Digestion

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Digestion
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Digestion

• Gastrointestinal (GI) tract
• Functions to
• Move nutrients, water, electrolytes from the
  external environment to the internal environment

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8 Functions of Digestive System

• 1. _________________occurs when foods enter the
  digestive tract through the mouth
• 2. ________________ physical manipulation of
  solid foods first by tongue and teeth, then by
  swirling and mixing motions of the digestive
  tract
• 3. ________________ chemical breakdown of food
  into small molecules that can be absorbed by the
  digestive tract lining or epithelium

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Functions of Digestive System

• 4. ______________ Movement of material in the
  GI tract as a result of muscle contraction
• 5. _______________
• A. Transfer of ions from the ECF to the lumen
• B. Release of water, acids, enzymes and buffers
  from the digestive lining and accessory organs

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Functions of Digestive System

• 6. ________________ movement of small organic
  molecules, electrolytes, vitamins and water
  across the digestive epithelium and into the
  interstitial fluid of the digestive tract
• 7. ________________ elimination of waste
  products from the body

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Functions of Digestive System

• 8. __________________
• Repels foreign invaders mucus secretion,
  digestive enzymes, acid
• Lymphoid tissue Gut associated lymphoid tissue
  (GALT)

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Processes of the Digestive System
Fig 21-1
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Review GI Tract Anatomy

• Salivary glands
• Esophagus
• Stomach
• Upper ____________
• Central ____________
• Lower ____________

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Fig 21-2b
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Review GI Tract Anatomy

• Small intestine
• Duodenum
• Jejunum
• Ileum
• Large Intestine
• Colon
• Rectum
• Anus
• Accessory glandular organs
• Liver
• Pancreas

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Fig 21-2a
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GI Tract Wall 4 Layers

• 1. Inner _______________
• A. Inner most layer of the mucosa
• _________________
• Single layer of epithelial cells Barrier
  function
• Mucous membrane consisting of an epithelial
  surface moistened by glandular secretions and an
  underlying loose connective tissue.

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Properties of Luminal Epithelial Cells
Enterocytes

• ____________________________ Move ions and
  water into the lumen and absorb ions, water, and
  nutrients
• _______________ Endocrine and Exocrine
• Mucosal/luminal/Apical surface release enzymes,
  mucus, and paracrines into the lumen
• Serosal/basolateral side secrete hormones or
  paracrines into the interstitial fluid

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Properties of Luminal Epithelial Cells

• Stomach Tight junctions
• Intestinal Leaky epithelium

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Mucosa (cont.)

• B. ______________ subepithelial connective
  tissue that holds the epithelium in place
• Contains nerve fibers and small blood and lymph
  vessels
• Immune cells
• In intestine, collections of lymphoid tissue form
  small nodules and larger Peyers patches that
  create visible bumps in the mucosa

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Mucosa (cont.)

• C. ___________________, thin layer of smooth
  muscle
• Affects surface area by moving villi back and
  forth

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GI Tract Wall 4 Layers

• 2. Middle ______________loose connective
  tissue that surrounds the mucosa
• Blood vessels and lymphatics
• Network of nerve fibers, sensory neurons and
  parasympathetic motor neurons Submucosal
  plexus, one of the two major nerve networks of
  the enteric nervous system
• Control and coordination of smooth muscle layers
  and the secretions of the digestive glands

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GI Tract Wall 4 Layers

• 3. Outer ___________________
• Two sets of smooth muscle
• Inner circular layer
• Outer longitudinal layer
• Responsible for paristalsis waves of
  contraction that push material along the length
  of the digestive tract
• Myenteric plexus The second network of the
  enteric nervous system lies between the two
  muscle layers

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Enteric Nervous System

• Submucosal (Meissners plexus) and the Myenteric
  (Auerbachs plexus)
• Receives input from both the autonomic nerves and
  sensory neurons located within the walls of the
  GI tract.
• Receives CNS input, but is capable of regulating
  many digestive functions independently

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GI Tract Wall 4 Layers

• 4. Outermost layer _________________
• Visceral peritoneum is continuous with the
  parietal peritoneum that lines the inner surfaces
  of the body wall
• Provides structural support
• Outer layer of epithelial tissue called
  mesothelium which secretes a watery lubricating
  fluid

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Fig 21-2c
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Fig 21-2e
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Intestinal Epithelial Cells
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Intestinal Mucosa Fingerlike extensions Villi
Increase surface area
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FUNCTION MOTILITY

• Motility
• 1. Move food from Mouth to Anus
• 2. Mechanically Mixing food to break it up

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Motility

• Single-Unit Smooth Muscle
• 1. Tonic contractions sustained for minutes or
  hours Sphincter muscles
• 2. Phasic contractions occur in the Posterior
  region of the stomach and small intestine
  Contraction-relaxation cycles that last only a
  few seconds

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GI Smooth Muscle Contraction

• 3 types of GI smooth Muscle Contraction Patterns
• 1. _________________________ Occurs between
  meals, when GI tract is empty. Begins in stomach
  and moves from section to section. Functions to
  sweep food remnants and bacteria out of the upper
  GI tract and into the large intestine.

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• 2. _________________ During and following a
  meal. Progressive waves of contraction that move
  from one section of the GI tract to another.
  Functions to propel material forward through the
  esophagus, and mixes food as it digests in the
  stomach.

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• 3. ________________________ Segments of
  intestine contract and relax. Can occur randomly
  or at regular intervals. Alternating segmental
  contractions churn intestinal content back and
  forth, miximing them and keeping them in contact
  with the absorptive epithelium.

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Secretions

• 1. ___________________
• Proenzymes Zymogen
• Activated by the GI lumen

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Secretions

• 2. _________________
• Viscous secretion of glycoproteins that are
  called mucins
• Made by Mucus cells in the stomach and Goblet
  Cells in the intestine
• Protective coating over the GI mucosa
  lubrication

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Secretions

• 3. ____________ Na, K, Cl-, HCO3-, and H

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Enzymes

• _____________ breaks down long glucose polymers
  into smaller glucose chains
• __________________ break interior peptide bonds
  to make smaller peptide fragments
• __________________ release single amino acids
  from peptides by chopping them off the ends

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Enzymes

• ______________ Digests fat
• ______________ Digests phospholipids

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21-22
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Cephalic Phase
Gastric Phase
21-23
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Digestion in Stomach

• __________ Kills bacteria and denatures
  proteins
• __________ protease that carries out initial
  digestion of proteins
• ________________ begin fat digestion, aided by
  lingual lipase
• Carbohydrate digestion that began in the mouth
  continues in stomach until mixing exposes the
  amylase to gastric acid that inactivates it.

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Secretion in Stomach

• __________ secrete HCl
• __________ secrete intrinsic factor Complexes
  with Vitamin B12 and is essential for B12
  absorption
• Absence of intrinsic factor causes pernicious
  anemia

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Fig 21-25
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Intestinal Phase

• Presence of of acidic chyme in the duodenum
  releases the hormone secretin
• Secretin inhibits acid production and gastric
  motility, slowing gastric emptying
• Secretin stimulates production of pancreatic
  bicarbonate
• Cholecystokinin (CCK) is secreted if fat is
  present
• GIP (glucose-dependent insulinotropic peptide) is
  released if carbohydrates are present

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Digestion and Absorption in the Intestine

• Most of the important digestive processes are
  completed in the small intestine
• Final products of digestionsimple sugars, fatty
  acids, and amino acidsare absorbed along with
  most of the ions vitamins and water.
• 80 reabsorption occurs in the small intestine
  through transporting absorptive cells
  enterocytes

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Brush border enzymes

• Intestinal enzymes that once secreted remain on
  the apical membrane
• Brush border enzymes include peptidases,
  disaccharidases, and enteropeptidase
• Enteropeptidases convert inactive trypsinogen
  secreted by the pancreas to active trypsin,
  activating a cascade of activation

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Accessory Organs in the Intestinal Phase

• Liver (hepatocytes) secretes bile
• Bile salts for fat digestion
• Bile pigments such as bilirubin that are the
  waste product of hemoglobin degradation
• Cholesterol
• Bile is secreted into hepatic ducts that lead to
  the gall bladder which stores and concentrates
  the solution. Gallbladder sends bile into the
  duodenum through the common bile duct.

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Accessory Organs in the Intestinal Phase

• Pancreas secretes
• pancreatic amylase
• Pancreatic lipase
• several proteases and exopeptidases
• Nucleases
• BICARBONATE

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Bicarbonate Secretion

• Bicarbonate secretion in small intestine is
  essential to neutralize the highly acidic chyme
  that enters from the stomach.
• Most bicarbonate comes from pancreas
• Bicarbonate production requires high levels of
  carbonic anhydrase.

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Cystic Fibrosis

• CFTR mutations result in no chloride and
  therefore, no fluid secretion. But goblet cells
  continue to secrete mucus. The result is
  thickened mucus that clogs the pancreatic ducts.

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Digestion and Absorption of Nutrients and Water
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Large Intestine

• Bacteria that inhabit the colon break down
  significant amounts of undigested complex
  carbohydrates and proteins through fermentation
• Responsible for absorbing most of the water that
  enters in the form of chyme

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Fig 21-31
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Ulcers

• See Running Problem in Textbook
• Website
• http//www.niddk.nih.gov/health/digest/pubs/hpylor
  i/hpylori.htm

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Lactose Intolerance

• See Website
• http//www.niddk.nih.gov/health/digest/pubs/lactos
  e/lactose.htm