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Flashcards in Physiology - Gastric Secretion Deck (23)

Whats the function of the Fundus of the stomach?


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


What is the function of the body of the stomach?

Mucous production
HCl Production
Pepsinigen production
Intrinsic factor production


What is the function of the antrum?

Gastrin Production

Substantial increase in thickness of the muscularis externa in the antrum


What is the cellular composition of gastric glands?

Mucous neck cells (secrete mucous)

Chief cells (pepsinogens)

Parietal cells (HCl and intrinsic factor)


What cells exist above the gastric gland?

Surface mucous cells.

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


Describe how HCl is produced by parietal cells

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


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

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

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

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


What molecules inhibit Hydrogen-Potassium ATPase?


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


What three mechanisms control gastric acid secretion?

Neurocrine (vagus/local reflexes)

Endocrine (gastrin)

Paracrine (histamine)


What are the two phases involved in gastric acid secretion?

The Cephalic Phase (in your head)

The Gastric Phase (once the food has been swallowed)


Describe the Cephalic phase in gastric acid secretion

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


Describe the Gastric Phase of gastric acid secretion

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


Describe how the Cephalic phase can inhibit gastric acid secretion

Stop eating and this decreases vagal activity


Describe how the gastric phase can inhibit gastric acid secretion

Increased HCl causes a decrease in pH.
This causes G cells to stop producing Gastrin


Describe how the intestinal phase can inhibit gastric acid secretion

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


What does GIP stand for?

Gastrin Inhibitory Polypeptide


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

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


What are the two effects that enterogastrones have?

Inhibit gastric acid secretion

Reduce gastric emptying (inhibit motility/contract pyloric sphincter)


What is the zymogen of pepsin?


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


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

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.


What are the mechanisms for pepsin control?

The mechanisms for pepsin control match the control of HCl secretion


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

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


What is intrinsic factor?

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