Physiology of Gastric Motility and Gastric Acid Secretion Flashcards
what is the role of the stomach
stores food before passing food on to small intestine as chyme
continues carbohydrate digestion
begins protein digestion
name the two regions of the stomach
orad region
caudad region
where is the orad region
fundus to proximal body
where is the caudad region
distal body and antrum
explain what happens in the orad region
vagus nerve causes relaxation leading to the opening of the lower oesophageal sphincter
this allows food to move in to the cuadad part of the stomach where it is stored
minimal mixing of contents leads. to
protein digestion begins here
carbohydrate digestion continues
how do contents from the orad region lead to the caudad region of the stomach
via weak phasic contractions
phasic contractions are weak due to relatively thin muscular wall
what is the role of gastrin
decreases the rate of gastric emptying
what happens at the caudad region
slow waves occur simultaneously but only those reaching threshold will illicit contraction
propels contents from mid stomach –> gastroduodenal junction
explain retropulsion
increased velocity of contraction approaching the gastroduodenal junction overtaking chyme movement
small amounts of chyme and small particles pass through
what is the slow wave eliciting the contraction AKA
antral wave or pump
what has an effect on gastric emptying
gastric and dodenal factors
list some gastric factors
consistency of chyme
proportional to volume of chyme
list some duodenal factors
fat - delay gastric emptying products of protein and carbohydrate digestion can draw water into the. small intestine and decrease plasma volume hormonal relex enterogatstric reflex acid - time for neutralisation
describe the gastric mucosa
surface lining the stomach
pits, invaginations of the surface
glands at the bottom of the its pits
the stomach can be split into
oxyntic gland and pyloric gland
pyloric gland has what kind of cells
D cells - secrete somatostatin
G cells - secretes gastrin
oxyntic mucosa has what kind of cells
chief cells - secrete pepsinogen
mucous cells - secrete histamine
parietal cells - HCL, intrinsic factor, gastroferrin
what is the role of HCl
pepsinogen –> pepsin
denature proteins
kill most micro-organisms
what type of cells does Hcl acid come from
parietal cells
intrinsic factor is responsible for
binding Vit B12
Gastroferrin is responsible for
binding Fe2+
What is pepsinogen
inactive precursor of pepsin
the role of histamine
stimulate HCl acid secretion
G cells
secrete HCl
Somatostatin
inhibits HCl
Mucus
Protective
How is hydrochloric acid secreted
CO2 + H20 join to form H2CO3 by carbonic anhydrase
H passes into the lumen via H/K ATPase
K then flows back across
K flows into the plasma via K+ channel and Na/KATPase
Na flows out from inside the cell
HCO3- exits the cell via HCO3/Cl antiporter
CL enters the cell
Cl enters the lumen via a chlorine channel CFTR
Cl and H fuse together to form HCl
what induces acid secretion
Acetylcholine
gastrin
histamine
how do acetylcholine, gastrin and histamine act
via direct and indirect mechanisms
Direct pathway
ACh, gastrin and histamine stimulates parietal cell –> brings H into lumen
indirect mechanism
ACh and gastrin stimulate ECL cells to secrete histamine and histamine then activates the parietal cell to bring hydrogen into lumen
by what mechanism does gastrin and acetylcholine work
activates PLC –> IP3 –> H
by what mechanism does histamine work
activates cAMP which activates PKa –> H
by which mechanisms do somatostatin and prostaglandin inhibit H secretion
preventing cAMP activating pKa and thereby preventing H entering the lumen
what are the 3 phases of gastric acid secretion
cephalic
gastric
intestinal
describe the stimulatory effects of gastric motility
cephalic - before food reaches stomach –> activation of the vagus nerve allows opening of the lower oesophageal sphincter
gastric - mix of chemical and physical mechanisms when food is in the stomach
intestinal - after food has left the stomach when chyme leaves the stomach
describe the inhibitory effects of gastric motility
cephalic - inhibition of vagus nerve CNX –> constriction of lower oesophageal sphincter
gastric - antral pH falls –> activates delta cells to release somatostatin –> stop gastric emptying
gastric - release of prostaglandin E2 –> prevent histamine from HCl secretion
Explain in detail the cephalic phase
slight smell, taste of food
vagal activation –> enteric neurones
Increased ACh increase parietal cells directly
Increased ACh can stimulate ECL cells to increase histamine and stimulate parietal cells
Increase in GRP can stimulate G cells to release gastrin and activate parietal cells
Parietal cells increase secretion
Increase in ACh can inhibit Delta cells from releasing somatostatin and therefore decrease its inhibitory effect on parietal cells
Explain in detail gastric phase
when food is in the stomach
Distension causes activation of mechanoreceptors which activate G cells to increase gastrin and thereby activate parietal cells
Distension also causes increase in ACh and thereby activates parietal cells
Increase pH causes inactivation of D cells and therefore decrease somatostatin and decrease inhibitory effect on parietal cells
Protein digestion activates G cells to increase gastrin and thereby parietal cells
