Water and balance in the GI tract Flashcards Preview

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Flashcards in Water and balance in the GI tract Deck (30)
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1
Q

How is water absorbed in the GI tract?

A

It is a passive process driven by the transport of solutes (particularly Na+) from the lumen of the intestine to the bloodstrea

2
Q

How many water enters and is absorbed by the GI tract every day?

A

9.3 liters enter the tract
8.3 litres are absorbed by the small intestie
1 litre enters large intestine of which 90% is absrobed

3
Q

What makes up faeces?

A

100ml water

50ml cellulose, billirubin and bacteria

4
Q

What is diarrhoea defined as?

A

Loss of lfuid and solutes from the GI tract in excess of 500ml per day

5
Q

What transporters are present throughout the small intestine?

A

Na+/glucose co-transport

Na+/ amino acid co-transport

6
Q

What transporters are presentin the duodenum and jejunum?

A

Na+/ H+ exchange

Stimulated by luminal HCO3-

7
Q

What transporters are present in the ileum and colon?

A

Parallel Na+/H+ and Cl-/HCO3- exchange

8
Q

What transporters are present in the colon?

A

Epithelial Na+ channels

Regualted by aldosterone

9
Q

What are the major mechanisms of postprandial Na+ in the jejunum?

A

Na+/glucose

Na+/ amino acid cotransporter

10
Q

What type of transport are the major co-transporters of postprandial Na+ absorption?

A

Seconadry active transport and are electrogenic, as is the Na+/K+ ATPase, the overall tranport of Na+ geneerates a transepithelial potential in which the lumen in negative - driving parallel absorption of Cl-

11
Q

What stimulates echange at the apical membrane of the jejunum (Na+/H+ exchnage) ?

A

Stimualted by the alkaline environemnt of the lumen (high pH = low proton conc) ue to the presence of bicarbonate from the pancreas

12
Q

What is the priamry mechanism of Na+ absorption in the interdigestie period?

A

Na+/H+ and Cl-/HCPO3- parallel exchange

13
Q

How are the Na+/H+ and Cl-/HCO3- exchangers regulated?

A

Intraellular cAMP, cGMP and Ca2+ all reducing NaCl absorption

14
Q

What does a reduction in NaCl absorption cause?

A

Diarrhoea (secretory diarrhoea due to infection with E.coli - heat stable enterotoxin from which activated adenylate cyclase and increases intracellular cAMP)

15
Q

What do epithelial Na+ channels mediate?

A

Electogenic Na+ absorption in the distal colon, highly efficient and important in Na+ conservation
Increased by aldosterone

16
Q

What are the actions of aldosterone?

A

Opens ENaC
Inserts more ENaC into membrane from intracellular vesicle pool
Increases synthesis of ENaC and Na+/K+ ATPase

17
Q

What are the cellular mechanisms of Cl- absorption?

A

Can occur passively via transcellular or paracellular routes
Driving force in the small intestine is provided by lumen negative potential due to electrogenic transport of Na+
Large intestine driving forve provided by negative potential due to electrogeenic movement of Na+ thorugh ENaC

18
Q

Where is Cl- secreted from?

A

Crypt cells rather than villus cells, important in many diarrhoeas

19
Q

When is the CFTR activated?

A

Bacterial entereotoxins (cholera toxin, enterotoxin, c diff toxin)
Hormones and neurotransmitters (VIP, guanylin, ACh, bradykinin, 5-HT)
Immune cells products (prostaglandins, histamine)
Laxatives (bile acids)

20
Q

What secondary messengers can activated CFTR?

A

cAMP (cholera toxin, VIP, histamine)
cGMP (enterotoxin, guanlylin)
Ca2+ (ACh, bradykinin, 5-HT)

21
Q

What does Cl- conductance mediated by CFTR result in?

A

Opening of channels at the apical membrane
Insertion of channels from intracellular vesicale sinto the membrane
Secretory diarrhoea

22
Q

What can cause diarrhoea?

A
Infectious agents - viruses, bacteria
Chronic disease 
Toxins
Drugs
Psychological factors
23
Q

What can diarrhoea result in?

A

Can involve small or large intestine

Result in dehydration (Na+ and h2o loss), metabolic acidosis, hypolakaemia

24
Q

How can severe diarrhoea be treated?

A

Maintenance of fluid and electrolye balance
Use of anti-infective agents
Use of non-antimicrobial antidiarrhoeal agents

25
Q

How does cholera cause diarrhoea?

A
Cholera toxin enters enterocyte
Enxymatically inhibits GTPase 
Increases activity of adenylate cyclase
Increases the conc of caMP 
caMP stimualtes CFR
Hypersectrion of Cl- with Na+ and water following
26
Q

What congenitcal defects can lead to diarrhoea?

A

Congenital chloridorrhoea - absence of Cl=/HCO3- exchanger)

27
Q

How does rehydration therapy work?

A

2Na+ bind
Affinity for glucose increases, glucose binds
Na+ and gluocse translocate from extracellular to intracellular
2 Na+ dissociate, affinity for glucose falls
Glucose dissociates
Cycle is repeated

28
Q

What do oral rehydration salts contain?

A
Gluocse
NaCl 
Sodium bicarbonate
Potassium chloride
Absoprtion of sodium and glucose by SGLT1 causes the accompanying absorpton of water
29
Q

What are the actions of opioids on the alimentary tract?

A

Inhibition of enteric neurones (hyperpolarization via activation of opioid receptors)
Decreased peristalsis , increased segmentation
Increased fluid absorption
Constriction of pyloric, ileocaecal and anal sphincters
Increased tone of large intestine

30
Q

What are major opioid agonists used in diarrhoea?

A

Coedine
Diphenoxylate
Loperamide