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?
They bind to receptors on parietal cells in the stomach epithelium
How does Gastrin affect HCl secretion in the stomach?
- > 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?
- 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?
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?
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?
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
A hormone released by glands in the duodenal mucosa to inhibit forward movement of gastric contents
Name 3 enterogastrones
Gastric Inhibitory polypeptide (GIP)
What triggers enterogastrone release?
A response to acid/fats(& products)/amino acids and a hypertonic solution in the duodenum
How do enterogastrones affect the gut?
They prevent excess build up of gastric contents (chyme) in the small intestine by:
- Inhibiting Gastric acid secretion
- Stimulating bicarbonate secretion to neutralize acid
- Inhibiting gastric motility & contracting pyloric sphincter to reduce gastric emptying
- Inducing secretion of pancreatic enzymes to break down chyme
Define a zymogen?
An inactive precursor to an enzyme, activated by another enzyme
Why are digestive enzymes stored as zymogens?
To prevent self digestion
Where is pepsinogen stored?
In chief cells in the gastric glands
What triggers pepsinogen release?
pepsin release is activated alongisde HCl release by Vagal tone and gastrin
What activate pepsinogen to pepsin?
In the acidic enviroment of gastric acid pepsinogen can cleave itself then pepsin continues cleaving more pepsin off the pepsinogen chains.
How is pepsin inactivated?
By a neutral pH, so it stops when acid stops too
What makes up the gastric mucous layer and what produces it on the surface of the gastric epithelium?
Mucous and bicarbonate
Surface epithelium and mucous neck cells
Whats the purpose of the gastric mucous layer?
Protecting the mucosa against gastric acid corrosion and pepsin digestion
What is the only essential (or non-compensated for) function of the stomach?
Intrinsic factor secretoin
What cells secrete intrinsic factor?
Parietal cells , same as HCl
What is intrinsic factor required for?
Vitamin B12 absorption. The complex of the 2 is absorbed in the ileum.
An intrinsic factor defect causes pernicious anemia as erythrocytes fail to mature.