Fluid and ion absorption
From Iusmphysiology
- started here on 02/22/11 at 11AM.
[edit] Fluid and ion absorption
[edit] Objectives
[edit] First image
- SI has vili and crypts; LI only has crypts.
- SI:
- Crypts for secretion
- Vili for absorption
- LI:
- Surface epithelium for absorption
- Crypts for secretion
- All the cells arise from progenitors that are found in the crypts.
- Make goblet, endocrine, epithlelial cells
- Cells divide as they move upward.
- Paneth cells:
- Secrete alpha defensisns
- Act like neutrophils
- Sit by the stem cells
[edit] Large suraface area for absorption
- SI: 6 meters, LI: 2.4 meters
- 200 square meters = SI, 25 square meeters = LI
- Note that the LI has no vili and does not absorb nutrients.
- Both absorb Na and K ions.
[edit] Inside the villus
- Each has its own blood supply: an artery, caps, and vein.
- Fats go to lymphatic system
[edit] Venous blood from the gut travels first to the liver...
- Venous blood of gut goes to liver via portal vein before going to the systemic circulation.
[edit] Fluid balance in the GI tract
- 8.5 liters enter the SI / day
- only a couple liters from the diet
- 6.5 liters of secretion
- So we have to reabsorb lots of it or we would rehydrate
- 2 liters enters the LI
- Only about 500 ml leave via feces
- 99% reabsorption
- Went over how much secretion from each part of the tract.
[edit] Net ion movement
- Depending on where you are along the intestine there are different ion transporters: both along the intestine as a whole and as vili versus crypts.
- SI absorbs: water, Na, Cl, K
- SI secretes: HCO3
- LI abosrbs: water, Na, Cl
- LI secretes: HCO3, K
- Epithelial cells are polar and have tight jxns so stuff has to go around or through.
- Para cellular is easier for charged ions (going through tight jucntions).
[edit] Na absorption
- Na absorption is driven by the basolater Na/K atpase; it maintains low Na intracellularly.
- The nutrient coupled transporters (think glucose and such) are how we absorb Na.
- This movement is independent of cAMP and Ca
- Impt b/c not affected by microbial toxins like cholera toxins.
- Between meals
- Use a Na-H and Cl-HCO3 exchange
- Ileum
- This is an electroneutral process.
- Stimulated by E-coli causing diarrhea.
- Regulated by cAMP and Ca
- Think microbes and their toxins which stimulate cAMP production and Ca production.
- Aldosterone is released in response to low blood volume
- Kidney and intestine keep water
- This is facilitated by retaining Na.
[edit] Fluid movement
- Water follows solutes via osmosis.
- In the jejunum the tight junctions can let fluid through.
- So as water goes through it can drag some ions through it.
- That is, the water is moving in the first place because of osmosotic forces.
- So resistance between tight junctions is what determines the ?
[edit] Cl absorption
- Cl is a voltage dependent absorption, driven by the Na gradient.
- So Cl movement is moved indirectly by Na gradient.
- This uses no transporters
- Cl-HCO3 exchanger:
- An antiporter
- HCO3 comes from CA activity
- Left over protons need to moved so a H pump moves it out on the basolateral surface.
- Cl-HCO3 exchanger with Na-H exchanger
- Coupled through changes in intracellular pH.
[edit] Electrogenic Cl secretion
- This process is stimulated by NTs, immune cells, bacterial toxins.
- These activate apical Cl channels which cause insertion into the membrane.
- This is a Na/K - Cl antiporter driven by the Na gradient.
- This is usually coupled with Na absorption through the tight junction to neutralize the charge changes.
[edit] K movement
- The small intestine absorbs K and the LI secretes it.
- Spaced out for a bit.
- In the distal portion, there is some active K absorption
- Uses an ATP-dependent pump to move K into the cell.
[edit] Regulation of intestinal ion transport
- Enteric nervous sytem:
- Ach from mucosal neurons can activate Cl secretion.
- Endocrine
- Aldosterone and angiotensin 2 can stimulate Na retention and absorption (and increases water).
- Paracrine
- 5Ht and serotoinin cause adjacent endothelial cells to...
- Immune cells
- CAn regulate epithelial transport
[edit] Immune cell regulation
- Prostaglandin O, oxygen radicals, bradykinin, etc. can act on epithelial cells:
- Direct:
- Indirect action:
- Cause cl secretion directly
- Activate enteric neurons which then act on the epith clels
[edit] Slide
- See slide
[edit] Calcium absorption
- Ca+'s absorptive limitation is movement across the epithelial cell.
- We don't want calcium absorption to screw with the cellular pathways of the epithelial cell so we bind it calbindin.
- Calbindin is made in the epithelil cells and requires active VitD, thus vitamin D is required for proper calcium absorption.
[edit] Iron absorption
- Absorbed through the epithelial cell.
- IN the diet, Fe comes as non-heme and heme.
- Non heme
- Binds to transferrin, internalized thorugh receptor mediated endocytosis, transferrin releases Fe, Fe picked up by mobiliferrin, carried acrros cell, dropped in plasma where it is picked upw ith transferrin
- Heme Fe:
- Heme oxygenase cuts out the fe, which is hten picked up by transferring.
- VitC helps to solublize Fe by reducing Fe2+ to Fe3+.
- Ion tablets should not be washed down with tea though because the tanins prevent the ions from being absorbed because of complexes formed.
[edit] Two major functons of the large intestine
- Absorption of fluid
- Formation, storage, and periodic elimination of the feces.
[edit] Formation of feces by the large intestine
- Allows smooth muscle to increase length when storage needed.
- xverse colon is primary site of relectrolyte and fluid removal.
- Lots of sgemented contractions to mix stuff around; hence haustra.
- First location of storage.
- 1-3 times per day we have mass peristolsis which moves feces into descending colon
- Called the gastric-colon reflex.
- Occurs after a meal
[edit] Major phases of the normal defecation process
- Delivery:
- MOves feces into recum
- Triggers reflex repsonse
- Detection
- Rectosphincteric reflex is triggered by distention
- Voluntary and involuntary
- Discharge:
- Relaxation of internal and external anal sphincter
- External is striated so pt has control over it.
- Moves rectum from angled to straight
- Holding it:
- Mostly the external anal sphincter
- Removes stimulus for defecation.
- stopped here on 02/22/11 at 12PM.
