2 - Large Intestine, Salt and Water Transport, Intestinal Secretion, Diarrhea Flashcards
Objectives: Describe the motility of the large intestine
Overview
Structure
- 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
Objectives: Describe the motility of the large intestine
Ileocecal Sphincter
Segmentation Contractions (most)
Peristaltic Contractions
Mass Movement
- 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
Objectives: Describe the motility of the large intestine
Distal Colon - Waste State; Movement of Waste
Rectum and Anal Canal
- 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)
Objectives: Describe the motility of the large intestine
Anal Sphincters
Defecation
Feces (composition)
- Anal Sphincters
-
Internal:
- Material Forced in = Internal Relaxed = Rectosphincteric Reflex
-
External:
- Tonically Contracterd; individuals will defecate who do not have control of rectosphincteric reflex
-
Internal:
- 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
Objectives: Describe pathophysiology of the large intestine
Hirschspring’s Disease (cause, results)
Constipation (cause, changes to colon)
- 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
- Cause: Absensce of enteric nervous system (normally keeps relaxation)
- 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
Objectives: Explain how water and electrolytes are absorbed in the small intestine
Overview
Reabsorption
Routes for Crossing of Water / Electrolytes
- 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
Objectives: Explain how water and electrolytes are absorbed in the small intestine
Na+ Transport:
Mechanisms (4x)
Extrusion from Enterocytes
-
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
Objectives: Explain how water and electrolytes are absorbed in the small intestine
Cl- Transport
Water Absorption
- Cl- Transport:
-
Passive Diffusion via Paracellular Route
- Gradient established by Na+
- Contransport with Na+ / K+
- Exchange with HCO3-
-
Passive Diffusion via Paracellular Route
- Water Absportion:
- Depends on Na – Where sodium goes, water follows
Objectives: Explain the mechanism of intestinal secretion
Overview
Mechanism (Cl- Secretion)
- 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
-
Crypt Cells use Na+-Cl- Contransport to bring Cl- INTO cell against gradient
-
Result: Secretion of NaCl; water follows to maintain isotonicity
- Stimulated by GI hormones, neurotransmitters
Objectives: Explain the etiology of diarrhea
Osmotic Diarrhea
- Accumulation within small intestine of nonreabsorbable solutes
- Excess solutes attract water from intestinal wall in volumes that exceed absorptive capacity of gut
Explain the Colonic Flora and their function
- 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
Explain Intestinal Gas
Sources
Components
- 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
-
CO2, H2, N2
Objectives: Explain the etiology of diarrhea
Secretory Diarrhea
Mechanism (channels?)
Causes (Infections?)
- 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
-
Cholerae
- Infection:
Objectives: Explain the etiology of diarrhea
Inflammatory/Infection Diarrhea
Causes
Mechanism
- Causes:
- Infection: Salmonella, Campylobacter Clostridium Difficile
- Virus: Rotavirus, Norovirus
- Mechanism:
- Inflammatory / immune response kills intestinal cells
- Loss of enterocytes results in inability to absorb
Objectives: Explain the etiology of diarrhea
Oral Rehydration Therapy
- 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
