GI S4 & S5 - The Stomach Flashcards
Why does food need to be stored in the stomach?
Food is ingested faster than it can be digested so chyme must be released into the duodenum at a controlled rate
What are the purposes of the stomach?
To store food
Physical disruption
Chemical disruption
Disinfection
What is the pH of the stomach?
Below 2.0
How is the acidic environment of the stomach maintained?
Hydrochloride acid concentration of up to 100mmol/L
Secreted by parietal cells located in gastric pits
How is acid secretion controlled?
ACh, Gastrin and histamine all increase acid secretion from parietL cells
How does ACh increase acid secretion from parietal cells
Food arrives in the stomach This causes gastric distension This stimulates post ganglionic parasympathetic nerves to secrete ACh Binds to M3 muscarinic receptors Increases H+ secretion
How does Gastrin increase acid secretion from parietal cells
Amino acid content in the stomach stimulates endocrine (G) cells Low pH inhibits G cells release Gastrin Gastrin binds to the CCK receptor Causes increased H+ secretion
How does histamine increase acid secretion from parietal cells
Mast cells are stimulated by ACh and Gastrin
This causes histamine release
Histamine diffuses locally to parietal cells
Binds to H2 receptors
Causes increased H+ secretion
What are drug targets for increasing stomach pH
H2 antagonists
Proton pump inhibitors
How do H2 antagonists work?
H2 receptor antagonists inhibit histamine binding to H2 receptors on parietal cells
Reduced stimulation of acid production
Reduced acid secretion
How do PPIs work
Proton pump inhibitors act on proton channels on the apical membrane of parietal cells so less H+ is secreted into the lumen of the stomach
How is H+ produced in parietal cells?
The mitochondria split water: H2O -> OH- + H+
The H+ is secreted into the stomach lumen via a proton pump on the apical membrane
CO2 binds to OH-: CO2 + OH- -> HCO3-
Bicarbonate diffuses into the blood via the basolateral membrane
Where exactly are proton pumps located
Stomach Gastric pits Parietal cells Apical membrane Canaliculi
What do chief cells secrete
Enzymes
Mainly pepsinogen, the inactive precursor of pepsin which is cleaved by acid in the lumen of the stomach
What are the phases of gastric secretion?
Cephalic
Gastric
Intestinal
Describe the cephalic phase of gastric secretion
The sight and smell of food and the act of swallowing cause parasympathetic stimulation
Therefore ACh is released
Describe the gastric phase of gastric secretion
Food reaches the stomach, causing distension and pH buffering
This causes ACh stimulation and Gastrin disinhibition
Acid secretion is stimulated
Gastric peptide concentration increases due to breakdown by acid and enzymes
This stimulates Gastrin, but low pH inhibits it
Describe the intestinal phase of gastric secretion
Chyme leaves the stomach
Low duodenal pH stimulates cholecystokinin (CCK) and gastric inhibitory peptide release
These reduce gastric acid secretion
How is the stomach protected from low pH?
Surface stomach mucosal cells secrete a thick alkaline mucus
This provides mechanical protection and chemical protection because H+ ions become trapped by HCO3 in the mucus
How are stomach defences controlled?
Prostoglandins stimulate mucus secretion
How does poetic ulceration occur?
When stomach defences are damaged or overwhelmed then acid reaches the stomach mucosal cells
This causes gastritis then ulceration
What can disrupt stomach defences?
Infection by Helicobacter pylori
NSAIDs
Excess alcohol
Briefly describe gastric motility
Swallowing induces a vagal reflex called receptive relaxation to reduce resting tension in the walls of the stomach so pressure remains low so oesophageal reflux is less likely
Peristalsis is induced by a pacemaker in the cardiac region. It moves from cardia to pylorus and both mixes contents and causes a small squirt of chyme to be deposited in the duodenum
What controls the rate of gastric emptying?
Feedback from the duodenum, induced by the presence of:
- Fats
- Low pH
- Hypotonicity
What is a gastric ulcer?
Mucosal inflammation and breakdown
How does the area of the stomach that is colonised by H pylori affect the patients symptoms and pathology?
If Antrum-predominant, most likely pathology is duodenal ulcer
If body-predominant, most likely pathology is gastric ulcer and cancer
If both Antrum and body predominant, most likely patient will be asymptomatic
