Gastric Secretion Flashcards
Whats the function of the fundus?
Storage, its got a lot very stretchy rugae
What are the functions of the stomach body?
- Storage production of: - Mucous - Gastric acid (HCl) - Pepsinogen - Intrinsic factor
What does Gastric acid do?
Begins protein digestion by acid hydrolysis and sterilizes food.
What can survive gastric acid?
Only Heliobacter Pylori can survive stomach acid
How does pepsinogen become pepsin and what does it do?
Pepsinogen is activated (converted to pepsin) by gastric acid.
Its a protease.
Whats the function of the antrum of the stomach?
- Mixing an Grinding stomach contents
- Gastrin production
What does gastrin do?
Its a peptide hormone that stimulates gastric acid production
What kind of cells are in a gastric gland and what do they produce?
Mucous neck cells -> mucous
Chief Cells -> Pepsinogens
Parietal Cells -> HCl & Intrinsic factor (contains receptors for the various chemicals that affect HCl secretion)
How is Gastric acid secreted from the epithelium?
1) CO2 from blood + water in the epithelial cell froms carbonic acid
2) degrades to bicarbonate and H+
3) Cl- enters the cell swapping with bicarbonate
4) Cl- exits into the stomach through a Cl- channel
5) the H+ from the carbonic acid is pumped into the stomach in exchange for K+ 1 for 1
How does the blood pH change after a meal and why?
Transient alkalisation due to the bicarbonate being pumped into the blood from the stomach epithelium in exchange for Cl- ions.
Part of gastric acid secretion
Does gastric acid secretion happen all the time?
No only when eating as its a high energy process
What 4 chemicals control the gastric acid secretion? Which are stimulatory and which are inhibitory?
What cell type do they act on to affect HCl secretion?
Stimulatory:
- Gastrin
- Histamine
- Acetylcholine
Inhibitory:
- Prostoglandin
They bind to receptors on parietal cells in the stomach epithelium
How does Gastrin affect HCl secretion in the stomach?
Gastrin:
- > Binds to receptor on epithelial cell
- > Receptor releases intracellular Ca2+
- > Ca2+ activates protein kinases
- > Induces H+K+ATPase initiating HCl secretion
How does histamine affect Gastric acid secretion?
The stomach has a unique histamine receptor (H2).
- > Binds to H2
- > Activates a g coupled protein (Gs)
- > Converts ATP to cAMP
- > Acts on protein Kinase
- > Stimulates HKATPase
How does acetylcholine affect HCl secretion?
- > Binds to receptor on parietal cell
- > Secretes intracellular Ca2+
- > Acts on protein kinase
- > Induces H+K+ATPase
Which two chemicals follow the same pathway to stimulate gastric acid secretion?
Gastrin & Acetylcholine both stimulate secretion of intracellular calcium in turn stimulate protein kinases
How does prostoglandin affect Gastric acid secretion?
- > Binds to receptor on parietal cell
- > activates Gi coupled protein
- > inhibits AC converting ATP -> cAMP
- > Basically disabling the histamine pathway
- > Reduction in protein kinase activty & so gastric acid secretion
How are vagal and local reflexes involved in gastric acid secretion?
They stimulate or inhibit the release of Acetylcholine, gastrin, histamine and prostoglandins depending on the reflex.
What phases are there to stimulating or inhibiting gastric acid secretion?
- Cephalic
- Gastric
- Intestinal (purely inhibitory)
What occurs during the cephalic phase?
The sight/smell/taste of food stimulates the vagus nerve
- > Increases Ach + acts on G cells to increase Gastrin
- > Both activate ECL cells which produce histamine
- > All 3 act on parietal cells to induce HCl secretion
When you stop eating the vagal tone decreases and all this is inhibited
Why is the cephalic stage so important to digestion?
If it didn’t occur food would hit your stomach acid less and bacteria would multiply and you’d get sick
What occurs during the gastric phase to stimulate gastric acid secretion?
Stimulatory:
1a) Stomach Distension -> Vagal/ENS reflexes -> Ach secretion
1b) Peptides in the lumen -> G cell activation -> Gastrin secretion
2) Both Gastrin & Ach act on ECL cells to induce histamine secretion
3) All 3 act on parietal cells to induce HCl secretion
What occurs in the gastric phase to inhibit HCl secretion?
Inhibitory:
The more acid released the lower the pH goes.
Lower pH inhibits gastrin release which lowers HCl secretion & inhibits histamine release from ECL cells
What occurs during the intestinal phase?
Two processes:
1) Acid in duodenum from stomach
- > Enterogastric (splanchnic) reflex + Secretin release
- > Reflex lowers gastrin secretion and secretin stimulates bicarbonate release
2) Fat in duodenum from stomach
- > GIP released (Gastric inhibitory polypeptide)
- > Lowers gastrin secretion and directly lowers parietal HCl secretion
