Water + Vitamins

Question 1

Site of water absorption in the body

Answer 1

In small intestine
- majority of water absorbed

In large intestine
- absorbs some water & form stool

Question 2

Process of water secretion into small intestine

Answer 2

Presence of digested nutrients & electrolytes in the intestinal lumen (↓water potential)

Water moves from higher water potential to lower water potential either:
- transcellularly, through aquaporins or
- paracellularly, through tight junctions between enterocytes

Question 3

Role of electrolyte in water secretion process into the small intestine

Answer 3

• Main electrolyte: chloride ions

Cl⁻ actively released in the intestinal lumen by crypt cells.

Electrical gradient created.

Na⁺ ions attracted into intestinal lumen paracellularly.

• increased conc. of electrolytes (Cl⁻ and Na⁺) in intestinal lumen ↓water potential

Question 4

How are Cl⁻ ions transported out of crypt cells into intestinal lumen?

Answer 4

Through CFTR channels, found on the apical membrane

Question 5

How do Cl⁻ ions enter crypt cells?

Answer 5

From basolateral side (blood side)

Via Na⁺-K⁺-2Cl⁻ cotransporter

Question 6

What happens when apical CFTR is defective?

Answer 6

impaired Cl⁻ secretion, and hence impaired water secretion

leads to:
- meconium ileus in newborns (especially in cystic fibrosis case)

Question 7

Regulation of water secretion process

Answer 7

Neural: Enteric nervous system
- neurotransmitters: acetylcholine, vasoactive intestinal peptide (VIP)

Hormonal: secretin, VIP

Local mediators: prostaglandins, serotonin

Pathological: bacterial toxins

Question 8

Primary site of water & electrolyte secretion in the small intestine

Answer 8

Crypts of Lieberkuhn

Question 9

Effect of vasoactive intestinal peptide (VIP) on secretion

Answer 9

Stimulates Cl⁻ and water secretion

• VIP binds to its receptor on crypt cell.
• ATP converted to cAMP
• ↑cAMP activates signaling pathway
• CFTR activated
• Cl⁻ secretion

Question 10

Consequence of excessive water secretion into large intestine

Answer 10

Secretory diarrhoea

Question 11

Pathological cause of excessive water secretion into large intestine

Answer 11

Bacterial toxins, such as Vibrio cholerae & certain E.coli

Question 12

What is Verner-Morrison syndrome?

Answer 12

Rare condition caused by a tumour that overproduces vasoactive intestinal peptide (VIP)

• leads to severe watery diarrhoea, hypokalaemia, dehydration

Question 13

Process of water absorption in small intestine

Answer 13

water follows Na+ via osmotic gradient
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