Intestinal Transport 1: Fluid and Electrolytes Flashcards
what are the order and lengths of the different small intestine portions?
duodenum (11 in)
jejunum (8 ft)
ileum (12 ft)
what is the surface area of the jejunum compared to the ileum? what causes this increase?
three times greater
caused by successive folding of the mucosal surface
what are the three amplifications of surface area of the small intestine?
macroscopic plica circularis (folds of kerckring), microscopic villi and crypts of Lieberkuhn and submicroscopic microvilli
how does the structure of the colon mucosa differ from the small intestine
there are crypts but no villi
how much of the intestines can be removed without compromising absorptive function?
half
what is celiac disease? what is another name for it?
gluten destroys absorbtive cells and decreases the number of functional villi. dehydration and malnutrition result
sprue
what is tropical sprue?
infectious disease associated with diarrhea, malabsorption and nutritional deficiencies
how long does it take for a enterocyte to differentiate and move to the tip of a villus? how long does it take to replace the intestinal epithelium normally?
5 days
a week
what do goblet cells do?
secret mucous in response to acetylcholine
which two cells have the same stem cell at the base of the crypt?
goblet cells and enterocytes
what do the undifferentiated cells at the base of the crypt do? how does this change as they mature?
they secrete NaCl from the blood into the lumen
take on absorptive function as they move up the villus
why does the intestinal mucosa have a great radiation sensitivity?
because there is a high turnover of the mucosa
what is the shunt pathway? what is it permeable to?
paracellular pathway through the tight junctions between enterocytes
permeable to H2O and cations (low permeability to anions)
what matrix is associated with the brush border? what is associated with it?
glycoprotein matrix
ectoenzymes are associated with it (enterokinase, disaccharidases and peptidases)
what is contained within the lamina propria of enterocytes?
lacteals and capillaries
which parts of the intestines have leakier tight junctions?
jejunum> ileum> colon
when are water and most nutrients absorbed by? what are the exceptions?
the end of the jejunum
vitamin B12 and bile salts- absorbed in distal ileum
how are bile salts removed from the lumen?
sodium dependent cotransport in the distal ileum
how is vitamin B12 absorbed?
it is absorbed bound to intrinsic factor (secreted by gastric parietal cells) in the distal ilium
what does the jejunal enterocyte absorb to prevent diarrhea? what happens to it
NaHCO3- transported out of the enterocyte into the blood
how does Na enter the jejunal enterocyte across the apical membrane? how does it travel across the basolateral membrane?
Na/ glucose and Na/AA cotransport & NA/H antiport
Na/K pump coupled with a K channel to dissapate the gradient
what is caused by the increase of the Na/K pump after a meal?
electrogenicity of the pump causes a potential to form across the serosal side of the cell
what are the 4 mechanisms of sodium absorption and where are they most active?
AA and glucose cotransport (jejunum)
Na/H exchanger (jejunum)
Na/H and Cl/CHO3 parallel exchange (ileum and colon)-Cl flows down concentration gradient into blood
epithelial Na channel (distal colon)
how is NaCl absorption controlled in the ileum and proximal colon?
cAMP inhibits absorption (stimulated by E coli and Vibriocholera toxin)
ACh stimulates cAMP along with VIP in vipomas
what is the function of CFTR in crypt cells? how is it influenced by cAMP
located on the apical membrane- allows Cl secretion into the lumen
activated by cAMP
what ion movement exists at the crypt cell basolateral membrane? what does this do?
Na/2Cl/K symporter from blood, K channel out and Na/K pump
creates a concentration gradient for K and Na to move out into the lumen
how does ACh stimulate NaCl excretion?
by increasing intracellular Ca- increasing K conductance
membrane hyperpolarization drives Cl out of cell
what does cholera toxin do to enterocytes? what causes worse diarrhea than cholera?
increases cAMP in cells
inhibits absorption of NaCl in ileum
increases secretion of NaCl by jejunal crypt cells
vipomas increase cAMP more–worse diarrhea
what is an oral rehydration solution?
a mixture of saline and glucose to enhance Na reabsorption (Na/glucose symporter)
what 8 endogenous factors promote intestinal secretion?
ACh, histamine, CCK, secretin, gastrin, GIP, motilin and VIP
what 5 endogenous factors promote intestinal absorption?
epinephrine and NE, dopamine, enkephalins, somatostatin and mineralcorticoids
how do absorptive and secretory factors relate?
substances that promote one tend to inhibit the other
what percentage of iron is absorbed from the diet? why not more?
5%
because most of ingested iron is ferric and cannot be absorbed
what happens at the brush border when body Fe is low?
number of transporters increases
what type of iron is most efficiently absorbed?
heme iron- absorbed as heme and released in cell
ferric iron is released and turned to ferrous
what transfers iron around the cell?
mobilferrin
how is Fe2+ released from food? how is its transformation to ferric iron prevented?
acidic pH of stomach releases it
ascorbate and citrate in stomach help reduce to ferrous
what is the intracellular storage of iron? how is it transported in the blood?
ferritin
transferrin
how is non heme iron absorbed in the cell?
it is absorbed by DMT1 in the duodenum, driven by H+ gradient (cotransport) formed by Na/H exchanger
in what oxidation state is iron carried by transferrin? what enzyme accomplishes this?
ferric (3+)
hephaestin
how is Ca transported in the blood?
half is free, the other half is bound to albumin
what transporter promotes calcium absorption?
a Ca ATPase that is activated by calmodulin- transports Ca from cell into bloodstream
what are the two mechanisms of calcium absorption?
paracellular absorption (unaffected by Vit D) channel absorption
once calcium enters an enterocyte, what happens?
it is bound to calbindin and taken up into organelles
extruded by Ca/Na exchanger and Ca pump
(affected by Vit D)
