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60 Cards in this Set
- Front
- Back
Undifferentiated cells in the small intestine migrate along
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villous-crypt axis and become villous absorptive cells, globet, enteroendocrine cells, paneth cells.
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Colon crypts feats
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No paneth cells here. Possibly the reason why colon infections are more prevalent.
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Cell junction in intestinal epithelia formed by
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tight junction
Adhering junction Gap junction Desmosome |
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Tight junctions in intestinal epithelia has 3 function
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1-provide selectivity for transport
2-make a fence: separate the membrane of the epithelial cells into apical vs basolateral 3-define polaririty |
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Adhering junction jobs
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Mediates cell-cell contact via Cell Adhesion Molecules (CAM)
Essential for proper organization of polarized epithelium Initiates assembly of subcortical cytoskeleton |
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Tight junctions of leaky epithelia are relatively
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more permeant to diffusion of ions than tight junctions of tight epithelia
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Which one is more leaky, jejunum or distal colon?
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jejunum
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Epithelia can become more leaky by factors such as
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proinflammatory cytokines
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Intercellular (Paracellular) transport process
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Solutes move passively between adjacent epithelial
cells via the tight junctions Driven by diffusion and convection |
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Transcellular transport process
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Solutes have to pass through two membranes (apical and basolateral) of the epithelial cells
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Fluid and electrolyte absorption and secretion occurs in both
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small and large intestine
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Net nutrient absorption occurs in which part of the GI?
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small intestine..proximal part
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GI feats regarding absorption
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1-tremendous capacity to absorb fluid
2-Net balance is towards absorptioon and very efficient |
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Absorption of electrolytes is proportional to
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water absorption
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Molecules that show parallel absorption along with water
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Na, Cl, HCO3
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Most of the Na+ is absorbed by
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villous cells of the small intestine
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Movement of Na+ from cell to blood due to
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action of Na+/K+ ATPase pump on the basolateral membrane
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Inwardly directed electrochemical gradient for Na+ is created by
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Na+/K+ ATPase pump across the apical membrane of epithelial cells
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Na/Amino acid transporter is electrogenic because
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it creates a current when Na goes across membrane
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Primary mechanism for Na+ absorption after a mediated by
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apical membrane transporters
-Na/Glucose cotransporter (SGLT1) -Na/Amino Acid cotransporters |
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Na-glucose, Na-AA transporters found in
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Jejunum and Ileum
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Na-H exchangers found in
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Duodenum
Jejunum |
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Types of Na-H exchanger
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-NHE3 and NHE2 isoforms on apical membrane
-NHE1 isoform on basolateral membrane |
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Na-H exchanger stimulated by
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luminal HCO3
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Na-H Exchange is uncoupled to
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Cl/HCO3 exchange process
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Na-H exchange is classified as...what type of transport
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secondary
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Na-H exchanger in terms of electric changes is
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electroneutral
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Na-H exchanger coupled to Cl/HCO3 process
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Na goes in through transporters
H+ goes out of cell More Alkaline in the cell since H+ is going out,leads to HCO3 going out and Cl coming in Net effect, Na and Cl come into the cell Major mechanism of NaCl absorption in intervals between meals |
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Epithelium Na channel (ENaC) restricted to
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very distal part of the colon close to rectum
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Cellular mechanisms of Cl absorption
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Voltage-dependent Cl- absorption coupled to electrogenic Na+ absorption (Na-nutrient cotransporters and ENaC)
Cl-/HCO3- exchange process mediated by members of SLC26 gene family of anion exchanger |
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SLC26A3 is down regulated during
SLC26A6 is a |
adenoma (DRA)
putative Anion Transporter-1 |
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Cl absorption in passive in
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Jejunum, distal colon, ileum. Passive due to pores,opened channels
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Cl/HCO3 for Cl transport used in
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Ileum, proximal colon, distal colon
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Cl/HCO3 function
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Cl goes in, bicarb out. Electroneutral, can work by itself or couple to Na/H exchanger
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Na/H and Cl/HCO3 work in parallel in
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Ileum, proximal colon
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Electrogenic Cl secretion occurs nmostly in
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crypt
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Cloride secretion is stimulated by
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secretagogues:
Bacterial exotoxins (Cholera toxin) Hormones and neurotransmitter (acetylcholine, VIP) Product of the immune system (histamine) Laxatives (bile acids) |
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Second messengers role in Cl secretion
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is stimulated by second messengers: ^[Ca++]i, ^cAMP and ^cGMP
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Chloride secretion across the apical membrane is via
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apical chloride channels
CFTR and/or calcium dependent-channel CIC |
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CTFR requires for proper Cl secretion
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ATP and phosphorylation
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Cl transport in basolateral membrane vua
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Na+/K+ATPase (Ouabain-sensitive)
NKCC (Na+-K+-2Cl- cotransporter,Furosemide-sensitive) K+ channels |
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HCO3- transport may occur via
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-anion exchangers such as Cl-/HCO3- exchangers: net Cl- absorption and HCO3- secretion.
-may also occur via channel (possibly CFTR) |
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HCO3 loading occurs via
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Na+-HCO3- cotransporters localized to the basolateral membrane of the epithelial cells
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HCO3 converted from CO2 via
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Carbonic Anhydrase
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Net K+ movement is
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absorptive in the small intestine and secretory in the colon
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Solvent drag
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K+ absorption in the small intestine
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Primary mechanism of K+ secretion in the colon
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Passive
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Active secretion of K+ in colon induced by
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aldosterone and cAMP
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Distal part of the colon, K+ net transport is
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active absorption
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cGMP net effect in Ion transport
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Decreases NaCl absorption
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Excess fluid in the stool is caused by
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Increase in fluid secretion or decrease in fluid absorption or both
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Osmotic Diarrhea
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Excess of non absorbed solutes in the lumen
example: lactase deficiency |
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Secretory Diarrhea
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Excessive secretion of fluid into the lumen
Infection: E.coli and cholera Secretagogue-producing tumors VIP, Serotonin, Glucagon |
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Congenital Chloride Diarrhea
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Rare genetic disease
Voluminous diarrhea Distended abdomen Fecal chloride concentration > 90 mmol/L vs 10-15 mmol/L in normal individuals Treatment: Salt replacement therapy |
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Defect in Congenital chloride diarrhea
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DRA is mutated.
Cl accumulates in lumen (luminal acidification) |
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Diarrhea ensues when
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the absorptive capacity of the GI tract has been surpassed
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Functions of Microflora or commensal microorganisms:
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Essential for the development of mucosal immune system
Help in performing reactions that do not occur in mammalian cells Inhibit the growth of competitor pathogenic bacteria |
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Microflora produced short chain fatty acids such as
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butyrate, propionate, acetate
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Why is butyrare important?
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It's the mos abundant anion in colonic lumen and it's the preferential energy source for the colonocytes
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Short fatty acids are transported across the membrane via
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HCO3 exchanger.
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