Session 4 - Functions of the stomach

Question 1

Where are gastric secretions produced? Name the 4 cell types found in this area and their secretions

Answer 1

Gastric pits

1) Chief cells - pepsinogen
2) Parietal cells - HCl
3) Neck cells - Mucus and alkali
4) G cells - Gastrin

Question 2

How is pepsinogen activated?

Answer 2

Activated by acid produced by parietal cells

Question 3

Explain the mechanism of secretion of stomach acid. What happens to the alkali?

Answer 3

Acid produced in parietal cells.

Parietal cells have high density of mitochondria which produces protons.

Canaliculi in the cells have proton pumps that secrete protons into the lumen of the stomach.

Alkali enters the blood and neutralises the acid in the duodenum.

Question 4

What 3 factors act on parietal cells to stimulate acid secretion?

Answer 4

• Gastrin – Hormone
• Histamine
• Acetylcholine

Question 5

How does gastrin function?

Answer 5

• Binds to surface receptor on parietal cell
• Stimulates acid and intrinsic factor (critical for absorption of vit b12) secretion via secondary messenger pathway

Question 6

How does histamine act on parietal cells? Where is it released from?

Answer 6

• Released from Mast cells
• Binds to H2 surface receptors on the parietal cells
• Stimulates acid secretion via cAMP.

Question 7

How does ACh act on parietal cells? Where is it released from? What blocks its action?

Answer 7

• Released from parasympathetic neurones
• Acts on muscarinic receptors
• Stimulates acid secretion by secondary messenger pathway
• Blocked by atropine

Question 8

What stimulates and inhibits gastrin secretion?

Answer 8

Stimulated by:

• Peptides in the lumen
• Ach released by nerves which are activated by the physical distension of the stomach

Inhibited by:

• Low pH in the stomach, important when there is no food in your stomach

Question 9

Why does blocking H2 receptors reduce acid secretions?

Answer 9

Histamine amplifies the effect of other messengers, therefore blocking histamine stops the amplification of the gastrin and ACh signal

Question 10

What stimulates mast cells?

Answer 10

Gastrin and ACh

Question 11

What occurs in the cephalic phase of gastric secretion?

Answer 11

The detecting and ingesting of food results in parasympathetic nerve stimulation, resulting in acid production in the stomach before food is ingested.

Question 12

What occurs in the gastric phase of gastric secretion?

Answer 12

• Food reaches the stomach and soaks up the hydrogen ions, so pH rises, disinhibiting gastrin
• Stomach actively relaxes to prevent pressure build up. Stretch receptors detect stretch and release Ach which stimulates more acid.
• Initial digestion releases peptides which stimulates more gastrin release.
• Ulcers hurt more at night due to acid build up without food to soak up the hydrogen ions.

Question 13

What stimulates ACh release?

Answer 13

distension of stomach

Question 14

What occurs in the intestinal phase of gastric secretion?

Answer 14

• Stomach begins to empty
• Accumulation of acid in empty stomach inhibits gastrin secretion.
• Chyme stimulates release of hormones from intestines which antagonise gastrin.

Question 15

Name 2 methods of reducing gastric acid secretion and their associated drug

Answer 15

Cimetidine – H2 receptor antagonist

Omeprazole (PPI) – proton pump inhibitors, even better.

Question 16

What is the luminal pH of the stomach

Answer 16

less than 2

Question 17

Explain how mucus protects the stomach for autodigestion.

Answer 17

Physical protection comes from mucus, mixtures of mucopolysaccharides, which are very sticky. This means that it adheres very tightly to the surface of the mucosa, exhibiting thixotropic property (forming a gap in the mucus means mucus will flow to fill the gap).

Mucus is basic and absorbs hydrogen ions:

• As hydrogen ions diffuse into the mucus they will eventually fill up the amine groups and overwhelm the buffering capacity
• However, due to stickiness an unstirred layer is formed which hydrogen ions find very difficult to diffuse through.
• Slow passage through unstirred layer allows hydrogen carbonate produced by the cells to neutralise the acid without it overwhelming the cells.
• CO2 produced, hence belching.
• Cells underneath tough enough to withstand pH of 6.

Question 18

What stimulates mucus and alkali secretions in the stomach?

Answer 18

Mucus and alkali secretion stimulated by prostaglandins which are promoted by most of the factors stimulating acid secretion.

Question 19

How does alcohol damage the stomach?

Answer 19

Alcohol dissolves the mucus. results in peptic ulcers

Question 20

How does H. pylori damage the stomach?

Answer 20

H. pylori gets into mucosa and prevents mucosa from defending itself, results in ulcers

Question 21

How does NSAIDS damage the stomach?

Answer 21

• NSAIDS inhibit prostaglandins and therefore reduce defences, e.g. aspirin which is converted into a non ionised form by stomach acid and then passes into the cells and re-ionises, killing them.
• Result of it all is peptic ulcers

Question 22

Explain the concept of receptive relaxation of the stomach, why it occurs, what triggers it, and what happens if you damage the vagus nerve

Answer 22

• Triggered by swallowing, which results in a neural reflex.
• Tension in resting stomach wall is actively relaxed
• Pressure therefore doesn’t build.
• If vagus nerve damaged you can’t get receptive relaxation and therefore can’t eat large volumes of food

Question 23

What drives the contractions of the stomach and how often does it fire?

Answer 23

Pacemaker in the cardiac region of the stomach. 3 times a minute.

Question 24

Describe the process which results in chyme being squirted into the duodenum.

Answer 24

o Due to wide to narrow nature of stomach, initially the wave of AP proceeds slowly, propelling before it the entire contents of the stomach towards the antrum

o As it moves past the widest part of the stomach it narrows and therefore starts to accelerate

o This leaves behind the lumps and propels forward the liquid, only propelling chyme towards the pyloric canal.

o A small squirt is then ejected and the wave reaches the pylorus and shuts it o The rest of the chyme returns to the stomach

Question 25

What affects the volume of each squirt of chyme into the duodenum?

Answer 25

o Rate of acceleration of peristaltic wave

o Hormones from intestine

Question 26

What 3 factors slow gastric emptying?

Answer 26

o Fat in duodenum

o Low pH in duodenum

o Hypertonicity in duodenum