Salt + Water Balance in the Intestines

Question 1

What are crypts of the intestine and what are their function?

Answer 1

Deep holes extending into epithelium of the colon.
Act as capillaries to suck water out of faeces.
Water absorption in crypts causes capillary suction action.

Question 2

Where does the majority of fluid absorption occur in the GI tract?

Answer 2

In the small intestines.(6700ml/day)

Large intestine only absorbs 1400ml/day.

Question 3

What are characteristics of stool?

Answer 3

65-85% Water - >85%=diarrhoea.

pH 5-6

Question 4

How is water absorption powered in the intestines?

Answer 4

Water absorption follow Na+ and other solute absorption by osmosis.

• Paracellular transport = via tight junction between cells
• Transcellular transport = across cell membranes via aquaporins.

Question 5

How is Na+ transported from gut lumen into enterocyte complex?

Answer 5

• counter transport in exchange for H+
• co-transport with amino-acids/monosaccharides
• co-transport with Cl-

Question 6

Name some colonic disorders.

Answer 6

Diarrhoea - congenital, bacterial infection, osmotic
Constipation - slow movement of faeces
Irritable bowel syndrome.

Question 7

What causes congenital diarrhoea?

Answer 7

Deficiency of normal ion transport.

• Mutation in chloride/bicarbonate exchange in ileum/colon.
• Cannot transport water into intercellular space
• Impairs Na+ transport

Question 8

How does cholera toxin cause diarrhoea?

Answer 8

Permanently activates adenylyl cyclase - elevates cAMP in crypt cells. Enhances Cl- secretion.
Opens CFTR channels - Cl- flows out of cell rather than into cell.

Question 9

What are the roles of intestinal flora?

Answer 9

• Synthesise and excrete vitamins e.g. vitamin K
• Prevent colonisation by pathogens by competing for essential nutrients.
• Antagonise other bacteria through production of substances which inhibit or kill non-indigenous species.
• Stimulate development of certain tissues.