3. Stomach Flashcards
What the key functions of the stomach?
- Digestion of macronutrients
- Storage reservoir
- Immunological protection
What type of epithelia lines the stomach?
Columnar epithelia
What are the anatomical regions of the stomach (downwards/clockwise)?
- Cardia - secretes mucous
- Fundus - secretes HCL, mucous and pepsinogen
- Body - secretes HCL, mucous and pepsinogen
- Pyloric antrum - secretes gastrin
- Pyloric canal - secretes mucous
Describe the stomach wall?
- Extra oblique layer of smooth muscle inside the circular layer (aids complex grinding motions)
- Rugae - folds created by the mucosa and submucosa (aids small amount of absorption), becomes flat when stomach is full
Compare the colour of the oesophageal to the stomach lining
- Oesophageal - light pink
* Stomach - bright red
What ensures the stomach can resist the low pH?
• Mucosal gel lining
- protects from corrosive acid
• Resistant stomach lining
What are gastric pits?
- Deep pores in the mucosa that line the stomach
* House functional secretory cells
What are the 7 different cell types in the stomach?
- Mucous cells
- Parietal cells
- Chief cells
- G cells
- Enterochromaffin-like cells
- D cells
- Gastric stem cells
Describe the function of the Mucous cells
- Secrete bicarbonate-rich mucus
- Helps protect stomach lining
- Keeps the pH next to the lining at 7 rather than 2-3
Describe the function of the Parietal cells
• Acid secreting cells
• Quiescent (dormant) until activated
• Tubovesicles in the cytoplasm fuse with small invaginations on the apical surface
• This makes a complicated canalicular surface, with a large SA for acid secretion
• Rich in mitochondria for membrane transport
• Strong HCL to:
- kill pathogens
- activate protease zymogens
- alter protein structure
- secrete intrinsic factor (glycoprotein for B12 absorption)
Describe the function of the Chief cells
• Produces protease zymogens (pepsinogen) and lipase (gastric lipase)
• Pepsinogen => pepsin [via HCL]
- secreted as a precursor to prevent auto-digestion
- pepsin breaks dietary proteins into smaller peptide chains
• Gastric lipase digests fats
- removes fatty acid => triglyceride molecule
Describe the function of the G cells
• Endocrine cells in the stomach and duodenum
• Found at the bottom of the gastric pit
• Release gastrin into the bloodstream in response to:
- vagus nerve stimulation
- peptide presence in the stomach
- stomach distension
• Gastrin:
- travels to stomach receptor cells to stimulate gastric secretion and motility
- causes stronger contraction and opening of the pyloric sphincter (=> duodenum)
- binds to pancreas and gall-bladder receptors (more pancreatic juice and bile)
Describe the function of Enterochromaffin-like cells
- Neuroendocrine cell deep in gastric glands
* Secrete histamine - aids secretion of acid from parietal cells
Describe the function of D cells
- Enteroendocrine cells
- Secrete somatostatin
- Inhibitory effect on gastrointestinal function
- Inhibit ECL production of histamine and parietal cell activity
- Found in the stomach, intesine and pancreas
Describe the function of Gastric stem cells
- Pluripotent
- Develop into all the types of cells in the stomach
- Development pathways vary for the different parts of the stomach
How do parietal cells produce HCl?
- CO2 diffuses down concentration gradient into parietal cells (basolateral surface)
- CO2 + water => carbonic acid [carbonic anhydrase]
- Dissociates into bicarbonate and proton
- Bicarbonate exchanged with chloride ion in the interstitial space
- Chloride moves down concentration gradient into lumen (chloride channels)
- K+ enters from basolateral side via sodium-potassium ATP-ase (swapping for Na+)
- K+ leaves via channels into the lumen
- K+ pumped back in exchange for H+
• H+ and Cl- join in the lumen to form HCL
What are the 3 phases of gastric secretion and motility in the stomach?
- Cephalic phase
- Gastric phase
- Intestinal phase
Outline the Cephalic phase
• Caused by sight, smell and taste of food
• Stimulates vagus nerve (ACh) => cell secretion via submucosal plexus
- mucous cells
- chief cells
- parietal cells
- G cells
- gastrin (from G-cells) and histamine (from ECLs) further stimulate parietal cell secretion
• Result: Small secretion for a few minutes (+ 20% of acinar secretion in pancreas)
Outline the gastric phase
- Caused by distension (mechanoreceptors), nutrients and low pH (chemoreceptors)
- Stimulates vagus nerve => secretions from cells
- Increased mobility via myenteric plexus
- Result: 3-4 hours of gastric activity and mechanical digestion (+ 10% of acinar secretion in pancreas)
Outline the intestinal phase
• Duodenal stretch and chemoreception of lower pH and duodenal distension
• Causes I-cells (in the duodenal enterocyte border) to release cholecystokinin (CCK), and S cells to secrete secretin into blood
• CCK stimulates the release of bile, the secretion of pancreatic enzymes and inhibits gastric motility and emptying
- allows downstream organs to deal with current contents
• Secretin inhibits parietal cell HCl secretion
(• 70% of acinar secretion and activates duct cells of pancreas)
