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?
Intrinsic factor production
What is the function of the antrum?
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)
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
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
Protects mucosal surface from mechanical injury
Neutral pH (HCO3) -> protects against gastric acid corrosion and pepsin digestion