gastrointestinal physiology Flashcards
function of the GI system:
digestion
motility
absorption
secretion
defence
excretion
stomach:
- secretion of acids, pepsinogen, intrinsic factor, mucus, bicarbonate and water
- Motility = churning to reduce particle size and to mix the secretions with chyme
- Beginning of digestion (pepsins and acid)
- Defence (acid)
intestine (with help from accessory organs - liver, gall bladder, pancreas)
absorbs nutrients and forms a barrier to luminal contents…
Function = digestion absorption of nutrition and ions secretion of fluid and HCO3- defence (flush out bacterial toxins)
water movement in the gut:
water transport is driven by osmotic and hydrostatic pressure
1) paracellular route in between cells through leaky tight junctions
2) transcellular route through aquaporins
3) isotonic water movement
1) paracellular water movement is…
in between cells
through tight junctions
according to the osmotic gradient
can occur in either direction
into the lumen if chyme is hypertonic
2) transcellular water movement is…
- driven by the osmotic gradient
- through aquaporins in the plasma membrane
3) isotonic water movement:
- When chyme becomes isotonic with plasma there is no osmotic gradient to drive water absorption
- the ileum isotonic water absorption occurs
- Isotonic water movement is based on creating a micro-hypertonic environment so that water continues to move out of the gut lumen
Absorption of water…
depends on absorption of nutrients through secondary active transport
Nutrient absorption…
by secondary active transport
sodium/potassium pump:
3 Na+ out
2 K+ in
osmotic diarrhoea / steatorrhea:
- non-digested carbohydrates and lipids can not be absorbed
- so the chyme maintains a high osmolarity compared to the plasma
- Water moves into the gut lumen down the osmotic gradient
Causes =
- pancreatic disease
- lactose intolerance
lactose intolerance:
Unabsorbed lactose causes an osmotic load in the gut so chyme retains water in the lumen which causes abdominal pain, diarrhoea and nausea
Unabsorbed lactose ferments in the colon exacerbating the symptoms
test for osmotic diarrhoea:
Hydrogen breath test:- unabsorbed lactose enters the colon where it is fermented by bacteria with release of hydrogen which is given off in the breath
secretory diarrhoea:
example = cholera
too much secretion
solution = promoting water absorption
secretion:
Water movement is osmotic
The movement of salt ions, Na+ through tight junctions and Cl- through cystic fibrosis transmembrane regulator (CFTR) controls water secretion
CFTR operation:
Stage 1 - Channel closed
Stage 2 - cAMP activates PKA, which then phosphorylates the regulatory domain of CFTR
Stage 3 - ATP is able to bind to nucleotide binding domains
Stage 4 - conformational change opens the pore of the channel allowing Cl- ions to leave cell
Stage 5 - ATP is hydrolysed to ADP causing channel to close.
Cholera toxin mechanism of action:
Gαs activated by cholera toxin which blocks GTPase activity thus preventing inactivation
Persistent activation of adenylyl cyclase leads to high levels of cAMP
Phosphorylation of CFTR by PKA = Increased flow of Cl- ions
Water follows the electrolyte movement
