Physio - Howell - Intestinal Transport I: Fluid and Electrolytes - 2/18

Question 1

How is the duodenum demarcated from the jejunum?

Answer 1

Ligament of Treitz

Question 2

T/F: The jejunum has 3x more SA than the ileum, consistent with greater absorption.

Answer 2

True

Question 3

What is the function of Paneth cells?

Answer 3

Paneth cells produce antimicrobial agents.

Question 4

Where are the macroscopic folds of Kerckring found (to increase SA)?

Answer 4

Small intestine

Question 5

Where are macroscopic semilunar folds found (to increase SA)

Answer 5

Large intestine

Question 6

How much of the absorptive area of the intestine can be removed without compromising function?

Answer 6

About 1/2

Question 7

What is tropical sprue?

Answer 7

Tropical sprue is an infectious disease present in certain areas of tropical countries associated with diarrhea, malabsorption and nutritional deficiencies.

Question 8

How are goblet cells stimulated to secrete their protective mucous?

Answer 8

PS fibers –> ACh –> goblet cells

Question 9

The villi are primarily made up of:

Answer 9

Columnar epithelium layer, enterocytes (absorptive cells)

Question 10

What is the role of the undifferentiated cells of the crypt?

Answer 10

Undifferentiated cells of the crypt secrete NaCl from the blood into the lumen (water flows by osmosis). They change their function as they travel up the crypt toward the villus and take on absorptive functions, absorbing NaCl or NaHCO3.

Question 11

Why do radiation victims experience intestinal bleeding, diarrhea, and slow death from dehydration and malabsorption?

Answer 11

Because of high turnover of cells in intestinal mucosa, they are very sensitive to radiation damage.

Question 12

What is the shunt pathway in the intestinal epithelium?

Answer 12

Materials can traverse the tight junction and extracellular space.

Question 13

Give 3 examples of ectoenzymes.

Answer 13

Enterokinase
Peptidase
Disaccharidase
All found at the brush border to help complete digestion

Question 14

Where are tight junctions leakiest?

Answer 14

Loosest in jejunum (fast uptake of water), becoming progressively tighter toward the colon

Question 15

Where is the most fluid absorbed after a meal?

Answer 15

Jejunum

Question 16

T/F: Removal of the jejunum is fatal due to malabsorptive effects.

Answer 16

False. The ileum can take over the function of the jejunum.

Question 17

What part of the ileum should never be removed, if possible, and why?

Answer 17

The distal ileum should never be removed if possible because it is the sole site of B12 absorption AND bile salt absorption.

B12 is absorbed in a complex with intrinsic factor (secreted by parietal cells)

Question 18

What metals are most dramatically absorbed in the duodenum?

Answer 18

Ca, Fe

Question 19

By what two methods does Na+ enter the jejunal enterocyte? How does Na then leave the cell (basolateral side)

Answer 19

1. Na/glucose or Na/aa cotransport
2. Na/H antiport –> helps to keep lumen of intestine at normal pH

Na then leaves the cell via the Na/K pump.

Question 20

What is the process by which NaHCO3 is abrosbed by the jejunal absorptive cell?

Answer 20

1. Na absorbed from lumen, leaves basolaterally via Na/K pump
2. H secreted to lumen
3. H combines with HCO3
4. H2CO3 –> H2O and CO2
5. CO2 diffuses into cell
6. CO2 + OH in cell –> HCO3
7. HCO3 leaves basolaterally

Question 21

T/F: ACh stimulates cAMP, which inhibits NaCl absorption by the ileum.

Answer 21

True. This is what gets overstimulated in the case of cholera or E. Coli infection.

Question 22

How is NaCl absorbed in the ileal absorptive cell?

Answer 22

1. Na enters cell via Na/H antiporter
2. Cl enters cell via Cl/HCO3 exchanger
3. Cl passes down BL membrane via electrochemical gradient to blood
4. Na leaves in the usual way, Na/K pump
5. HCO3 combines with H in lumen
6. H2CO3 –> H2O and CO2

Question 23

What is a vipoma, and what are the effects in the intestine?

Answer 23

A vipoma is a VIP-secreting tumor. The amount of VIP is greatly increased, stimulating cAMP, which inhibits NaCl absorption, leading to an osmotic shift, causing diarrhea.

Question 24

What is the role of the apical CFTR channel?

Answer 24

To permit movement of Cl from the crypt cell to the lumen

Note that increased cAMP increases the behavior of this channel in jejunal crypt cells.

The CFTR channel is deficient in patients with cholera, thus it is expected that cholera would be ineffectual in CF patients.

Question 25

How do Na, K and Cl get into crypt cells on the BL membrane?

Answer 25

1 Na/1 K/2 Cl channel

Question 26

Why is oral rehydration therapy with saline and glucose better than plain water in treating cholera?

Answer 26

Intestinal Na/glucose cotransporter helps to absorb Na, decreasing diarrhea and dehydration.

Question 27

T/F: Substances that promote secretion tend to inhibit absorption and vice versa.

Answer 27

True

Question 28

Name 5 endogenous intestinal secretory stimuli.

Answer 28

``` ACh Histamine CCK Secretin Gastrin GIP VIP Motilin ```

Question 29

Name 5 exogenous intestinal secretory stimuli.

Answer 29

``` E. Coli Vibrio cholera Salmonella Bile salts and fatty acids Laxatives ```

Question 30

What do these substances have in common? Epi, Norepi and other alpha-adrenergic agonists Dopamine Enkephalins ie morphine Somatostatin Mineralcorticoids ie aldosterone (in the colon)

Answer 30

They are all endogenous promoters of absorption

Question 31

Congenital hemochromatosis involves:

Answer 31

too much iron absorption

Question 32

T/F: If body Fe is low, brush border transporters increase. If body Fe is high, the brush border transporters decrease AND ferritin increases.

Answer 32

True

Question 33

Iron absorption occurs by 2 pathways: heme iron and non-heme iron. Explain the absorption of iron in either case.

Answer 33

1. DcytB reduces Fe3+ to Fe2+ for absorption (body cannot take up Fe3+) 2. Fe2+ and H+ enter duodenal cell via DCT1 (aka DMTI) cotransporter 3. Fe combines with mobilferrin 4. Mobilferrin-Fe leaves cell on BL side via IREG1 5. Fe gets oxidized to Fe3 again 6. Fe3 binds to transferrin OR 1. Heme enters cell 2. Heme oxygenase releases Fe3+ 3. Fe3+ reduced to Fe2+ 4. See above step 4.

Question 34

What vitamin can aid in iron absorption?

Answer 34

Vitamin C, bc it reduced Fe3+ to Fe2+ For the same reason, patients with low gastric acid absorb less iron and can become anemic.

Question 35

What is the major dietary source of iron?

Answer 35

Red meat (bc it contains a lot of heme)

Question 36

The small intestine absorbs Ca by two mechanisms. The passive, paracellular absorption (through tight junctions) of Ca occurs throughout the small intestine and is not under vitamin D control. The second mechanism, the active transcellular absorption of Ca, occurs only in the duodenum. Explain this active process.

Answer 36

1. Ca enters duodenal enterocyte 2. Ca binds to calbindin, and is taken up by organelles as needed 3. Ca leaves on BL side via Ca/Na exchanger, with a net effect of Ca absorption. Vitamin D stimulates these actions by stimulating the synthesis of proteins involved in Ca uptake.

Question 37

What factors influence Ca absorption?

Answer 37

1. Increased fatty acids due to formation of soap 2. Oxalate reduces bioavailability of Ca (leafy greens) 3. Bile salts form complexes with Ca which facilitate absorption

Question 38

Where does Vitamin D become active?

Answer 38

In the kidney, after a second hydroxylation (stimulated by PTH) First hydroxylation occurs in the liver.

Question 39

Fill in "increase" or "decrease" Increase in Ca in plasma --> __1__ PTH secretion --> __2__ formation of 1,25-OH2-VitD3 --> __3__ synthesis of calbinding --> __4__ Ca absorption

Answer 39

All decrease