Gastric Secretion Flashcards
Function of fundus?
Storage
Functions of stomach body?
Storage and production of:
- Mucous
- Gastric acid (HCl)
- Pepsinogen
- Intrinsic factor
What does HCL do?
Begins protein digestion by acid hydrolysis and sterilizes food.
What can survive gastric acid?
Only Heliobacter Pylori
What is pepsinogen?
The inactive version of protease enzyme pepsin
Pepsin is in this form to stop it digesting protein of actual stomach
Function of the antrum?
Mixing/grinding
Making gastrin - a hormone that acts on cells in the body to control acid and pepsinogen secretion
What kind of cells are in a gastric gland and what do they produce?
Mucous neck cells - make mucus
Chief cells - make pepsinogens
Parietal cells - make HCL and intrinsic factor
What do parietal cells contain to help in HCL secretion?
Receptors for the various chemicals that affect HCl secretion
How is HCL secreted from the epithelium?
1) CO2 from blood + water in the epithelial cell froms carbonic acid via carbonic anhydrase enzyme
2) CO2 quickly degrades to bicarbonate and H+
3) Cl- enters the cell swapping with bicarbonate which enetrs the blood
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+
How does the blood pH change after a meal and why?
Alkalisation due to the bicarbonate being pumped into the blood from the stomach epithelium in exchange for Cl- ions.
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?
Stimulatory:
- Gastrin
- Histamine
- Acetylcholine
Inhibitory:
- Prostoglandin
Where do these 4 chemicals act?
Bind to receptors on parietal cells in the stomach epithelium
How does Gastrin affect HCl secretion?
Gastrin enters circulation and passes through the heart and heads towards stomach
- Binds to its g-protein coupled receptor on epithelial cell
- This causes a rise in Ca2+ ions inside the cell
- This stimulates the H+/K+-ATPase to pump more H+ into the stomach
How does histamine work to increase HCL secretion?
Stomach has a unique histamine G-protein coupled receptor
Once bound it is activates adenosine cyclase which converts ATP to cAMP
cAMP acts on its protein kinase A
This causes H+ to be pumped into stomach
How does Ach work?
Acts on cholinergic muscarinic m3 receptors
M3 acts on a g-protein that is coupled to the IP3 system
This causes release of Ca2+ which acts on protein kinase C
H+ pumped into stomach
For all the stimulatory systems, what else is pumped and where?
K+ is pumped into the cell
How does prostaglandin inhibit HCL ?
It turns off adenosine cyclase and stops it turning ATP to cAMP
By what mechanisms is HCL secretion controlled?
Neurocrine
Endocrine
Paracrine
Describe what neuro responses exist.
Vagal and local reflexes
Desc. what endocrine responses exist.
Gastrin response
What paracrine response exists? What does paracrine mean?
Histamine
paracrine means locally released
What is the cephalic phase of gastric acid secretion?
Vagus nerve stimulation that occurs before food even reaches stomach
Vagus nerve stimulates release of Ach and g cells which activate gastrin - all these go on to activate parietal cells
Presence of gastrin and Ach causes local release of histamine
How can we stop the H+ ion pumps?
Proton pump inhibitors - omeprazole
Describe the gastric phase of HCL secretion.
Stomach distends due to food which activates vagal/enteric reflexes further
- Ach released
- Presence of peptides in stomach causes G-cells and gastrin release
- Gastrin and Ach cause histamine release
What inhibits HCL secretion in the cephalic phase?
Reduced vagal stimulation
What reduces HCL in the gastric phase?
A decreased ph due to high levels of HCL = lowered gastrin secretion
What inhibits HCL production once food enters the intestinal phase?
If acid covered food is detected in the duodenum the enterogastric/splanchnic reflex activates the secretion of secretin
Secretin reduces stimulation of gastrin and also reduces gastrins effect on parietal cells
How can fat in the duodenum inhibit HCL secretions?
Causes release of GIP which inhibits G-cells
Why is acid in the duodenum such a bad thing?
Pancreatic enzymes can’t work in acidic conditions
What are enterogastrones?
Hormones released by gland cells in the duodenal mucosa in response to acid/food
- Secretin
- CCK
- GIP
How does secretic, cholecystokinin (CCK) and GIP work?
Inhibit gastric acid secretion and reduce gastric emptying by contracting pyloric sphincter
This is all to stop acid in duodenum
What secretes pepsinogen?
Chief cells
What is a zymogen enzyme?
An inactive precursor of the enzyme
What converts pepsinogen to pepsin?
A low pH acts on pensinogenvia acid hydrolysis
What happens to the cleaved off zymogen part of the enzyme?
Is digested by pepsin
What stops pepsin?
When all food is digested and HCL is stopped being secreted - pepsin will inactivate at neutral pH
What makes gastric mucus?
Surface epithelial cells
Mucous neck cells
Gastric mucus role?
Protects wall against mechanical injury, acid corrosion or pepsin digestion
What does mucus have to help it protect?
High bicarbonate
What is the only essential/non-compensatory role of stomach?
Production of intrinsic factor from parietal cells
-needed for B12 absorption
