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Medsci 205 > Lecture 3: Water and electrolyte absorption and secretion in the GI tract > Flashcards

Lecture 3: Water and electrolyte absorption and secretion in the GI tract Flashcards

1
Q

What is the main purpose of the small intestine? how is this achieved?

A

absorption, increase surface area
- microvilli and crypts of lieberkuhn
- folds of kerching
- submicroscopic microvilli

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2
Q

What is the main purpose of the large intestine? What anatomical features are present

A

reasbsorption of water
- semilunar folds
- crypts but no vili
- microvilli

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3
Q

How much fluid is reabsorbed in the small intestestine vs large intestine?

A

6.5L/day
2L/day

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4
Q

What is the main difference in asbirption and secretion between the SI and LI?

A

SI - non-electrolyte nutrients asborbed
SI and LI - water and electrolytes absorbed

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5
Q

In detail, what electrolytes are absorbed/secreted in the SI and LI respectively

A

SI
- absorbs water, Na, Cl, K
- secretes HCO3

LI
- absorbs water, Na, Cl
- secretes K, HCO3

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6
Q

What is the main function of ATPase

A

make sure that the Intracellular environement is deficient in sodium

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7
Q

explain the difference between transcellular and paracellular movment of solutes/water

A

para - movement passive via tight junctions, diffusion
trans - cross apical and basolateral membrane in series, for solutes at least one membrane is active

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8
Q

Explain the process of absorption of water

A
  • entriely by osmosis, coupled to solute movement
  • transcellular or paracellular (paracellular predominant)
  • Primarily in the jejunum
  • solvent drag (bulk movement) responsible for considerable Na and urea absorption in jejunum
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9
Q

Where does sodium absorption occur in the SI and LI respectively?

A

villus epithelial cells in SI
surface epithelial cells of LI

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10
Q

What mechanisms ensures Na can enter the cell. Explain

A

Na-K ATP ase on basolateral membrane, maintains low intracellular Na concentration, provides electrical gradient/force for Na movement from lumen across apical membrane via Na/glucose cotransporter

Na/K ATPase - 3 Na out, 2 K in
symporte - 2 Na, 1 glucose

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11
Q

What four ways can sodium be absorbed in the body?

A
  1. Na/glucose or Na/amino acid cotransporters
  2. Na-H exchanger
  3. Parallel Na-H and Cl-HCO3 exchangers
  4. Epithelial Na channel
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12
Q

Explain the process of sodium absorption via Na/glucose or Na/amino acid cotransporters

A
  1. Na/K ATPase pump
  2. SGLT1 –> Na + glucose, apical membrane
  3. Na + amino acid, apical membrane
  4. GLUT 2 –> glucose to ECF, basolateral membrane
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13
Q

Explain the process of sodium absorption via the Na-H exchanger

A
  1. Na/K ATPase pump
  2. NHE1 –> H to ICF from ECF, Na to ICF, baso
  3. NHE3 –> Na to ICF + H out to lumen, apical
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14
Q

Explain the process of sodium absorption via the Parallel Na-H and Cl-HCO3 exchangers

same process used for chloride absorption

A
  1. Na/K ATPase
  2. NHE3 –> Na lumen to ICF, H ICF to lumen
  3. Cl-HCO3 –> Cl lumen to ICF, HCO3 ICF to lumen
  4. ClC-2 –> Cl ICF to ECF
    –> HCO3 by CO2/H2O mechansim in cell
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15
Q

Explain the process of sodium absorption via the Epithelial Na channel

A
  1. Na/K ATPase
  2. Na channel inserts when high aldosterone level in distal colon (sodium reabsorption when deficient)
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16
Q

What are the three mechanisms of Cl absorption by the intestine?

A
  1. voltage dependent (passive diffusion)
  2. Cl-HCO3 exchanger
  3. parallel Na-H and Cl-HCO3 exchangers
17
Q

Explain the process of chloride absorption via passive diffusion

A

paracellular or transcellular
- paracellular: driven by lumen-negative transepithelial voltage

18
Q

Explain the process of chloride absorption via the Cl-HCO3 exchanger

A
  1. Na/K ATPase
  2. DRA apical membrane (exchanger)
  3. conversion of CO2 (from bloodstream) to carbonate in the cell, carbonate goes to lumen via DRA and Cl into the cell via DRA
  4. CIC-2 Cl in cell then goes to ECF

chloride absorption and secretion

19
Q

Explain the process of chloride absorption via parallen Na-H and Cl-HCO3 exchangers

A

responsible for electroneutral NaCl absorption and can mediate Cl abosrption inbetween meals
1. NHE3 and DRA apical membrane
2. NA/K ATPase and CIC-2 basolateral membrane
3. co2 and h20 - carbonic anhydrase makes carbonate in cell

20
Q

Explain the difference between absorption and secretion of potassium in the gut

A
  • passive absorption and secretion via paracellular pathway
  • active secretion by potassium channled as NKCC1 (basolateral cotransporter)
  • active asborption via K/H ATPase
21
Q

Explain how chloride secretion works in the gut

A
  1. sodium-potassium ATPase creates as sodium deficient environement within the cell
  2. potassium leakes out of the cell to ECF via channels
  3. NKCC1 allows sodium, potassium and chlordide to enter the cell from ECF
  4. chloride is then secreted via CFTR in the apical membrane
  5. **Note: CFTR is only inserted when cAMP levels are high, overstimulation of cAMP pathway can cause diarhhea **
22
Q

Explain what happens when cAMP is overstimulated and the effect that has on chloride secretion

A

cAMP overstimulated –> increased CFTR insertion –> increased chloride secretion (out of body) –> sodium moves paracellualrly to maintain charge –> water moves with it –> diarhhea

23
Q

How can analysis of diarhhoea give us indication of gut problems?

A
  • voluminous, lots of H20 lost –> small intestine
  • small volule –> large intestine
24
Q

describe the difference between osmotic and secretory diarhhoea

A

osmotic - results from disturbances of absorption (unabsorbed substance draws water from plasma into intestinal lumen along osmotic gradient)
secretory - results from disturbances in secretion (disordered elecrtolyte transport, CFTR, H20 lost)

25
Q

What generally causes osmotic diarhhoea and give examples of conditions that can lead to this

A

macronutrient malabsirption retaining osmotic pressure in the lumen and thus retaining water “osmotic load”

lactose intolerance, coeliac disease

26
Q

What causes secetory diarhhoea and what mechanism underlies it?

A
  • increase in active secretion via CFTR
  • generally caused by cholrea or E.coli
  • enterotoxins produced by the microorganism raie intracellular cAMP, cGMP or Ca2+
  • this leads to stimulaton of anion secretion, specifically Cl –> process after this is explained previously (na, h2o follows etc)
27
Q

Explain some physiological changes that occur in respone to secretory diarhhoea

A
  • blood volume drops as water follows Na out of body
  • blood pressure drops as baroreceptors not stretched
  • BP is Co x tpr –> co needs to go up to maintian bp, so Hr up
28
Q

Explain how cholera works

A

permanently activates adenylate cyclase thereby causing an elevation in cAMP which opens CFTR cahnnels - prolonges cl, na and water secretion can be fatal

Medsci 205 (5 decks)

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