L28 Flashcards
what are the components of gastric secretions
acid
pepsin
intrinsic factor
mucus
what are the functions of the acid in gastric secretion
protective role (kills bacteria)
denature proteins (chemical digestion)
optimum pH for many digestive enzymes (pepsin requires a low pH to be activated)
render fluid isosmotic (150 mM HCl - HCO3-
-neutralised part)
what is the osmolarity of the gastric secretions. why
200mOsmol
This is because of the bicarbonate in the stomach (this is here fore protection of the epithelial lining of the stomach)
what is the pH of the stomach
1
what is the main digestive enzyme in the stomach
pepsin
why do we need intrinsic factor
for absorption of vitamin B12
what is B12 important for
(DNA, red blood cells, pernicious anaemia)
what os pernicious anaemia
B12 effects the production of RBC
therefore if you are severally lacing in intrinsic factor it causes anemia
why is mucus an important gastric secretion
protects against acid and mechanical forces
makes the lining elastic
what are the 3 different functions anatomical regions of the stomach and what do they secrete
Lower esophageal sphincter and cardia = mucus and HCO3-
fundus and body = H+, intrinsic factor, mucus, HCO3-, pepsinogen, lipase
antrum and pylorus = mucus and HCO3-
NOTE: fundus and body secret lipase but there is no fat digestion until the intestine
where is gastrin secreted from and what is it secreted into
gastrin is secreted by the late part of the stomach (antrum and pylorus) into the BLOOD (its a hormone) which then causes acid secretion in the upper parts of the stomach (fundus and body)
what are the structures of the stomach
lumonal to interstitial
laminar propria (contains gastric glands)
muscularis mucosa
sub mucosa
muscularis externa
serosa
where are the gastric glands located
in the body of the stomach (in the lamina propria)
describe the structure of the gastric glands
gastric pit = opening
surface cells
mucus secreting cells
oxyntic/parietal cells
chief cells
enterochromaffin like cells (ECL cells)
what do surface cells secrete. why is that important
The surface cells produce bicarbonate which is important for protecting the epithelial lining from the acid
what do mucus cells secrete. why is that important
Mucus cells are important for making it elastic and to withstand the mechanical factors. They also protect againsted the acids
what do parietal cells secrete
acid and intrinsic factor
what do chief cells secrete
enzymes such as pepsinogen
what do ECL cells secrete
ECL cells release histomine which is important in the control of gastric acids secretion
Parietal cells and chef and ecl cells are in close proximity
why is this
for control of the parietal cells
what is the volume and composition of gastric secretions
2 - 3 litres per day
composition varies depending on eating or fasting
between meals what is the rate of gastric secretion
15-30 mL/hour
isosmotic fluid is secreted by surface cells. what is the composition of this
isosmotic solution with similar [Na+] as plasma but higher [HCO3-]
also mucus
about 200mOsmol
what happens to the volume of gastric secretion when we are eating
it is superimposed on basal secretion are much larger volumes of
secretion
produced at 150 mL/hour (5-10x higher)
how are gastric secretions isosmotic
isosmotic (150 mM) solution of HCl produced by parietal cells
remember surface cells secrete HCO3- (H+ + HCO3 –> H2CO3 –> H2O + CO2)
therefore final gastric secretion has an osmolality of ≈ 200 mOsmol/L
what happens to the composition of gastric secretions while eating
more…
isosmotic fluid (HCl)
pepsinogen - secreted by chief cells
intrinsic factor - oxyntic (parietal) cells
more mucus (There is a lot of mechanical force in the stomach therefore we need lots of mucus to keep them alive)
as the rate of secretion increases what happen to Cl-
it slightly increases (linear)
as the rate of secretion increases what happen to H+
it parabolically increase (HUGE increase) and then tappers off towards the end
as the rate of secretion increases what happen to Na+
it decreases parabolically (but not as much as H+ increases)
as the rate of secretion increases what happen to K+
says the same (slight increase but negalable)
Gastric secretions
A. consist of bile acids.
B. between meals have a rate of 3 mL/min.
C. of surface epithelium are mucus and bicarbonate.
D. of chief cells contain intrinsic factor.
E. increase upon stimulation by a factor of 100.
A. from liver and don’t get until duodenum
B. 15-30mL/hour
D. chief cells = enzymes, parietal cells = intrinsic factor
E. 5-10x
C is correct
what needs to happen to the parietal cells in order for acid to be secreted
they undergo a number of structural changes
describe a non secreting parietal cell
small intracellular canaliculi
short stubby microvilli at apical pole
extensive tubulovesicular system (these hold the components for acid secretion, but they are quiescent)
there are also few mitochondria
describe a secreting parietal cell
disappearance of
tubulovesicular system
development of extensive intracellular canaliculi
appearance of large apical microvilli
increase surface area of apical membrane by 50-100-fold
you also get a Huge increase in mitochondria because you need to produce the enzyme we need to feed which requires a H/K ATPase which requires energy (more H/K ATPases get inserted into the membrane)
what is the main change in a secreting parietal cell
the addition of more H/K ATPases into the apical membrane
describe H/K ATPase in the secreting parietal cell
is responsible for acid secretion
primary active transporter
utilises ATP to actively transport H+ out of cell in exchange for K+ into the cell. (primary active transport)
80% of the protein in the tubulovesicular membrane is the H+/K+ ATPase
what % of the proteins in the tubulovesicular membrane is the H+/K+ ATPase
80%
H/K ATPase is sitting in vesicles under the membrane when it is inserted into the membrane the driving force is K and the conc of K is maintained with another K channel in the apical membrane
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describe the events of the parietal cell at the apical membrane
K+ and Cl- diffuse down their electrochemical gradients into the lumen via channels
K+ is recycled back across the apical membrane via H+ ,K+ -ATPase
this causes the secretion of HCl and water follows passively
The secretion of hydrochloric acid (HCl)
A. is dependent on an apical (luminal) K+ gradient.
B. requires an apical bicarbonate gradient.
C. requires an apical Na+ gradient.
D. requires the change of the parietal cells from large intracellular canaliculi to an extensive tubulovesicular system.
E. requires a secondary active apical transport system.
B and C aren’t involved in HCl secretion
D. should say small instead of large
E. primary active transport
A is correct
Function of basolateral transporters –
basal (unstimulated) conditions
Na+/K+ ATPase maintains K+ in cell above equilibrium
K+ channel recycles K+ and generates membrane potential
Na+/H+ and Cl-/HCO3- exchanger maintain pH homeostasis and maintenance of Cl above
equilibrium
describe both membranes in the unstimulated state of the parietal cell
the basolateral membrane is always set up and the H/K ATPase is in vesicles under the apical membrane
when the parental cell is stimulated the H/K ATPases are inserted into the apical membrane
you also produce more H+ which causes more HCO3- to be formed
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what is omeprazole
Drug given to people with gastric ulcers/reflux (people that make too much acid) it is very specific in the fact that it only targets the H/K ATPase
describe apical transporters in stimulated conditions of the parietal cell
on stimulation fusion of tubulovesicular membrane inserts H+/K+ ATPase in apical membrane
activation of apical K+ and Cl- channel therefore K+ and Cl- diffuse into the lumen
presence of K+ in the lumen stimulates the H+/K+ ATPase to exchange H+ for K+
H+ obtained from hydration of CO2 by carbonic anhydrase
water driven out of cells by the efflux of ions
result is a secretion of HCl plus some KCl into the lumen
describe basolateral transporters stimulated conditions of the parietal cell
Na+/K+ ATPase maintains K+ above equilibrium
K+ channel contributes to membrane potential
Na+/H+ exchanger plays a minimal role
Cl-/HCO3 exchanger means that for each mole of H+ ion secreted an equivalent amount of base is
produced. this is extruded across the basolateral membrane by the Cl-/HCO3 exchanger (alkaline tide after a meal)
the exchanger is what provides the Cl- ion that is secreted with H+
describe pepsins
proteolytic enzymes (cleaves proteins)
secreted as inactive precursor pepsinogen by chief cells
converted to active pepsins by acidic pH and pepsins
describe the mucus and HCO3- secretions
mucus secreted by mucus neck cells in glands and surface cells
HCO3- rich solution secreted by surface cells
get a layer of alkaline mucus that protects the stomach from
abrasion
acid pH
the stomach secretes bicarbonate for protection
what is another way the stomach can get this bicarbonate
This bicarbonate is also coming from the blood because it is alkaline tide (from basolateral membrane) therefore the bicarbonate released into the blood through the anhydrase reaction is recycled and secreted out into the mucus of the stomach
