Water and balance in the GI tract

Question 1

How is water absorbed in the GI tract?

Answer 1

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

Question 2

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

Answer 2

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

Question 3

What makes up faeces?

Answer 3

100ml water

50ml cellulose, billirubin and bacteria

Question 4

What is diarrhoea defined as?

Answer 4

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

Question 5

What transporters are present throughout the small intestine?

Answer 5

Na+/glucose co-transport

Na+/ amino acid co-transport

Question 6

What transporters are presentin the duodenum and jejunum?

Answer 6

Na+/ H+ exchange

Stimulated by luminal HCO3-

Question 7

What transporters are present in the ileum and colon?

Answer 7

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

Question 8

What transporters are present in the colon?

Answer 8

Epithelial Na+ channels

Regualted by aldosterone

Question 9

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

Answer 9

Na+/glucose

Na+/ amino acid cotransporter

Question 10

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

Answer 10

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-

Question 11

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

Answer 11

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

Question 12

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

Answer 12

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

Question 13

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

Answer 13

Intraellular cAMP, cGMP and Ca2+ all reducing NaCl absorption

Question 14

What does a reduction in NaCl absorption cause?

Answer 14

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

Question 15

What do epithelial Na+ channels mediate?

Answer 15

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

Question 16

What are the actions of aldosterone?

Answer 16

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

Question 17

What are the cellular mechanisms of Cl- absorption?

Answer 17

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

Question 18

Where is Cl- secreted from?

Answer 18

Crypt cells rather than villus cells, important in many diarrhoeas

Question 19

When is the CFTR activated?

Answer 19

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)

Question 20

What secondary messengers can activated CFTR?

Answer 20

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

Question 21

What does Cl- conductance mediated by CFTR result in?

Answer 21

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

Question 22

What can cause diarrhoea?

Answer 22

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

Question 23

What can diarrhoea result in?

Answer 23

Can involve small or large intestine

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

Question 24

How can severe diarrhoea be treated?

Answer 24

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

Question 25

How does cholera cause diarrhoea?

Answer 25

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

Question 26

What congenitcal defects can lead to diarrhoea?

Answer 26

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

Question 27

How does rehydration therapy work?

Answer 27

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

Question 28

What do oral rehydration salts contain?

Answer 28

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

Question 29

What are the actions of opioids on the alimentary tract?

Answer 29

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

Question 30

What are major opioid agonists used in diarrhoea?

Answer 30

Coedine
Diphenoxylate
Loperamide