L4: secretions of the stomach

Question 1

Functions of the stomach

Answer 1

• stores food - compliant
• kills bacteria with acid
• liquefies food
• mixes chyme with gastric secretions
• kneading of food particles to less than 1mm
• regulates chyme release into duodenum
• produces intrinsic factor
• very little digestion (proteins) and absorption (alcohol, aspirin) takes place in the stomach

Question 2

Microscopic view of the stomach

Answer 2

Has many indents which are gastric pits which contain cells that secrete mucous, hydrochloric acid and enzymes into the stomach.

Question 3

What cells make up the gastric pits and what substances do they secrete?

Answer 3

Mucous neck cells - mucous and bicarbonate
Parietal cells - gastric acid (HCl) and intrinsic factor
Enterochromaffin-like (ECL) cell - histamine
Chief cells - pepsin and gastric lipase
D cells - somatostatin
G cells - gastrin

Question 4

What secretes mucous and what is the function?

Answer 4

Goblet (surface and mucous neck cells)

They neutralise acid at the lining of the stomach to prevent damage to the stomach wall

Question 5

What secretes gastric acid and what is it’s function?

Answer 5

Parietal cells
Mechanical digestion - denatures proteins
Chemical digestion - pepsinogen into pepsin which denatures and digests proteins
Converts poorly absorbed ferric iron (Fe3+) into absorbable ferrous iron (Fe2+)
Causes iron deficiency anaemia without acid

Question 6

What secretes pepsinogen and what is it’s function?

Answer 6

Chief (zymogen) cells

Starts protein digestion in the stomach

Question 7

What secretes histamine and what is it’s function?

Answer 7

Entereochromaffin-like cells

Stimulates acid secretion from parietal cells

Question 8

What secretes gastrin and what is it’s function?

Answer 8

G - cells
Stimulates acid secretion from parietal cells
Stimulates ECL. Ells to release histamine

Question 9

What secretes somatostatin and what is it’s function?

Answer 9

D-cells

Inhibits acid secretion from parietal cells

Question 10

What secretes intrinsic factor and what is it’s function?

Answer 10

Parietal cells
Aids absorption of vitamin B12 from the ileum - vitB12 is essential for RBC maturation
So causes pernicious anemia without the intrinsic factor

Question 11

How is acid secreted?

Answer 11

HCl production is is stimulated by gastrin, ACh and histamine
Stimulates parietal cells to secrete HCl
Increases hydrogen potassium ATPases and chloride channels on parietal cells
Energetically expensive process

Question 12

How are stimulators signals delivered to the parietal cell to induce HCl secretion?

Answer 12

Paracrine - histamine released from a neighbouring gastric ECL cell
Endocrine - gastrin released from stomach G cells into the blood, where it comes back to the stomach and stimulates the parietal cell
Neurocrine - ACh release from neurones of the vagus nerve to act on parietal cell

Synergistic response

Question 13

What are the receptors on the parietal cells?

Answer 13

Histamine H₂ receptor
Gastrin receptor CCK’B
ACh M3 receptor (muscarinic 3)

Question 14

What is the most potent acid secretor?

Answer 14

Histamine, but when all 3 stimulator present, maximum secretion occurs

Question 15

Resting parietal cell properties

Answer 15

Full of mitochondria as acid production uses lots of energy
Has lots of tubulovesicles containing proton pumps which can come together to increase the parietal cell surface area when acid needs to be secreted.

Question 16

Stimulated parietal cell properties

Answer 16

Histamine, gastric and ACh will signal tubulovesicles to come together to produce canaliculi which increase the cell surface area ready for acid secretion and insert proton pumps on the surface

Question 17

How does HCl get to the stomach lumen?

Answer 17

• when eating, parietal cells are stimulated with gastrin, histamine and ACh
• the tubulovesicles put many proton pumps/hydrogen potassium ATPases on the stomach lumen side of the parietal cell
• carbon dioxide from the blood will enter the parietal cell and bind with water forming carbonic acid using carbonic anhydrase which dissociates to bicarbonate and a proton
• the proton gets pumped out into the stomach lumen, using energy
• the bicarbonate moves out into the blood (causing alkaline tide) and chloride moves through the parietal cell and out to the stomach lumen

Question 18

Why does the alkaline tide occur?

Answer 18

Effluent of lots of bicarbonate ions from the parietal cells into the blood after eating a meal disturbs the plasma buffer system.
The bicarbonate ions would react with the circulating protons, increasing plasma pH, detected by a temporary rise in pH = alkaline tide

Question 19

Why does acid secretion need to be controlled?

Answer 19

Acid should only be secreted when food is in the stomach. If the stomach is empty, acid secretion should not occur as it could irritate the stomach lining.

Question 20

What is the cephalic phase of acid secretion?

Answer 20

An entirely neural phase in which the thought, sight, smell or approach of food leads to ACh release from the vagus nerve. Chewing or swallowing can do this too. Produces a small amount of acid.

Question 21

What is the gastric acid phase of acid secretion?

Answer 21

Once food is in the stomach, it can distend it. This is detected by mechanoreceptors that lead to ACh release. Also, the products of protein digestion (peptides and amino acids) in the stomach are detected by chemoreceptors that can lead to gastrin release. Largest phase and lasts the longest, hours.

Question 22

What is the intestinal phase of acid secretion?

Answer 22

Primarily an acid inhibitory phase. Once food enters the duodenum, HCl secretion needs to be turned off. Presence of HCl in the duodenum leads to somatostatin, secretion and CCK (cholecystokinin) release, which all inhibit acid secretion from parietal cells.
Secretion also stimulates bicarbonate release from the pancreas, which neutralises acid entering the duodenum.
CCK is also stimulated by lipids entering the duodenum. CCK can also stimulate digestive enzyme and bile release from pancreas and gallbladder respectively.

Question 23

Where are secretin and CCK released from?

Answer 23

Secretin is released from duodenal S-cells
CCK is released by duodenal I-cells
Both endocrine release

Question 24

What stops the efflux of acid into the oesophagus and unnecessary leakage of acid into the duodenum?

Answer 24

Lower Oesophageal sphincter (LOS) stops the efflux of acid into the oesophagus
Pyloric sphincter stops leakage of acid into duodenum

Question 25

What is Barrett’s Oesophagus?

Answer 25

Pre-cancerous condition bought about by persistent acid reflux

Question 26

What are duodenal ulcers?

Answer 26

Erosions on the first part of the small intestine due to acid

Question 27

Peptic ulcer treatments (anti-acids)

Answer 27

H₂-receptor antagonists block the action of histamine receptor, e.g. cimetidine, ranitidine
Proton-pump inhibitors block ATP driven hydrogen and potassium ion exchange. More effective than hydrogen receptor antagonists. E.g. omeprazole, lansoprazole

Question 28

How can reflux disease affect the mouth?

Answer 28

Reflux disease causes tooth decay
Dentist is often the first to notice it
Patient may not be aware - asymptomatic reflux
If acid attacks the teeth aggressively: high sensitivity rate, lots of cavities, lose vertical dimension
Could wear mouth guard at night
Bulimia has similar effects

Question 29

Helicobacter pylori

Answer 29

• bacterium strongly associated with peptic ulcer disease (PUD)
• linked with gastric cancer
• rate of re-infection after antibiotics is high
• route of transmission unclear, oral-oral?
• may survive on dental plaque
• dentists may be at greater risk of infection and may aid transmission