2 - Large Intestine, Salt and Water Transport, Intestinal Secretion, Diarrhea Flashcards

1
Q

Objectives: Describe the motility of the large intestine

Overview

Structure

A
  • Large intestine absorbes some of elctrolytes; most of fluids passed from small intestine
  • Haustra - sac-like segments; locations not fixed; present when empty; appear during, disappear following contractions of specific segment
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2
Q

Objectives: Describe the motility of the large intestine

Ileocecal Sphincter

Segmentation Contractions (most)

Peristaltic Contractions

Mass Movement

A
  • Ileocecal Sphincter
    • When ileum distended, sphincter relaxes (OPENS FLOW)
    • When colon distended, sphincter contracts (PREVENTS BACKFLOW)
  • Segmentation Contractions
    • Proximal colon, appearance of haustra
    • Mix contents; expose to absorptive surfaces
  • Peristaltic Contractions
    • Move chyme slowly along
  • Mass Movement
    • Peristaltic Wave; 1-3/day
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3
Q

Objectives: Describe the motility of the large intestine

Distal Colon - Waste State; Movement of Waste

Rectum and Anal Canal

A
  • Distal Colon
    • Most water absorbed in proximal colon; waste becomes more solid
    • Segmentation Contraction prominent; result in little movement
    • Mass Movements most important for moving material
  • Rectum and Anal Canal
    • Usually empty
    • Frequency of Contractions > Distal Sigmoid Colon
      • Accounts for retention/absorption of suppositories (backflow)
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4
Q

Objectives: Describe the motility of the large intestine

Anal Sphincters

Defecation

Feces (composition)

A
  • Anal Sphincters
    • Internal:
      • Material Forced in = Internal Relaxed = Rectosphincteric Reflex
    • External:
      • Tonically Contracterd; individuals will defecate who do not have control of rectosphincteric reflex
  • Defecation
    • If does not occur, internal sphincter contracts, rectum relaxes to accommodate fecal material within
    • Urge to defecate subsides
  • Feces
    • Inorganic material, undigested plant fibers, bacteria, water
    • Composition relatively unafected by variations in diet; large fraction non-dietary in origin
    • Starving will still defecate
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5
Q

Objectives: Describe pathophysiology of the large intestine

Hirschspring’s Disease (cause, results)

Constipation (cause, changes to colon)

A
  • Hirschspring’s Disease (congenital megacolon)
    • Cause: Absensce of enteric nervous system (normally keeps relaxation)
      • Normal inhibitory; w/out = tonic contraction
    • Results in constriction; loss of coorinated movement
    • Dilation of colon proximal to constriction
  • Constipation
    • Increased time of passage or transit of material through colon
    • Changes to Function:
      • Increased storage capacity of cecum, ascending, transverse colon
      • Decreased propulsive capacity of descening, sigmoid colon
    • Diminish Frequency of Bowel Movements
      • Ignoring urge to defecate
      • Lack of Exercise
      • Narcotics, Antidepressants
      • Old Age
      • Long-term use of laxatives
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6
Q

Objectives: Explain how water and electrolytes are absorbed in the small intestine

Overview

Reabsorption

Routes for Crossing of Water / Electrolytes

A
  • Small Intestine absorbed dietary and biological water / electrolytes (salivary, gastric, biliary, pancreatic)
    • Essential for preventing dehydration
  • Routes for Transport (Absorption)
    • ​Transcellular - Move through cells
    • Paracellular - Move between cells
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7
Q

Objectives: Explain how water and electrolytes are absorbed in the small intestine

Na+ Transport:

Mechanisms (4x)

Extrusion from Enterocytes

A
  • Duodenum / Jejunum:
    • Na+-Glucose / Na+-Amino Acid Contransport
    • Na+-H+ Exchange
  • Ileum:
    • Cl- Contransport
  • Colon:
    • Restricted Na+ Diffusion through Channels
  • Extrusion (Keeps Intracellular Low)
    • Na+ - K+ ATPase Pump
    • Keeps intracellular Na+ levels lower than in lumen; promotes transport of Na+ into cell
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8
Q

Objectives: Explain how water and electrolytes are absorbed in the small intestine

Cl- Transport

Water Absorption

A
  • Cl- Transport:
    • Passive Diffusion via Paracellular Route
      • Gradient established by Na+
    • Contransport with Na+ / K+
    • Exchange with HCO3-
  • Water Absportion:
    • Depends on Na – Where sodium goes, water follows
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9
Q

Objectives: Explain the mechanism of intestinal secretion

Overview

Mechanism (Cl- Secretion)

A
  • Small / Large Intestines secrete water and electrolytes from crypt cells; secrete mucus from goblet cells
  • Mechanisms:
    • Crypt Cells use Na+-Cl- Contransport to bring Cl- INTO cell against gradient
      • NaK ATPase keeps intracellular Na low
    • Cl- Channels in Apical membrane activated by increase in cAMP or Ca2+ levels
      • Mutated in cystic fibrosis
    • Cl- diffuses from cell into lumen through channels
      • Resting conditions normally closed
    • Na secreted into lumen by following Cl-
      • Paracellular
  • Result: Secretion of NaCl; water follows to maintain isotonicity
    • ​Stimulated by GI hormones, neurotransmitters
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10
Q

Objectives: Explain the etiology of diarrhea

Osmotic Diarrhea

A
  • Accumulation within small intestine of nonreabsorbable solutes
  • Excess solutes attract water from intestinal wall in volumes that exceed absorptive capacity of gut
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11
Q

Explain the Colonic Flora and their function

A
  • Numerous bacteria reside in large intestine; stable
  • Functions:
    • Digest carbs that enter large intestine
    • Form secondary bile acids and deconjuagate bile acids
    • Generate Short-Chain Fatty Acids (SFCA) such as acetate, propionate, and butyrate that are absorbed by colon
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12
Q

Explain Intestinal Gas

Sources

Components

A
  • Sources:
    • Swallowed Air, bacterial action in ileum and colon, diffusion of gas from the bloodstream
  • Small Intestine:
    • Usually from swallowed air; passed on to colon
  • Large Intestine:
    • Colonic Gas (flatus) produced in large volumes
    • Produced from breakdown of nutrients
  • Main Components:
    • CO2, H2, N2
      • ​N2 = Air, CO2 = Acids, H2 = Carbs
    • CH4 may be produced
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13
Q

Objectives: Explain the etiology of diarrhea

Secretory Diarrhea

Mechanism (channels?)

Causes (Infections?)

A
  • Mechanism:
    • Excess stimulation of secretory cells in crypts of small intestine and colon
    • Cl- Channels ALWAYS OPEN; increased secretion of Cl- into lumen
      • Increases secretion of Na+, Water
  • Causes: Infections, Tumors
    • Infection:
      • Cholerae
        • Toxin keeps Cl Channel Open
      • E. coli
        • Enterotoxins activate guanylin receptors
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14
Q

Objectives: Explain the etiology of diarrhea

Inflammatory/Infection Diarrhea

Causes

Mechanism

A
  • Causes:
    • Infection: Salmonella, Campylobacter Clostridium Difficile
    • Virus: Rotavirus, Norovirus
  • Mechanism:
    • Inflammatory / immune response kills intestinal cells
    • Loss of enterocytes results in inability to absorb
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15
Q

Objectives: Explain the etiology of diarrhea

Oral Rehydration Therapy

A
  • Secretory Diarrhea: Secretion > Absorption
  • Fluid and Electrolyte loss can be reduced by oral administration of Na+ and Glucose (or amino acids)
    • Na+ Absorbed via Na+-Glucose/Amino Acid Contransporter
    • Cl- and H2O Follow
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