Physiology - Gastric Secretion

Question 1

Whats the function of the Fundus of the stomach?

Answer 1

Storage

Smooth muscle at the top near the fundus is quite thin so lots of stretching

Question 2

What is the function of the body of the stomach?

Answer 2

Storage
Mucous production
HCl Production
Pepsinigen production
Intrinsic factor production

Question 3

What is the function of the antrum?

Answer 3

Mixing/Grinding
Gastrin Production

Substantial increase in thickness of the muscularis externa in the antrum

Question 4

What is the cellular composition of gastric glands?

Answer 4

Mucous neck cells (secrete mucous)

Chief cells (pepsinogens)

Parietal cells (HCl and intrinsic factor)

Question 5

What cells exist above the gastric gland?

Answer 5

Surface mucous cells.

These are arranged in the gastric pit which leads to the gastric gland

Question 6

Describe how HCl is produced by parietal cells

Answer 6

CO2 exists as a metabolic waste product in blood.
Moves into parietal cells
With the help of carbonic anhydrase it combines with water to form carbonic acid.
This is only semi stable so quickly dissociates to H+ and bicarbonate.
H+ pumped out of cell for K+ my Hydrogen-Potassium ATPase.
Bicarbonate pumped into blood in exchange for Cl.
Chlorine moves into lumen with H and forms HCl.

The movement of ions causes water to move pancellularly and hydrochloric acid is formed

Question 7

What are the 3 signalling molecules which stimulate hydrogen-potassium ATPase?

Answer 7

Gastrin (through cell receptor
->Ca ->Protein Kinase)

Histamine (G protein -> ATP become cAMP -> Protein Kinase)

Acetylcholine (through cell receptor -> Ca -> Protein Kinase)

Question 8

What molecules inhibit Hydrogen-Potassium ATPase?

Answer 8

Prostaglandins

Receptor inhibits adenylyl cyclase which produces cAMP from ATP.
Prevents Histamine producing protein kinases

Question 9

What three mechanisms control gastric acid secretion?

Answer 9

Neurocrine (vagus/local reflexes)

Endocrine (gastrin)

Paracrine (histamine)

Question 10

What are the two phases involved in gastric acid secretion?

Answer 10

The Cephalic Phase (in your head)

The Gastric Phase (once the food has been swallowed)

Question 11

Describe the Cephalic phase in gastric acid secretion

Answer 11

Sight, smell or taste of food activates the parasympathetic system.
Increased vagal tone has 2 effects:
-Acetyl Choline released
-G Cells produce gastrin
Both these stimulate parietal cells

Gastrin/ACh also stimulates ECL cells to produce histamine which also stimulates Parietal cells

Question 12

Describe the Gastric Phase of gastric acid secretion

Answer 12

Distension of the stomach due to the arrival of food causes Vagal/Enteric reflexes.
ACh is produced

Peptides in the lumen stimulate G cells to produce gastrin

Gastrin/ACh stimulates ECL cells to produce histamine

Question 13

Describe how the Cephalic phase can inhibit gastric acid secretion

Answer 13

Stop eating and this decreases vagal activity

Question 14

Describe how the gastric phase can inhibit gastric acid secretion

Answer 14

Increased HCl causes a decrease in pH.

This causes G cells to stop producing Gastrin

Question 15

Describe how the intestinal phase can inhibit gastric acid secretion

Answer 15

Acid enters the duodenum as the stomach empties.
This has 2 effects:
-Enterogastric (splanchnic) reflex
-Secretin release

This decreases gastric secretion and so decreases the stimulation of parietal cells

Fat in the duodenum also causes the release of GIP.
This decreases gastrin secretion

Question 16

What does GIP stand for?

Answer 16

Gastrin Inhibitory Polypeptide

Question 17

What are Enterogastrones?
Give some examples
What do they do?

Answer 17

Hormones released from gland cells in duodenum mucosa (Secretin, Cholecystokinin (CCK), GIP)

Released in response to acid, hypertonic solutions, fatty acids or monoglycerides in duodenum

Act collectively to prevent further acid build up in the duodenum

Question 18

What are the two effects that enterogastrones have?

Answer 18

Inhibit gastric acid secretion

Reduce gastric emptying (inhibit motility/contract pyloric sphincter)

Question 19

What is the zymogen of pepsin?

Answer 19

Pepsinogen

It is secreted by chief cells
By storing a zymogen cellular digestion is prevented

Question 20

Unlike most other enzymes what happens to pepsin at neutral pH’s?

Answer 20

Pepsin is inactivated at neutral pH’s

Only works at very low pH.
This means that if it enters the duodenum it doesn’t cause havoc.

Question 21

What are the mechanisms for pepsin control?

Answer 21

The mechanisms for pepsin control match the control of HCl secretion

Question 22

What is the function of Gastric mucous and how is it produced?

Answer 22

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

Question 23

What is intrinsic factor?

Answer 23

Only essential (non-compensated) function of stomach

Produced by parietal cells and required from Vitamin B12 absorption

Intrinsic factor/B12 complex absorbed from ileum

Defect is called Pernicious Anaemia