GI 6 - The Stomach Flashcards
(31 cards)
Which gastric cells secrete histamine?
Enterochromaffin-like cells
What acts to stimulate Gastrin secretion in the stomach?
- Peptides/ amino acids in lumen
- Vagally released Acetylcholine
- Vagally released Gastrin-releasing peptide (GRP)
What is the function of intrinsic factor? Which gastric cell produces it?
- Necessary for Vitamin B12 absorption in the ileum
- Parietal cells
Why do parietal cells change shape when stimulated to produce acid?
Tubulovesicular structure rearranges into a canicular structure = massively increases surface area for secretion
Describe the mechanism of HCl secretion:
- Carbonic anhydrase combines CO2 + OH- to form HCO3- + H+ in parietal cell
- Cl- moves into parietal cell via HCO3-/Cl- antiporter
- Cl- moves into stomach lumen via apical Cl- channel, down conc. grad.
- H+ is pumped into lumen via H+/K+-ATPase
- Basolateral Na+/K+-ATPase sets up Na+ conc. grad.
- Na+/H+-ATPase maintains pH of parietal cell
- Excess K+ leaves via apical/basolateral K+ channels
What is the alkaline tide that occurs after eating? What causes it?
- Temporary increase in pH after eating
- Caused by efflux of HCO3- by parietal cells during the secretion of acid into the stomach
Name the channels used in acid secretion on the basolateral parietal wall:
HCO3-/Cl- exchanger
Na+/K+-ATPase
Na+/H+-ATPase
K+ channel
Name the channels used in acid secretion on the apical parietal wall:
H+/K+-ATPase
Cl- channel
K+ channel
What directly stimulates the production of gastric acid?
- Vagus nerve secretes Ach -> M3 on parietal cells
- Histamine
- Gastrin
How does the vagus nerve stimulate the production of acid both directly and indirectly?
Directly: stimulates parietal cells via Ach -> M3
Indirectly:
- Stimulates enterochromaffin-like cells to secrete Histamine = stimulates parietal cells
- Stimulates G cells to secrete Gastrin = stimulates parietal cells
How is acid secretion from parietal cells inhibited?
Luminal acid stimulates D cells to secrete Somatostatin, which:
- Directly inhibits Parietal cells
- Inhibits Gastrin production from G cells, which indirectly inhibits parietal cells
Name the 3 phases of digestion, and the percentage of total HCl secretion caused by each stage:
1) Cephalic ~ 30% of total HCl production
2) Gastric ~ 60%
3) Intestinal ~ 10%
Describe the cephalic stage of digestion:
- Anticipation of food (sight, smell, taste, chewing) stimulates the parasympathetic fibres on the Vagus nerve
- This directly stimulates Parietal cells = acid secretion
- and G cells = gastrin production
Somatostatin is released when D cells detect luminal acid. Why is somatostatin release inhibited when food is present in the stomach?
Food acts as a buffer, causing pH to rise, inhibiting D cells
What is the Enterogastric reflex?
- Lipids and gastric acid in the duodenum reduces vagal stimulation
- Inhibition of gastrin secretion in the stomach
- This reduces stomach motility, reducing the rate of emptying from the stomach
Chyme in the duodenum stimulates the release of:
- CCK
- Secretin
- Gastric Inhibitory Hormone
How does the duodenum control the rate of gastric emptying?
- Lipids and gastric acid in the duodenum reduces vagal stimulation
- Inhibition of gastrin secretion in the stomach
- This reduces stomach motility, reducing the rate of emptying from the stomach
How can gastric acid secretion be reduced via drugs?
- H2 blockers - prevent Histamine stimulating Parietal cells
- Proton-pump inhibitors - prevent H+ moving from the Parietal cell into the stomach lumen
Name some drugs which can reduce gastric acid secretion:
H2 blockers: Cimetidine, Ranitidine
PPI: Omeprazole
How do gastric epithelia survive the pepsins and low pH of the lumen?
Covered in an alkaline mucous layer, secreted by mucous neck cells and surface epithelial cells
= Diffusion barrier for H+ and pepsins
The peristaltic contractions of the stomach are stimulated by a pacemaker in which area of the stomach? How frequently do these contractions occur?
Cardia
3x/min
What are the common causes of acute gastritis?
- NSAIDs
- Alcohol
- Chemotherapy
- Bile reflux
- Stress
How does heavy NSAIDs use lead to gastritis?
- NSAIDs reduce the number of prostaglandins
- Decreases blood flow
- Less nutrients to cells
- Reduced mucous production
- Reduced protection of gastric epithelia
What can cause chronic gastritis?
- Helicobacter pylori
- Cytomegalovirus
- Some parasites
- Autoimmune
- Chronic bile reflux
- Chronic NSAIDs use
- Chronic alcohol intake
- Chronic radiation
- Crohn’s
- Sarcoidosis
- Wegener granulomatosis
- Coeliac disease
