Absorption of ions and water

Question 1

Which part of the GI tract is exclusively responsible for the absorption of dietry nutrients?

Answer 1

Small intestine

Question 2

What cells are found in the villus of small intestine?

Answer 2

1. enterocyte- nutrient and electrolyte absorption
2. goblet- mucous secretion

Question 3

What cells are found in the crypt of small intestine?

Answer 3

1. Crypt - enterocytes- secretion
2. enteric endocrine cell- hormone secreting
3. stem/ progenitor cell- essential for crypt-villous axis
4. paneth cell- host defence against microbes

Question 4

Small intestine surface area is increased by…

Answer 4

Microscopic folds of Kerckring- plicae circulares Villi Microvilli

Question 5

Absorptive process in the small intestine are enhanced in the —- state

Answer 5

Postprandial

Question 6

What is transcellular movement?

Answer 6

Solute cross two cell membranes in series. Active transport implicated

Question 7

What is paracellular movement?

Answer 7

Solute moves passively between adjacent epithelial cells via tight junctions

Question 8

What does the absorption of water depends upon?

Answer 8

Absorption of Na+ and Cl-

Question 9

How does water absorption occur?

Answer 9

Transport of Na+, Cl- and HCO3 into the lateral intercellular spaces

The resulting high NaCl near the apical end of the intercellular space causes it to be hypertonic which causes an osmotic flow from the lumen, and via tight junctions into intercellular spaces

Question 10

What happens to Na+ in the GI tract?

Answer 10

Active absorption in jejunum, ileum and colon

Question 11

What happens to K+ in the GI tract

Answer 11

Passive absorption in jejunum and ileum

Net secretion in colon

Question 12

What happens to Cl- in the GI tract?

Answer 12

Absorption in jejunum, ileum and colon

Question 13

What happens to HCO3- in the GI tract?

Answer 13

Absorption in jejunum, secretion in ileum and colon

Question 14

What are the four routes of entry for Na+?

Answer 14

1. Na/gucose transport, Na/amino acid transport
2. Na-H exchanger
3. Parallel Na-H and Cl- HCO3 exchange
4. Epithelial Na+ chanel

Question 15

Which transporter is involved in Na/glucose transport?

Answer 15

SLT-1

Question 16

Where in the GI tract has the highest net rate of absorption of Na+ and why?

Answer 16

Jejunum- presence of glucose, galactose and neutral amino acids in the lumen

Question 17

Explain the first route of entry for Na+

Answer 17

1. Active export of Na+ from epithelial cells by the Na+,K+ ATPase in the basallateral plasma membrane
2. Na+ crosses membrane down the concentration gradient
3. This provides energy for moving glucose, galactose and neutral amino acids into the epithelial cells against concentration gradient

Highest in jejunum

Important post-prandial

Question 18

Explain the second route of entry for Na+

Answer 18

1. Na-K pump extrude Na+, lowers intracellular Na+ conc
2. Na-H exchanger couples Na+ uptake with proton extrusion

Mostly in Jejunum

Process is enhanced by decrease in intracellular pH and increase in luminal pH

Question 19

Why does luminal pH increase?

Answer 19

luminal HCO3- secretion by pancreatic, biliary and duodenal issues

Question 20

Explain the third route of entry for Na+

Answer 20

Parallel Na-H and Cl-HCO3- exchange (electroneutral)

1. Bicarb leaves in exchange for Cl-
2. H+ leaves in exchange for Na+
3. Gradient maintained by Na-K pump

Occurs in ileum and large intestine

not affected by luminal glucose or pH

Primary route of absorption in fasted state

Question 21

What would reduce NaCl absorption?

Answer 21

Increase in:

1. cAMP- enterotoxins elevate cAMP
2. cGMP
3. intracellular Ca²⁺

Question 22

Explain the fouth route of extry for Na+

Answer 22

entry occurs across the apical membrane via ENaC channels that are highly specific for Na+

Mainly in distal colon

Question 23

How does mineralcorticoids increase Na+ absorption?

Answer 23

1. increase opening of apical Na+ channels
2. insertion of channel from vesicle
3. increased synthesis of apical pumps

Question 24

Describe Cl- transport

What are the 3 models proposed?

Answer 24

occurs throughout the small and large intestine and is often closely linked to Na+ absorption

1. passive Cl- absorption
2. parallel Na-H and Cl-HCO3 exchange
3. Cl-HCO3 exchange

Question 25

Cl- secretion

Answer 25

* in small and large bowel crypt cells * mediated by cAMP and Ca²⁺

Question 26

Explain the first model for Cl- absorption

Answer 26

Passive Cl- absorption 1. A passive process driven by the electrochemical gradient for Cl- either by a paracellular of transcellular route 2. The driving force for both is derived from either Na+glucose transport in the small intestine and epithelial Na+ channels in the distal end of the colon

Question 27

Explain the second model for Cl- absorption

Answer 27

Parallel Na-H and Cl-HCO3 exchange electroneutral NaCl absorption also mediates Cl- absorption in the **ileum and proximal part of the colon**

Question 28

Explain the third model for Cl- absorption

Answer 28

Cl-HCO3 exchange In the absence of Na-H exchange occurs in the **ileum and in surface epithelial cells in the large intestine**

Question 29

What is congenital chloridorrhea?

Answer 29

* The congenital absence of an apical Cl-HCO3 exchanger * autosomal recessive disorder * diarrhoea with an extremely high Cl- * Because secretion of HCO3 is reduced, patients are alkalotic * Treatment- PPI- get rid of alkaline tide

Question 30

What are the 4 categories of secretagogues?

Answer 30

1. bacterial enterotoxins 2. hormones and neurotransmitters 3. products of the immune system 4. laxatives

Question 31

Absorptogogues promote absorption. Give some examples Where and how does it act?

Answer 31

**Angiotensin and aldosterone** Increase BP by stimulating Na+ absorption Angiotnesion- Small intestine - enhances electroneutral NaCl ansorption by upregulating Na-H exchange Aldosterone- Colon - stimlates absorption through ENaC

Question 32

How does cholera cause secretory diarrhoea? What is an effective treatment and why?

Answer 32

* increases cAMP * increases Cl and K secretion * inhibits electroneutral NaCl absorption * Second messengers do not alter the function of nutrient-coupled Na+ absorption, **oral rehydration solution containing glucose and Na+** is effective

Question 33

How do immune products cause diarrhoea? What are the treatments?

Answer 33

**Histamine** and **prostaglandins** released from mast cells induce diarrhoea through **cAMP** e.g. in IBD _Treatments_ 1. **opiod receptor against** acts on myenteric plexus (Loperamide)- decrease smooth muscle motility, more time for water reabsorption 2. **somatostatin analogues**

Question 34

What is Verner-Morrison syndrome?

Answer 34

Affects hormone/NT mediated secretogogues mostly pancreatic tumours that produce **VIP which increases cAMP** --\> diarrhoe

Question 35

Describe how calcium is absorbed

Answer 35

**Passive** paracellular pathway, not under control of VDR **Active** 1. active transcellular uptake via apical Ca2+ channels, 2. binds to calbindin which acts as a buffer to keep conc low. 3. Basolateral Na-Ca exhanger extrude Ca2+ from cells 4. Active VitD activates all three steps