gastric acid secretion - regulation Flashcards
what are the contents of normal gastric juice in the fasting state
cations: na, k, mg, h
anions: cl, hpo4, so4
pepsins, lipase
mucus
intrinsic factor
normal pH of fasting gastric juice
about 3.0
what are secreted by the fundus and body of the stomach
mucus
HCl
pepsinogen
ALL EXOCRINE
what are secreted by the antrum
dec HCl
inc gastrin
what are the walls of tubular glands in epithelial cells lined with
parietal cells
what do parietal cells secrete
HCl and intrinsic factor
What do enterochromaffin-like cells (ECL) secrete
paracrine agents eg histamine
how is gastric acid made in the stomach lumen
bicarb exhanged for Cl- in the blood - acidity of venous blood from stomach is dec c.f. blood serving it.
excess Cl- diffused into stomach through cl channels; H+ pumped into stomach via K+ATPase
results in net flow of H+ and Cl- out of parietal cell and into lumen
how does mucus protect against H+ secretion
inc HCO3 forms water insoluble gel on epithelial surface
effect of lipase on triglycerides
converts into fatty acids and triglycerol
what does intrinsic factor do
prevents pernicious anemia - absorbs Vit B12
what does HCl do
kills bacteria
denatures digested food
activates PEPSINOGEN
What are the phases of HCl secretion by parietal cells
cephalic
gastric
intestinal
what regulates HCl secretion
neuronal pathways, duodenal hormones
how do neuronal pathways and duodenal hormones regulate secretion of HCL
- directly, on parietal cell
- indirectly influence gastrin and histamine secretion
BOTH UPREGUALTE ACID SECRETION
how is gastric acid secretion regulation in the cephalic phase
ach released
stimulated release of histamine from ECL cells
ach acts directly on parietal cells - inc HCl
how is gastric acid secretion regulated in the gastric phase
stomach distension inc pepitde conc
inc acidity/ lowering of [H+]
why is the acidity of the lumen of the stomach higher before a meal
no bueffers
what do proteins do to luminal acidity
act as buffers in the gastric lumen - so HCl secretion increases
mechanism for effect of proteins on luminal acidity
proteins remove free H+ - their conc decreases; proteins remove inhibitory effect of HCl on gastrin and therefore acid secretion
gastrin-mediated acid secretion is increased
summary of gastric phase
food in stomach:
- distension - neural reflex - ach on parietal cells
- buffers acid - blocks stimulation of somatostatin secretion
- peptides, AAs, act on G cells for gastrin release
what increases gastrin secretion
luminal distension, inc activity of PSNS nerves to stomach
what inhibits gastrin secretion
increased HCl secretion
what does gastrin secretion promote
histamine secretion, acid secretion from parietal cells
what can act on parietal cells to increase acid secretion
gastrin, histamine
increased activity of PSNS nerves on stomach
function of the intestinal phases of gastric acid secretion
balances secretory activity of stomach and digestive/absorptive capacities of the SI
how does the high acidity of the duodenal contents affect acid secretion
reflexly inhibits it
what can inhibit acid secretion
duodenal distension hypertonic solution AA fatty acids monosaccharides
what does inhibition of acid secretion in the SI depend on
chyme volume and composition
how is acid secretion inhibited in the intestinal phase
short and long neuronal reflexes, hormones eg secretin and CCK inhibit parietal cell acid secretion
what can lack of HCl result in
failure of protein digestion - production of gastric acid in the stomach is absent or low
symptoms of HCl deficiency
undigested food in stool
flatulence, bloating
lack of intrinsic factor
treatment of HCl deficiency
bitter herbs
lemon juice, vinegar, vit b1
all stimulate HCl secretion
other ways HCl secretion can be stimulated
caffeine alcohol nsaids nicotine h. pylori stress
effect of histamine on gastric acid secretion
inhibition
effect of histamine / ach/ gastrin on binding to parietal cells -
increased HCl secretion
where is pepsin secreted from
chief cells as pepsinogen
how is pepsinogen activated
high [proton] - high acidity exposes its active site
how is pepsinogen inactivated
entry of food into SI - HCO3 - and peptides neutralise H+
what stimulates pepsinogen secretion
inputs to chief cells from nerve plexus
stimulators/inhibitors of acid secretion in cephalic and intestinal phases exert the same effect on pepsinogen secretion
what does pepsin do
initiates protein digestion - degrades proteins in food into peptides
pepsin in not required for food digestion, but what is?
intrinsic factor - secreted by the parietal cells
how do NSAIDS eg aspirin affect acid secretion
impair barrier properties of mucosa
supress gastric prostaglandin synthesis - COX inhibtion
inhibit platelet aggregation
