Gastric secretion Flashcards
what 3 mechanisms control gastric acid secretion
neurocrine (vagus/local reflexes)
endocrine (gastrin)
paracrine (histamine)
what is the cephalic phase of gastric secretion?
occurs before food enters the stomach, especially while it is being eaten.
Results from the sight, smell, thought, or taste of food;
greater appetite = more intense stimulation.
what is the gastric phase mediated by?
vagus nerve and the release of Gastrin
How would you inhibit the cephalic phase?
stopping eating as this decreases vagal activity
How would you inhibit the gastric phase?
Decreasing the release of gastrin. Gastrin release is inhibited by a low pH ie when HCL in the lumen is too high.
What is the intestinal phase and how does it inhibit gastric acid secretion?
The duodenum reacts to the arrival of chyme to by moderating gastric acid activity. It initially enhances gastric secretion, but soon inhibits it.
The ways it inhibits gastric acid secretion is:
If it detects acid in duodenum - enterograstric reflex and secretin release kicks in - decreased gastrin secretion and decreased gastrin stimulation of parietal cells (ie less HCL moving into lumen).
If it detects FA’s/CHO in the duodenum - GIP is released which decreases gastrin secretion and decreases parietal HCL secretion
What are enterogastrones?
Hormones released from gland cells in duodenal mucosa.
They are released in response to acid, hypertonic solutions, FA’s or monoglycerides in the duodenum and work together to prevent acid build up here.
They inhibit gastric acid secretion or reduce gastric emptying.
Give some examples of enterogastrones? (3)
secretin
cholecystokinin (CCK)
GIP
Why are enterogastrones produced/ what is their function?
They are produced in response to acid, hypertonic solutions, fatty acids or monoglycerides in duodenum.
They act collectively to prevent further acid build up in duodenum.
what are the 2 ways in which enterogastrones prevent further acid build up in the duodenum?
inhibit gastric acid secretion
reduce gastric emptying (inhibit motility/contract pyloric sphincter)
What secretes pepsinogen?
chief cells
when are pepsins inactivated?
at a neutral pH
what is the role of gastric mucus
Produced by surface epithelial cells and mucus neck cells to carry out cytoprotective role
Protects mucosal surface from mechanical injury
Neutral pH (HCO3) => Protects against gastric acid corrosion and pepsin digestion
what is required for vitamin B12 absorption
intrinsic factor produced by parietal cells
where is the intrinsic factor/ B12 complex absorbed from
the distal ileum (final section of the small intestine)
What do parietal cells secrete?
HCl and intrinsic factor
Describe the mechanism that creates an acidic environment in the stomach lumen
CO2 moves from the blood into parietal cell and binds with water within the cytoplasm
This creates H2CO3 which is very unstable and dissociates quickly to make 1 H+ ion and HCO3.
HCO3 is pushed out the cell in exchange for a Cl- which then moves into the stomach lumen through a channel.
H+ ion moves into lumen and K+ comes in 1:1
H+, Cl- are now in the lumen and all that is needed is H20 - this moves into the lumen due to the osmotic gradient that is built up
What type of hormone is histamine?
paracrine - released into blood and acts locally
Increased vagal activity causes the release of Ach which in turn does what?
stimulates parietal cells to secrete HCl
What type of hormone is gastrin?
Endocrine
distension of your stomach (food arriving) has what affect on vagal activity etc?
it increases vagal activity which releases Ach - stimulates Parietal cells - releases HCl which works to digest the food
A decrease in pH has what effect on Gastrin?
this would mean an increase in HCl which would cause a decrease in gastrin
What is the enterogastric reflex?
The enterogastric reflex is stimulated in the stomach by a pH of 1.5.
Upon initiation of the reflex, release of gastrin by G-cells in the antrum is shut off. This in turn inhibits gastric motility and the secretion of gastric acid + the stimulation of parietal cells
When fat or carbohydrates enter the duodenum, gastric-inhibitory peptide (GIP) is released- what effect does this have?
decrease in gastrin secretion
decrease in parietal HCl secretion
What is pepsinogen?
a substance secreted in the stomach and converted into the enzyme pepsin by gastric acid
pepsinogen prevents cellular digestion - it only works in low pH (stomach) it is inactivated at a neutral pH
What is a zymogen
an inactive substance which is converted into an enzyme when activated by another enzyme
ie pepsinogen is the zymogen for pepsin
Which cells produce gastric mucus?
Surface mucous cells and mucus neck cells
How does mucus play a cyoprotective role?
Protects mucosal surface from mechanical injury
As the layer of mucus is of a neutral pH (HCO3 - pH is 7 at cell surface) it protects against gastric acid corrosion and pepsin digestion
What is intrinsic factor?
Only essential (non-compensated) function of stomach
Its secretion is stimulated via all pathways known to stimulate gastric acid secretion: histamine, gastrin, and acetylcholine.
produced by parietal cells and needed for vitamin B12 absorption
Intrinsic factor/B12 complex absorbed from ileum
If a patient has a defect in intrinsic factor secretion what can occur?
Pernicious Anaemia (failure of erythrocyte maturation)
