Secretions of GI Tract & Pancreas

Question 1

Gastric Secretion: Cephalic Phase

Answer 1

• Accounts for 30% of total HCl secreted in response to a meal
• Stimuli: smelling, tasting, chewing, swallowing, conditioned relexes

Question 2

Gastric Secretion: Cephalic Phase - Mechanisms

Answer 2

(1) Vagus N –> Parietal Cell
- Vagus releases ACh to Parietal Cells which stimulates the secretion of HCl

(2) Vagus N –> Gastrin –> Parietal Cell
- Vagus releases GRP to G Cells which triggers them to release Gastrin into the circulation
- Gastrin stimulates HCl secretion from Parietal Cells in the stomach

Question 3

Gastric Secretion: Gastric Phase

Answer 3

• Accounts for 60% of total HCl secreted in response to a meal
• Stimuli: distention of stomach (activates mechanoreceptors in the mucosa of oxyntic and pyloric glands), presence of broken down proteins, AAs, small peptides

Question 4

Gastric Secretion Gastric Phase - Mechanisms

Answer 4

(1) Vagus N –> Parietal Cell
- Released ACh stimulates HCl secretion

(2) Vagus N –> Gastrin –> Parietal Cell
- GRP released from Vagus stimulates Gastrin release from G Cells into circulation
- Gastrin stimulates HCl release from Parietal Cells

(3) Distention of Antrum
- Local reflexes –> ACh –> Parietal Cell and G Cell activation

(4) Amino Acids and Small Peptides
- Gastrin –> Parietal Cell

Question 5

Gastric Secretion: Intestinal Phase

Answer 5

• Accounts for 5-10% of total HCl secreted in response to a meal
• Distention of SI stimultes acid secretion
• Digested protein stimulates acid secretion via: Gastrin –> Parietal Cells; Direct effect on parietal cell; Entero-oxyntin

Question 6

Pepsinogen –> Pepsin: Optimal pH

Answer 6

1.8 - 3.5

Question 7

Pepsinogen –> Pepsin: Reversibly Inactivated pH

Answer 7

> 3.5 - 5

Question 8

Pepsinogen –> Pepsin: Irreversibly Inactivated pH

Answer 8

> 7 - 8

Question 9

Gastric Mucosa: Protective Factors

Answer 9

• HCO3-
• Mucus
• Prostaglandins
• Mucosal blood flow
• Growth Factors

Question 10

Gastric Mucosa: Damaging Factors

Answer 10

• H+
• Pepsin
• H. Pylori
• NSAIDs
• Stress
• Smoking
• Alcohol

Question 11

Helicobacter Pylori (H. Pylori)

Answer 11

• Major acquired factor in the origin of both Gastric Ulcers and Duodenal Ulcers
• Bacteria releases cytotoxins that breakdown the mucosal barrier and underlying cells

Question 12

Helicobater Pylori: Urease

Answer 12

• Enzyme that allows the bacteria to colonize the gastric mucosa
• Converts urea to ammonia (NH3) which alkalinizes the local environment
• Produces ammonium NH4+ which is a major cause of cytotoxicity (damages epithelial cells and breaks the mucosal barrier)

Question 13

Two Main Components of the Exocrine Pancrease:

Answer 13

• Aqueous solution containing HCO3- (secreted by Centroacinar and Ductal Cells)
• Enzymatic secretion (secreted by Acinar Cells)

Question 14

Pancreatic Secretion: Cephalic Phase

Answer 14

• Initiated by smell, taste and conditioning
• Mediated by Vagus N
• Produces mainly an enzymatic secretion

Question 15

Pancreatic Secretion: Gastric Phase

Answer 15

• Initiated by distention of the stomach
• Mediated by the Vagus N
• Produces mainly an enzymatic secretion

Question 16

Pancreatic Secretion: Intestinal Phase

Answer 16

• Accounts for 80% of pancreatic secretions
• Enzymatic and Aqueous secretions are stimulated