Gastric Secretion Flashcards
What is the function of the fundus of the stomach?
Storage
What are the functions of the body of the stomach?
Storage Mucus HCl (stomach acid) Pepsinogen Intrinsic factor
What are the functions of the antrum?
Mixing/grinding
Gastric
Why does the stomach not digest itself?
Enzymes are stored in inactive precursor forms
What is the content of mucous neck cells?
Mucus
What is the content of chief cells?
Pepsinogens
What are contents of parietal cells?
HCl
Intrinsic factor
What regulates low pH in stomach and higher pH in blood?
CO2 combines with water in cytoplasm (carbonic anhydrase present) = H2CO3
H ion pumped out against massive gradient as K moves into cell using ATP hydrolysis
Bicarbonate is exchanged for chloride which moves through cell and out with H ion forming osmotic gradient
Causes pH in stomach to drop = acidic
Bicarbonate in blood causes increase in blood pH
What are the 3 stimulatory signalling substrates involved with gastric secretion?
Gastrin
Histamine
Acetylcholine
What is an inhibitory signalling substrate involved with gastric secretion?
Prostaglandin
What effect does gastrin have on proton pump?
Increases activity
How is cyclase produced?
Type II histamine receptors couple with stimulator gene
What type of cells have receptors for gastrin? + effect
Parietal cells
Causes rise in intracellular calcium
Where does acetylcholine come from?
Vagus and enteric nerves
What happens to acetylcholine when it binds with receptor?
Generates into calcium which adds to protein kinases
What effect does prostaglandin have?
Prostaglandins couple up with glandin 3 receptors which are bound up to inhibitory gene which prevents ATP turning into cAMP
Affects production of protein kinases = inhibits proton pump
What 3 mechanisms control gastric acid secretion?
Neurocrine (vagus/local reflexes)
Endocrine (gastrin)
Paracrine (histamine)
What happens in the cephalic phase? + result
Sight, smell, taste and or thought of food causes stimulatory response -> increase vagus nerve stimulation -> acetylcholine
Increase vagus nerve stimulation also causes G cells to produce gastrin
Gastrin and ACh trigger ECL cells to release histamine
Result = more HCl
How is cephalic phase inhibited?
By stopping eating which decreases vagal activity
What happens in the gastric phase?
Distension of stomach causes vagal/enteric reflexes triggers release of ACh
Peptides in lumen causes G cells to release gastric
Gastrin/ACh causes ECL cells to release histamine
How is gastric phase inhibited?
Decrease in pH (increase in HCl) causes decreased gastrin
How is intestinal phase inhibited?
Acid in the duodenum causes enterogastric reflex and secretin release which causes decrease in gastrin secretion and decrease in gastrin stimulation of parietal cells
Fat/CHO in duodenum causes gastric inhibitory peptide release which results in decrease gastrin secretion and parietal HCl secretion
Why does HCl need to be reduced in small intestine?
Enzymes secreted from the pancreas have an optimum pH of about neutral - acidic pH from HCl would denature those enzymes
What are enterogastrones?
Hormones released from gland cells in duodenal mucosa (e.g. secretin, cholecystokinin (CCK), GIP)
When are enterogastrones released?
In response to acid, hypertonic solutions, fatty acids or monoglycerides in the duodenum
What is the function of enterogastrones?
Act collectively to prevent further acid build up in duodenum
What are the 2 strategies for enterogastrones to carry out their function?
Inhibit gastric acid secretion
Reduce gastric emptying (inhibit motility/contract pyloric sphincter)
What causes hypertonicity in duodenum?
Molecules coming out of the stomach
What is pepsinogen secreted by?
Chief cells
What in the inactive precursor of pepsinogen?
Zymogen
What is the effect of low pH < 3 on pepsinogen?
Turns into pepsin through acid hydrolysis
What does the storage of zymogen prevent?
Cellular digestion
What are pepsins inactivated?
At a neutral pH
What is gastric mucus produced by?
Surface epithelial cells and mucus neck cells
What is the cytoprotective role of gastric mucus?
Protects mucosal surface from mechanical injury
Neutral pH protects against gatric acid corrosion and pepsin digestion
What is the only essential function of the stomach?
Intrinsic factor
What is intrinsic factor produced by?
Parietal cells
What is intrinsic factor required for?
Vitamin B12 absorption
Where is the intrinsic factor/B12 complex absorbed from?
Tail end of the ileum
