor absorbed
  • Unpleasant side effects
  • Absorption of fat sol. vitamins pot. problem

Phases of Digestion/Absorption










  • Anticipation
    • Begins with sensory input that is sent to the cerebral cortex and then to the hypothalamus to stimulate the PNS
    • Salivation, relaxation of the GE sphincter, relaxation of stomach muscles, increase gastric motility
  • Mastication
    • Mechanical digestion
    • Teeth, tongue
  • Deglutition
    • Bolus formation for swallowing
  • Peristalsis in esophagus
salivary glands
Salivary Glands
  • Parotid Glands
    • Secretes Mucous
      • Allows for smooth swallowing of bolus
      • Mumps
  • Sublingual and Submandibular glands secrete Amylase
    • Breaks down complex carbohydrates into simple carbohydrates

Reflux Esophagitis = Heartburn = GERD

Gastroesphogeal Reflux Disease

Lower esophageal sphincter dysfunction

Why reflux against gravity?


GERD, cont’d

  • Intrapleural pressure  during inspiration

 Wall of esophagus expands

  • Subatmospheric pressure in esophageal lumen sucks acidic stomach contents into esophagus
  • May also occur during pregnancy or obesity


  • Gastric
  • Intestinal
  • Defecation
  • Storage
    • Relaxation or contraction as needed.
  • Digestion
    • HCl - Hydrochloric Acid (Parietal cells)
    • Pepsin (Chief cells)
    • Mechanical
  • Immune Protection
    • Ingested pathogens
    • Respiratory mucus
mechanical digestion
Mechanical Digestion
  • Oblique, Longitudinal and Circular Gastric Muscles
    • Esophagus does not contain oblique muscles
  • Allows for gastric motility to occur
  • Muscles are sensitive to
    • Parasympathetic input
    • Gastrin hormone
  • Rugae are used along with the Gastric Muscles for mechanical digestion
chemical digestion
Chemical Digestion
  • Gastric Pits
    • Invaginations of the stomach wall
    • Lined with several different types of cells
  • Chief Cells
    • At the bottom of the pits
    • Secretes pepsinogen
      • Inactive form of the enzyme pepsin
      • Pepsinogen is converted into pepsin upon mixing with HCl
    • Pepsin breaks down proteins into amino acids
chemical digestion cont
Chemical Digestion Cont.
  • Parietal Cells
    • Above Chief Cells
    • Secret H+ and Cl- into the Gastric Pits
    • Used for Digestion of foodstuff
  • Gastric Glands or G cells
    • Secretes the hormone Gastrin
    • Gastrin secretion increases when the contents of the stomach is basic
    • Gastrin secretion decreases when the contents of the stomach is acidic
    • Gastrin’s functions are
      • To increase HCl secretion from the Parietal Cells
      • To increase Gastric Motility
mucous cells
Mucous Cells
  • Secrete Mucous to coat the lumen of the stomach
    • Prevents ulceration
      • H. Pylori also responsible for ulcers
  • 18 year old White Female Adult enters into the emergency room.
  • She has been vomiting for over 72 hours.
  • You observe she is also hypoventilating.
  • How is the vomiting and hypoventilation tied together?
  • List two of the possible problems are that your patient is experiencing.
  • HINT: Don’t forget the Parietal Cell


  • Gastric
  • Intestinal – Chyme (ingesta) enters small intestine
  • Defecation
  • Neutralization of HCl
    • NaHCO3 from pancreas
    • Running Problem: Peptic Ulcer and antacids, including H2 antagonists and proton pump inhibitors
  • Pancreatic enzymes
    • Digest protein, CHO
  • Bile acids
    • Emulsification of Fat
  • Absorption of H2O
mechanical digestion1
Mechanical Digestion
  • Peristalsis and Segmentation
    • Occurs in the small intestine
    • Food is basically shaken back and forth in small segments along the length of the small intestine
chemical digestion1
Chemical Digestion
  • During the segmentation process the food scrapes along the tips of the villi and causes cells to explode off
    • Release Digestive Enzymes
    • Rapid Mitosis in the Crypts of Lieberchun or Intestinal crypts
      • Easily affected by chemo/radiation therapy due to rapid mitosis
chemical digestion2
Chemical Digestion
  • Release of Digestive Enzymes
    • Pancreas releases digestive enzymes into the duodenum
  • Release of Bile
    • Emulsification of Fat
    • Storage and Release by Gall Bladder
    • Production of Bile by Liver
    • Bile contains:
      • Cholesterol
      • Bile Salts
      • Lecithin
      • Billirubin


  • Gastric
  • Intestinal
  • Defecation
  • Bacterial fermentation of CHO and proteins
  • Lactate, some vitamins are digested and/or absorbed
  • More H2O absorption
  • Osmotic diarrhea vs. secretory diarrhea
    • Osmotic-solutes prevent H2O reabsorption
      • Lactose intolerance
    • Secretory- bacterial toxins ("flush out' pathogens)
    • Defecation Reflex
  • Three Major Functions of the Large Intestine
    • Absorption of H2O
    • Storage of Feces
      • Usually 24 hours
    • Production of Vitamin K
      • From E. Coli
      • Used in the Blood Clotting Process

Lactose Intolerance

  • Lactose = glucose + galactose
  • Lactase only found in juvenile mammals
  • Adaptive (dominant) mutation in populations with dairy-based cultures
  • Lactose intolerance in
    • 95% of Native Americans,
    • 90% of Asian Americans
    • 70% of African Americans
    • 50% of Mexican Americans