GI Physiology - The Stomach

Question 1

Function of the stomach.

Answer 1

Preliminary digestion and destroying any potential pathogenic microorganisms that may have been ingested.

Question 2

pH of the stomach.

Answer 2

It is an acidic environment with a pH that can vary between 1.5-3.5.

Question 3

Which cell type produces hydrochloric acid?

Answer 3

Parietal cells

Question 4

How is acid secretion from the stomach increased?

Answer 4

• ACh, which is released from the vagus nerve
• gastrin which is secreted from G cells in the stomach
• enterochromaffin like cells in the stomach secrete histamine which binds to H2 receptors on the parietal cells

Question 5

What stimulus results in the release of ACh?

Answer 5

• released during the cephalic phase of digestion, which is activated upon seeing or chewing food, leading to direct stimulation of parietal cells via the vagus nerve.
• also produced during the gastric phase of digestion when intrinsic nerves detect distension of the stomach, stimulating the production of ACh by the vagus nerve.

Question 6

What results in the release of gastrin?

Answer 6

G cells are activated by:

• the vagus nerve
• gastrin related peptide
• peptides in the stomach lumen produced via protein digestion

Question 7

Function of gastrin.

Answer 7

• Gastrin is released into the blood and travels through the blood until it reaches the parietal cells.
• Gastrin binds to CCK receptors on the parietal cells which also elevates calcium levels causing increased vesicular fusion.

Question 8

What stimulus results in the release of histamine from enterochromaffin like cells?

Answer 8

These cells release histamine in response to the presence of gastrin and ACh.

Question 9

Function of histamine in digestion.

Answer 9

Histamine leads to increased fusion however it is via the secondary messenger cAMP as opposed to calcium in the other methods.

Question 10

How is acid production decresased?

Answer 10

• accumulation of acid in the empty stomach between meals
• the presence of chyme within the duodenum

Question 11

How does accumulation of acid in the stomach lead to decreased acid production?

Answer 11

• an increase in acid leads to a lower pH within the stomach
• D cells produce somatostatin to inhibit the secretion of gastrin

Question 12

How does the presence of chyme in the duodenum lead to decreased acid production?

Answer 12

• enterogastric reflex
  
  • sends inhibitory signals to the stomach via the enteric nervous system
  • signals to medulla result in reduced vagal stimulation
• entero-endocrine cells release cholecystokinin and secretin, both of which inhibit gastric acid secretion.

Question 13

What is the function of gastric mucus?

Answer 13

It acts as part of a barrier that protects the stomach wall from the acid and digestive enzymes within the stomach lumen.

Question 14

Which cells in the stomach is muscus secreted from?

Answer 14

Foveolar cells, found in the necks of the gastric pits.

Question 15

What do parietal cells produce?

Answer 15

• HCL
• Intrinsic factor

Question 16

What do cheif cells produce?

Answer 16

pepsinogens

Question 17

Why is the biocarbonate element of stomach mucus important?

Answer 17

It allows an increased pH local to the epithelial cells, protecting them from the highly acidic stomach environment.

Question 18

How are the biocarbonate ions in stomach acid produced?

Answer 18

• They are formed in the mucus-secreting cells by reacting carbon dioxide with water, using the enzyme carbonic anhydrase.
• The bicarbonate ions are then pumped into the mucus layer by exchanging them with chloride ions.