Diarrhoea Flashcards
How long do villus tips live for?
Villus tips - cells die and shed into lumen after 3-5 days.
How many times do villi and microvilli increase SA for absorption?
Villi = 10 fold
Microvilli = 20 fold
(Total of 200 fold)
What is the function of the villous tips?
(With regards to fluid dynamics in the normal SI)
Mature enterocytes absorb sodium, glucose and amino acids across the brush border on the apical surface. odium is pumped out of the cell on the basolateral aspect. This results in net absorption of fluid.
What is the function of the crypt base with regards to fluid dynamics in the normal SI?
Stem cells (secreting cells with leaky intercellular junctions) allow sodium to leak back into the intestinal lumen. This results in net secretion of fluid.
How does the net absorption at villus tips compare to secretion in crypts in normal animals vs animals with diarrhoea.
Normal animals = Net absorption at villus tips > net secretion in crypts
In diarrhoea = Decreased absorption and/ or increased secretion.
What is diarrhoea?
Increased volume and fluid content of faeces.
What are the 4 basic mechanisms for pathogenesis of diarrhoea?
1) Altered epithelial cell transport
2) Altered structural or permeability of mucosa
3) Osmotic effects
4) Altered motility
What is the role of luminal and basolateral membrane transporters and intracellular signalling mechanisms?
All are involved in intestinal fluid absorption and secretion by enterocytes.
The electrochemical driving force for this process is the basolateral Na+/K+ -ATPase.
How do bacteria alter epithelia cell transport and cause diarrhoea?
1) Some types of bacteria attach to villous enterocytes and produce enterocytes that block the activity of the sodium pump.
2) Blocking of the sodium pump results in net loss of sodium into the intestinal lumen
3) Water follows the sodium resulting in massive fluid loss into the lumen of intestine
4) Produces a profuse, watery diarrhoea
5) The condition is self-limiting as the mature villous epithelial cells are shed at the normal turnover rate after 3-5 days.
What is the role of enterocytes in digestion?
Enterocytes play a key role in the absorption and terminal digestion of nutrients and this function is reliant on healthy enterocytes.
How do bacterial infections alter the structure or permeability of mucosa?
- Some bacteria produce toxins (cytotoxins) that damage and kill cells.
- Death of enterocytes allows blood ,fluid and other contents of the villus to leak into the intestinal lumen.
- The villus will no longer be absorptive as mature enterocytes have been lost.
- There is clearly altered structure and altered permeability with loss of normal function.
What is the term for the appearance of blood and shreds of intestinal tissue in the diarrhoea?
Dysentery
How do cellular infiltrates alter structure or permeability of mucosa?
A massive increase in the cellularity of the villus core will cause the width of the villus to increase and the height of the villus to decrease - there aren’t enough epithelial cells to cover a tall fat villus.
What happens if epithelial cells stretch to cover the altered villus?
They flatten and lose their brush borders. Loss of surface area and enzymes of final stage of digestion.
This results in lack of absorption of nutrients and a loss of normal function.
What is osmotic diarrhoea?
The lack of specific brush border enzymes results in accumulation of small osmotic solutes in the intestinal lumen, drawing water from the bloodstream into the intestines.
