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Digestive > 28 - Water & electrolyte absorption and secretion in the GI tract > Flashcards

28 - Water & electrolyte absorption and secretion in the GI tract Flashcards

1
Q

Enteric nervous system

A

Myenteric and submucosal plexi

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2
Q

SI surface is amplified at 3 levels by

A

plicae circularis
villi + crypts of lieberkuhn
microvilli

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3
Q

LI surface is amplified at 3 levels by

A

Semi lunar folds (and haustra)
Crypts of Lieberkuhn (NO villi)
Microvilli

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4
Q

LI vs SI pathologies?

A

SI - voluminous/large volume

LI - less fluid but more frequent fluid loss

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5
Q

Does active Na absorprtion occur in the LI and SI?

A

Yes in both

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6
Q

Does active K+ secretion occur in SI and LI?

A

NOT in SI only in LI

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7
Q

Does nutrient absorption occur in SI and LI?

A

SI yes LI no - the absorption of non-electrolyte nutrients occurs in the SI while BOTH the SI and LI absorb water and electrolytes (na/Cl)

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8
Q

What is the net absorption/secretion in the SI/LI

A
  • SI absorbs net amounts of water, Na, Cl and K and secretes HCO3-
  • the LI absorbs net amounts of water, Na, CL and secretes K and HCO3-
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9
Q

Net amounts of secreted and absorbed fluid?

A 
Saliva - 1.5 L
Gastric secretion - 2 L
Bile - 0.6 L
Abs SI - 6.5 L
Abs LI - 1.9 L
Faecal fluid - 0.1 L
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10
Q

Na/K ATPases

A
  • basis of membrane transport and membrane potential
  • ACTIVELY remove 3 Na+ and bring in 2 K+
  • use ATP
  • means there is always a deficiency in cell
  • conc and electrical gradient
  • gradient for action and membrane potentials, transport of AA and glucose
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11
Q

Describe the transport of glucose in the cell?

A
  • Na/K atpase (apical)
  • SGLT
  • GLUT (basolateral)
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12
Q

Block Na/K ATPases?

A
  • diminish conc gradiet
  • no glucose move into cells
  • cells starve
  • membrane potential affected as normally driven by sodium (intracellular is negative - electrical gradient)
  • electrical gradient (membrane potential) lost if ATPases lost
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13
Q

Transepthelia movement of water is either

A

Paracellular or transcellular

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14
Q

Transcellular

A

Across 2 membranes in series
At least 1 membrane is active
The other is usually facillitated by solutes like glucose or AAs i.e. the active membrane creates an osmotic gradient for water to follow to osmotic movement

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15
Q

paracellular

A

Moves passively between cells via tight junctions

Paracellular movement of water predominates

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16
Q

Absorption of water

A
  • Is entirely by osmosis

- Often coupled with solute movement

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17
Q

Majority of water absorption occurs in the

A

Jejunum of the SI

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18
Q

How does water help sodium and urea absorption in the jejunum

A

Solvent drag allows for considerable uptake of urea and sodium at the jejunum

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19
Q

Where does sodium absorption occur

A

The SI villus epithelial cells and surface epithelial cells of the LI

20
Q

How is sodium absorbed

A

The transcellular absorption of sodium is mediated by the Na/K atpase at the BL surface which maintains a low intracellular conc
This provides the gradient/energy for Na movement by diffusion from the lumen into the cell across the apical surface and then into the ECF (by ATPase)
The apical movement of Na is mediated by Na coupled transporters OR Na channels

21
Q

How does sodium move across the apical surface

A

either by cotransporters or sodium channels including..

  1. Na/glucose co transporters
  2. Na/H EXCHANGERS (H+ in duo to counteract bicarbonate)
  3. Na/H AND Cl-/HCO3- exchangers (Na and Cl in cell, HCO3- and H+ out of the cell)
  4. Epithelial Na channels
22
Q

Where are you going to find Na epithelial channels

A

They are very specific and found in the distal colon

Sparse and only contribute to 5% of sodium absorption

23
Q

How is most sodium absorbed across the apical membrane

A

Glucose co transporter

24
Q

What CONTROLS sodium absorption etc

A

Sodium needs to be controlled as it is important in maintaining osmolarity, structure of the cell and enable solutes to move in/out of the cell
Controlled by aldosterone

25
Q

Cl- absorption

A

Tends to move with Na+ to neutralise

  1. Passive absorption/electrical gradient with Na+ (secondary active sodium movement sodium across apical membrane causes intracellular to be positive so CL follows to maintain membrane potential)
  2. Cl/HCO3- exchanger
  3. Na/H and HCO3-/Cl parallel exchangers
26
Q

Where is chloride absorbed

A

jejunum, ileum and proximal colon

27
Q

Is chloride secreted and absorbed passively or actively

A

secreted ACTIVELY

absorbed passively with sodium

28
Q

How is chloride secreted

A

BL : na/k ATPase powers
Na/K/2Cl- CO transporter to bring Na, K, Cl into cell
Apical :CFTR facilitates passive diffusion of cl out of the cell into the lumen
The movement of Cl- into the lumen, osmotically drags Na and H20 through paracellular movement FROM the ECF

(movement of Cl causes an osmotic drag - can see how an increase in Cl secretion from CTFR would cause increased Na H20 paracellular secretion from the ECF causing secretory diarrhoea)

29
Q

Is sodium absorbed or secreted

A

ABSORBED in both si and li

30
Q

What can cause excessive cl secretion

A
  • cholera toxin causes excessive cAMP to bind to and stimulate CTFR (like secretion normally does)
  • cAMP
  • Ca
31
Q

How does potassium move in the gut

A
  • absorbed in SI
  • Secretion in LI
    Passive movement
32
Q

What 3 systems control absorption and secretion in the gut

A
  1. Enteric nervous system
  2. Endocrine system
  3. Paracrine system
33
Q

How does the Enteric nervous system, Endocrine system and Paracrine system control abs/secretion

A
  1. enteric releases ACh, VIP and other secratogogues
  2. endocrine - aldosterone
  3. paracrine - 5HT
34
Q

What does the enteric system release

A

Ach
VIP
Other secretagogues

35
Q

Solvent Drag?

A

Where the solute may be moved by coupled to fluid movement as it is swept away by the bulk movement of the solute

36
Q

Anything that causes an increase in secretion or decrease in absorption can cause

A

diarrhoea

37
Q

Def of diarrhoea

A

more than 200g/0.2L or more than 2 liquid bowel movements a day

38
Q

Diarrhoea is the .. most common cause of infant death worldwide

A

2nd

39
Q

Difference between osmotic and secretory

A

osmotic - Is a disturbance of absorption
- often due to enzymes malfunction so increase in osmotic particles/load

secretory - is a disturbance of secretion

  • increased channel activity and fluid loss from the body
  • often due to infection or tumour
40
Q

Osmotic Diarrhoea

A

Macronutrient malabsorption causes an increase in osmotic load in lumen cause water to be retained

  • Can occur is pancreatic disease/enzyme insufficiency, large intake of sugar or alcohol, intolerances, celiac
  • lactose intolerance (more than 5% of the world more common in asia and africa)
41
Q

Secretory Diarrhoea

A
  • an increase in active secretion
42
Q

How does cholera/E.Coli cause secretory diarrhoea

A
  • cholera/e coli most common causes
  • release an enterotoxin that causes an increase in cAMP/cGMP/Ca++
  • stimulation of CFTR
  • Cl secretion and paracellular H20 and Na movement to maintain electrical and osmolarity balance
43
Q

Besides cholera what else can cause secretory diarrhoea

A

Can get an absence of the CL/hco3 exchanger

44
Q

How does secretory diarrhoea impact Na absorption

A

No impact on nutrient absorption or Na coupled co exchangers
Basis for ORT
Treat with gluc/na/h20 to utilise these co transporters to get water back into the ECF

45
Q

What kind of fluid loss does secretory diarrhoea cause

A

ISO-OSMOTIC fluid loss

  • essentially tap from ECF as losing both electrolytes and water some osmolarity
  • this fluid is coming from the blood resulting in patients quickly becoming dehydrated
46
Q

What does secretory diarrhoea result in

A
  • loss of ECF/blood plasma
  • loss of blood volume (and venous return so can lead to shock)
  • decrease in blood pressure
  • increase in HR as compensated for by baroreceptors and increase in sympathetic stimulation
47
Q

What does ORT consist of any why

A
  • water (rehydrate fluid loss)
  • na (enable uptake via co transport gradients to restore Na and glucose loss)
  • glucose
  • HCO3 - losing fluid/ECF so losing the HCO3 that is usually secreted. This means the pH drops so need to restore HCO3- to prevent acidosis

Digestive (24 decks)

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