Gastric secretion

Question 1

Describe the structure and various functions of the stomach

Answer 1

• Fundus
	- Storage
	• Body
	- Storage
	- Mucous
	- HCl
	- Pepsinogen
	- Intrinsic factor
	• Antrum
	- Mixing/grinding
	- Gastrin
	• Mucous neck cells
	- Mucus
	• Chief cells
	- Pepsinogen 
	• Parietal cells
	- HCl
	- Intrinsic factor

Question 2

What are the hormonal and neural mechanisms responsible for the control of the secretion of gastric acid?

Answer 2

• Neurocrine (vagus/ local reflexes)
• Endocrine (gastrin)
• Paracrine (histamine)
• The sight, smell and taste of food stimulates the vagus nerve
→ This causes the release of acetyl choline and also stimulates the G cells which produce gastrin
→ Both the gastrin and the acetyl choline stimulate the parietal cells
→ Gastrin and acetyl choline also stimulate ECL cells which release histamine
→ Histamine stimulates the parietal cells
→ The parietal cells secrete gastric acid
• The vagus is stimulated by
- Smell, sight and taste of food
- Distension of stomach
• The G cells are stimulated by
- The stimulation of the vagus
- Peptides in the lumen
• ECL is stimulated by
- Gastrin/ acetyl choline
• Stopping eating decreases vagal activity
• The decreasing pH decreases the production of gastrin
• Acid in the duodenum causes the enterogastric reflex and secretin release which decreases gastric secretion
• Fat in the duodenum causes GIP release which decreases gastric secretion

Question 3

What are the hormonal and neural mechanisms responsible for the secretion of pepsin?

Answer 3

• Stored as pepsinogen the zymogen (inactive precursor) of pepsin (it is converted by an activated zymogen called trypsin which is activated by enterokinase)
  
  • Zymogen storage prevents cellular digestion
  • Pepsin activated at low pHs
  • Pepsins are inactivated at neutral pH
    * Mechanisms for control of pepsin secretion parallel HCl secretion (CCK)

Question 4

What is the role of intrinsic factor in vitamin B12 absorption?

Answer 4

• This is the only essential (non-compensated) function of the stomach
  
  • Produced by parietal cells
  • Intrinsic factor/ B12 complex is absorbed from the ilium
  • A defect causes pernicious anaemia

Question 5

What are the functions of gastric mucous?

Answer 5

• Produced by surface epithelial cells and mucous neck cells
• Cytoprotective role
- Protects mucosal surface from mechanical injury
- Neutral pH (HCO3)→ protects against gastric acid corrosion and pepsin digestion