Intestinal Transport 1: Fluid and Electrolytes

Question 1

what are the order and lengths of the different small intestine portions?

Answer 1

duodenum (11 in)
jejunum (8 ft)
ileum (12 ft)

Question 2

what is the surface area of the jejunum compared to the ileum? what causes this increase?

Answer 2

three times greater

caused by successive folding of the mucosal surface

Question 3

what are the three amplifications of surface area of the small intestine?

Answer 3

macroscopic plica circularis (folds of kerckring), microscopic villi and crypts of Lieberkuhn and submicroscopic microvilli

Question 4

how does the structure of the colon mucosa differ from the small intestine

Answer 4

there are crypts but no villi

Question 5

how much of the intestines can be removed without compromising absorptive function?

Answer 5

half

Question 6

what is celiac disease? what is another name for it?

Answer 6

gluten destroys absorbtive cells and decreases the number of functional villi. dehydration and malnutrition result
sprue

Question 7

what is tropical sprue?

Answer 7

infectious disease associated with diarrhea, malabsorption and nutritional deficiencies

Question 8

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?

Answer 8

5 days

a week

Question 9

what do goblet cells do?

Answer 9

secret mucous in response to acetylcholine

Question 10

which two cells have the same stem cell at the base of the crypt?

Answer 10

goblet cells and enterocytes

Question 11

what do the undifferentiated cells at the base of the crypt do? how does this change as they mature?

Answer 11

they secrete NaCl from the blood into the lumen

take on absorptive function as they move up the villus

Question 12

why does the intestinal mucosa have a great radiation sensitivity?

Answer 12

because there is a high turnover of the mucosa

Question 13

what is the shunt pathway? what is it permeable to?

Answer 13

paracellular pathway through the tight junctions between enterocytes
permeable to H2O and cations (low permeability to anions)

Question 14

what matrix is associated with the brush border? what is associated with it?

Answer 14

glycoprotein matrix

ectoenzymes are associated with it (enterokinase, disaccharidases and peptidases)

Question 15

what is contained within the lamina propria of enterocytes?

Answer 15

lacteals and capillaries

Question 16

which parts of the intestines have leakier tight junctions?

Answer 16

jejunum> ileum> colon

Question 17

when are water and most nutrients absorbed by? what are the exceptions?

Answer 17

the end of the jejunum

vitamin B12 and bile salts- absorbed in distal ileum

Question 18

how are bile salts removed from the lumen?

Answer 18

sodium dependent cotransport in the distal ileum

Question 19

how is vitamin B12 absorbed?

Answer 19

it is absorbed bound to intrinsic factor (secreted by gastric parietal cells) in the distal ilium

Question 20

what does the jejunal enterocyte absorb to prevent diarrhea? what happens to it

Answer 20

NaHCO3- transported out of the enterocyte into the blood

Question 21

how does Na enter the jejunal enterocyte across the apical membrane? how does it travel across the basolateral membrane?

Answer 21

Na/ glucose and Na/AA cotransport & NA/H antiport

Na/K pump coupled with a K channel to dissapate the gradient

Question 22

what is caused by the increase of the Na/K pump after a meal?

Answer 22

electrogenicity of the pump causes a potential to form across the serosal side of the cell

Question 23

what are the 4 mechanisms of sodium absorption and where are they most active?

Answer 23

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)

Question 24

how is NaCl absorption controlled in the ileum and proximal colon?

Answer 24

cAMP inhibits absorption (stimulated by E coli and Vibriocholera toxin)
ACh stimulates cAMP along with VIP in vipomas

Question 25

what is the function of CFTR in crypt cells? how is it influenced by cAMP

Answer 25

located on the apical membrane- allows Cl secretion into the lumen activated by cAMP

Question 26

what ion movement exists at the crypt cell basolateral membrane? what does this do?

Answer 26

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

Question 27

how does ACh stimulate NaCl excretion?

Answer 27

by increasing intracellular Ca- increasing K conductance | membrane hyperpolarization drives Cl out of cell

Question 28

what does cholera toxin do to enterocytes? what causes worse diarrhea than cholera?

Answer 28

increases cAMP in cells inhibits absorption of NaCl in ileum increases secretion of NaCl by jejunal crypt cells vipomas increase cAMP more--worse diarrhea

Question 29

what is an oral rehydration solution?

Answer 29

a mixture of saline and glucose to enhance Na reabsorption (Na/glucose symporter)

Question 30

what 8 endogenous factors promote intestinal secretion?

Answer 30

ACh, histamine, CCK, secretin, gastrin, GIP, motilin and VIP

Question 31

what 5 endogenous factors promote intestinal absorption?

Answer 31

epinephrine and NE, dopamine, enkephalins, somatostatin and mineralcorticoids

Question 32

how do absorptive and secretory factors relate?

Answer 32

substances that promote one tend to inhibit the other

Question 33

what percentage of iron is absorbed from the diet? why not more?

Answer 33

5% | because most of ingested iron is ferric and cannot be absorbed

Question 34

what happens at the brush border when body Fe is low?

Answer 34

number of transporters increases

Question 35

what type of iron is most efficiently absorbed?

Answer 35

heme iron- absorbed as heme and released in cell | ferric iron is released and turned to ferrous

Question 36

what transfers iron around the cell?

Answer 36

mobilferrin

Question 37

how is Fe2+ released from food? how is its transformation to ferric iron prevented?

Answer 37

acidic pH of stomach releases it | ascorbate and citrate in stomach help reduce to ferrous

Question 38

what is the intracellular storage of iron? how is it transported in the blood?

Answer 38

ferritin | transferrin

Question 39

how is non heme iron absorbed in the cell?

Answer 39

it is absorbed by DMT1 in the duodenum, driven by H+ gradient (cotransport) formed by Na/H exchanger

Question 40

in what oxidation state is iron carried by transferrin? what enzyme accomplishes this?

Answer 40

ferric (3+) | hephaestin

Question 41

how is Ca transported in the blood?

Answer 41

half is free, the other half is bound to albumin

Question 42

what transporter promotes calcium absorption?

Answer 42

a Ca ATPase that is activated by calmodulin- transports Ca from cell into bloodstream

Question 43

what are the two mechanisms of calcium absorption?

Answer 43

``` paracellular absorption (unaffected by Vit D) channel absorption ```

Question 44

once calcium enters an enterocyte, what happens?

Answer 44

it is bound to calbindin and taken up into organelles extruded by Ca/Na exchanger and Ca pump (affected by Vit D)