3. Stomach

Question 1

What the key functions of the stomach?

Answer 1

• Digestion of macronutrients
• Storage reservoir
• Immunological protection

Question 2

What type of epithelia lines the stomach?

Answer 2

Columnar epithelia

Question 3

What are the anatomical regions of the stomach (downwards/clockwise)?

Answer 3

• Cardia - secretes mucous
• Fundus - secretes HCL, mucous and pepsinogen
• Body - secretes HCL, mucous and pepsinogen
• Pyloric antrum - secretes gastrin
• Pyloric canal - secretes mucous

Question 4

Describe the stomach wall?

Answer 4

• 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

Question 5

Compare the colour of the oesophageal to the stomach lining

Answer 5

• Oesophageal - light pink

* Stomach - bright red

Question 6

What ensures the stomach can resist the low pH?

Answer 6

• Mucosal gel lining
- protects from corrosive acid
• Resistant stomach lining

Question 7

What are gastric pits?

Answer 7

• Deep pores in the mucosa that line the stomach

* House functional secretory cells

Question 8

What are the 7 different cell types in the stomach?

Answer 8

• Mucous cells
• Parietal cells
• Chief cells
• G cells
• Enterochromaffin-like cells
• D cells
• Gastric stem cells

Question 9

Describe the function of the Mucous cells

Answer 9

• Secrete bicarbonate-rich mucus
• Helps protect stomach lining
• Keeps the pH next to the lining at 7 rather than 2-3

Question 10

Describe the function of the Parietal cells

Answer 10

• 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)

Question 11

Describe the function of the Chief cells

Answer 11

• 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

Question 12

Describe the function of the G cells

Answer 12

• 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)

Question 13

Describe the function of Enterochromaffin-like cells

Answer 13

• Neuroendocrine cell deep in gastric glands

* Secrete histamine - aids secretion of acid from parietal cells

Question 14

Describe the function of D cells

Answer 14

• 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

Question 15

Describe the function of Gastric stem cells

Answer 15

• Pluripotent
• Develop into all the types of cells in the stomach
• Development pathways vary for the different parts of the stomach

Question 16

How do parietal cells produce HCl?

Answer 16

• 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

Question 17

What are the 3 phases of gastric secretion and motility in the stomach?

Answer 17

• Cephalic phase
• Gastric phase
• Intestinal phase

Question 18

Outline the Cephalic phase

Answer 18

• 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)

Question 19

Outline the gastric phase

Answer 19

• 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)

Question 20

Outline the intestinal phase

Answer 20

• 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)