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DIG > Absorption of ions and water > Flashcards

Absorption of ions and water Flashcards

1
Q

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

A

Small intestine

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2
Q

What cells are found in the villus of small intestine?

A
  1. enterocyte- nutrient and electrolyte absorption
  2. goblet- mucous secretion
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3
Q

What cells are found in the crypt of small intestine?

A
  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
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4
Q

Small intestine surface area is increased by…

A

Microscopic folds of Kerckring- plicae circulares Villi Microvilli

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5
Q

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

A

Postprandial

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6
Q

What is transcellular movement?

A

Solute cross two cell membranes in series. Active transport implicated

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7
Q

What is paracellular movement?

A

Solute moves passively between adjacent epithelial cells via tight junctions

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8
Q

What does the absorption of water depends upon?

A

Absorption of Na+ and Cl-

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9
Q

How does water absorption occur?

A

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

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10
Q

What happens to Na+ in the GI tract?

A

Active absorption in jejunum, ileum and colon

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11
Q

What happens to K+ in the GI tract

A

Passive absorption in jejunum and ileum

Net secretion in colon

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12
Q

What happens to Cl- in the GI tract?

A

Absorption in jejunum, ileum and colon

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13
Q

What happens to HCO3- in the GI tract?

A

Absorption in jejunum, secretion in ileum and colon

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14
Q

What are the four routes of entry for Na+?

A
  1. Na/gucose transport, Na/amino acid transport
  2. Na-H exchanger
  3. Parallel Na-H and Cl- HCO3 exchange
  4. Epithelial Na+ chanel
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15
Q

Which transporter is involved in Na/glucose transport?

A

SLT-1

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16
Q

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

A

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

17
Q

Explain the first route of entry for Na+

A
  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

18
Q

Explain the second route of entry for Na+

A
  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

19
Q

Why does luminal pH increase?

A

luminal HCO3- secretion by pancreatic, biliary and duodenal issues

20
Q

Explain the third route of entry for Na+

A

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

21
Q

What would reduce NaCl absorption?

A

Increase in:

  1. cAMP- enterotoxins elevate cAMP
  2. cGMP
  3. intracellular Ca2+
22
Q

Explain the fouth route of extry for Na+

A

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

Mainly in distal colon

23
Q

How does mineralcorticoids increase Na+ absorption?

A
  1. increase opening of apical Na+ channels
  2. insertion of channel from vesicle
  3. increased synthesis of apical pumps
24
Q

Describe Cl- transport

What are the 3 models proposed?

A

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
25
Q

Cl- secretion

A
  • in small and large bowel crypt cells
  • mediated by cAMP and Ca2+
26
Q

Explain the first model for Cl- absorption

A

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
27
Q

Explain the second model for Cl- absorption

A

Parallel Na-H and Cl-HCO3 exchange

electroneutral NaCl absorption also mediates Cl- absorption in the ileum and proximal part of the colon

28
Q

Explain the third model for Cl- absorption

A

Cl-HCO3 exchange

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

29
Q

What is congenital chloridorrhea?

A
  • 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
30
Q

What are the 4 categories of secretagogues?

A
  1. bacterial enterotoxins
  2. hormones and neurotransmitters
  3. products of the immune system
  4. laxatives
31
Q

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

A

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

32
Q

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

A
  • 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
33
Q

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

A

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

34
Q

What is Verner-Morrison syndrome?

A

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

35
Q

Describe how calcium is absorbed

A

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

DIG (22 decks)

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