GI 9 Flashcards
2 Important Reasons for a Fluid Environment
- Protect the intestinal lining
- Needed for enzymatic digestion and diffusion to site of absorption
Does the majority of fluid come from the diet or from secretion?
secretion
What % of fluid is absorbed by the small intestine?
80%
What % of fluids are excreted in the feces?
1%
Can absorption and secretion occur simultaneously?
yes
Of villus and crypt cells in the intestinal epithelium, which are responsible for absorption and which are responsible for secretion?
Absorption = Villus
Secretion = Crypt
2 Paths for Fluid Transport
- Transcellular path (electrolytes, mainly)
- Paracellular path (water, some ions like Cl-, Na+, K+)
2 Types of Tight Junctions
- Leaky epithelia
- Tight epithelia
Leaky epithelia location
small intestine
Tight epithelia location
colon
How does fluid decide what transport mechanism to use? (2)
- Does it need energy?
- Does it cause net movement of charge across the membrane?
Passive transcellular transport
ion channels responding to an electrochemical gradient
Site for major Na+ absorption?
small intestine (specifically jejunum)
Two Methods of Na+ absorption in the Jejunum?
- Sodium cotransporters (electrogenic)
- Na+/H+ Exchanger (electroneutral)
What compound is absorbed in the ileum?
sodium chloride (NaCl)
Is Na+ absorption in the ileum electrogenic or electroneutral?
electroneutral (Na+/H+ exchanger and Cl-/bicarbonate exchanger)
Is bicarbonate positive or negative charged?
negative
During what phase of digestion does sodium absorption in the ileum occur?
interdigestive phase
What is there a net absorption of in the small intestine?
Na+, K+,Cl-, and water
What is there a net absorption of in the large intestine?
Na+, Cl-, and water
What is there a net secretion of in the large intestine?
bicarbonate and potassium
What hormone enhances sodium absorption in the colon?
aldosterone (helps increase efficiency during dehydration)
Does sodium use a transporter in the colon?
nope, just a channel and an electrochemical gradient generated by primary active transport (restricted diffusion)
3 Mechanisms of Chloride Absorption in the Intestine
- Passive
- Bicarbonate/Cl- exchangers
- Coupled/parallel Na+/H+ and Bicarbonate/Cl- exchangers
Passive chloride absorption and main location(s)
either via the electrochmeical gradient to follow sodium into cell, or paracellular pathway in the jejunum (electrogenic); jejunum and distal colon
Chloride Absorption via Bicarbonate/Cl- exchanger and main location
high conc. of bicarbonate in the cell, bicarbonate is pumped out to lumen so chloride can enter cell (electroneutral); proximal colon
Chloride Absorption via parallel exchangers and location
both exchangers on apical side, allows for add’l absoprtion in ileum and proximal colon
Mechanism of K+ absorption
passive via tight junctions (with water in small intestine)
Is potassium being absorbed in the colon?
Yes, just less than what is being secreted, via channels in the epithelium
What does intestinal secretion center on?
active transport of Cl-
2 Types of Secretions in the Crypt
- Chloride
- Bicarbonate
Bicarbonate is secreted in response to what?
acid in the duodenum
Chloride Secretion in Crypt Cells
- Hormones/NTs bind
- Ca2+/cAMP increase
- More CFTR on luminal surface
- Chloride channels open
- Chloride secreted into lumen, followed by sodium and water (Na+ and H2O via paracellular pathway)
Osmotic Forces
generated by transport of solutes/ions and will cause water absorption and secretion
Is water transcellular or paracellular?
paracellular (mainly)
