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
Pancreas:
Secretes digestive enzymes from acinar cells into duodenum
And hormone secreting cells in chyme stimulate pancreatic duct cells to secrete bicarbonate and water = neutralise stomach acidic chyme so that digestion enzymes will work more effectively
endocrine part of pancreas:
composed of islets of Langerhans
Islets produce hormones such as insulin for adsorption, storage and metabolism
Dysfunction of endocrine pancreas causes diabetes mellitus
exocrine part of pancreas:
is composed of acini
acini secrete pancreatic juice into the duodenum through pancreatic ducts
Pancreatic juice contains enzymes important for digestion of food
Disorders of the exocrine pancreas result in pancreatitis
The Sphincter of Oddi:
- Place where pancreatic juice and bile enters the duodenum at the ampulla of Vater
- The sphincter is smooth muscle that can be contracted to seal the junction or relaxed to allow flow of juices and bile to into the duodenum to aid digestion
- Block at the sphincter of Oddi and pancreatic ducts causes pancreatitis
Zymogen granules function as an intracellular protective mechanism:
- Inactive secretory enzymes called zymogens
- Enzymes are stored in secretory granules:- compartmentalisation
- Granules contain a protease inhibitor called serine protease inhibitor Kazal type 1 (SPINK1) which can inhibit trypisin
- Non-digestive proteases in the zymogen granule can inactivate trypsin
- Zymogen granule pH is below the optimum for trypsin and lipase activity
caliculi:
Caliculi are stones made of protein and calcium carbonate precipitate
block small ducts
Lithostathine prevents caliculi formation
20% of fluid secreted by pancreas…
NaCl and digestive enzymes are secreted from acinar cells
80% of fluid secreted by pancreas…
duct cells secrete H20, NaCl and HCO3-
acute pancreatitis
failure to drain due to blockage and hyper-secretion
caused by acute inflammation of the pancreas
can be caused by:
- alcohol
- gall stones
- trauma
- tumor
- hereditary
- infection e.g. round worm
- autoimmune
- scorpion sting
symptoms:
- abdominal pin
- nausea
- fever
- coagulation
- shock
