physiology of gastric secretion Flashcards
1
Q
what are the anatomical divisions of the stomach?
A
fundus
body —-> largest portion
antrum
2
Q
what are the regions of stomach?
A
orad region ——> upper 2/3 of the body + the fundus
caudad region ——-> lower 1/3 of the body + the antrum
3
Q
describe the orad region ?
A
it is the upper 2/3 of the body
thin wall and it is large
4
Q
what is the function of orad region ?
A
food storage
5
Q
describe the caudad region?
A
lower 1/3 of the body + the antrum
the wall is much thicker
6
Q
what is the function of caudad region?
A
thicker wall for stronger contractions for food mixing
7
Q
what is the function of pyloric sphincter?
A
regulates the passage of partially digested food from the stomach to the duodenum and prevents backflow
8
Q
compared to the GIT how many muscle layers do we have in stomach?
A
muscularis externa :
in GIT : 2 layers
in the stomach : 3 layers
9
Q
what are the 3 muscle layers of the stomach?
A
outer longitudinal
middle circular
inner oblique —> unique to the stomach only
10
Q
what is the function of outer longitudinal layer ?
A
peristalsis and stomach contractions ( assist )
11
Q
what is the function middle circular layer?
A
forms the pyloric sphincter and help with mixing food
12
Q
what is the function of the inner oblique layer?
A
enhances churning and mixing food with gastric juices ( only present in the stomach )
13
Q
what happens to the thickness as you go from proximal to distal parts of the stomach?
A
the muscle wall thickness increase as you go from proximal to the distal stomach
to support stronger contractions for digestion
14
Q
what are the motor functions of the stomach ?
A
storage of large quantities of food
mixing of food and propulsion
slow emptying chyme
15
Q
describe the storage of large quantities of food motor function of the stomach?
A
stretch of the stomach with food ——–> vagovagal reflex ——–> initiate and reduce the muscle tone wall —–> the muscles become stretched and flabby —> Expand and accommodate large quantities of food up to 0.8 - 1.5 liters
16
Q
what reflex is responsible for the storage function?
A
vagovagal reflex
17
Q
describe the mixing and propulsion motor function of the stomach?
A
when the food reaches the DISTAL parts of the stomach —->
slow waves originating from interstitial cells OF CAJAL ( stomach pacemaker ) —-> initiates the basic electrical rhythm ( BER ) ——->
as the food and the waves reach the antrum the waves become stronger and develop into peristaltic waves —–>
during the this process the action potentials are generated with spike potentials forming on top of the slow waves —> the spikes become full blown action potentials —> peristalsis
when peristalsis start , constrictors rings form to propel the food towards the antrum
18
Q
what structures play role when the food reach pylorus?
A
1- pyloric pump ( aka pyloric canal )
2- pyloric sphincter
19
Q
what is the function of pyloric pump?
A
functional pumping mechanism that help regulate the flow of food in the duodenum
20
Q
what is the function of pyloric sphincter?
A
thickened muscular ring that controls the passage of chyme ( Digested food )
21
Q
together what is the ultimate function of both pyloric pump and pyloric sphincter?
A
regulate the amount of chyme entering the duodenum and aid in mixing
22
Q
what happens to food that is not been mixed properly?
A
pushed back into the stomach in a process called like RETROGRADE MOVEMENT/ RETROPULSION to ensure proper mixing with the gastric juices before it can pass through the pyloric sphincter
22
Q
describe the slow emptying of chyme motor function of the stomach?
A
chyme is emptied gradually into the small intestine
the small intestine is narrower than stomach and it is not designed for food storage
the primary function of small intestine is to further digest food and absorb nutrients
23
Q
what is required for the small intestine to absorb and digest food ?
A
food consistency : paste like form —> large food particles are undesirable because they can damage the delicate mucosa of the duodenum ( they are retropulsed )
TIME : it needs to be controlled and slow and it needs TIME to perform digestion and absorb efficiently
24
what are the components of gastric juices?
hydrochloric acid ( HCL )
Pepsinogen
intrinsic factors
mucus
25
what is the function of hydrochloric acid ?
sterilization of meals -----> eliminates pathogen ingested with food
hydrolysis of dietary macromolecules -----> break down macromolecules
converts pepsinogen to pepsin -----> convert inactive pepsinogen into active pepsin in the acidic environment , eventually leading to protein digestion
26
what is function pepsinogen?
once becoming pepsin it will initiates protein digestion and its completed later in the intestine
activated by HCL
27
why is pepsinogen secreted in the inactive form and not active ?
to prevent self digestion of gastric cells
28
what produces intrinsic factor ?
parietal cells
29
what is the function of intrinsic factor ?
facilitates the absroption of vitamin B12 in the ILEUM
B12 is used for maturation of RBCS ---> lack of intrinsic factor ---> pernicious anemia
30
what secretes mucus?
mucosal glands in the stomach
31
what is the function of mucus?
protects the gastric mucosa from the corrosive action of HCL
provide lubrication of gastric contents for smooth passage
act as a barrier to prevent pathogens from reaching the gastric lining
32
what happens when you have low mucus or damage to mucus barrier ?
gastric secretion penetrating the mucosa = Ulcers
33
what are the type of glands found in the stomach?
oxyntic glands : found in the body and fundus of the stomach
antral/pyloric gland : located in the antrum , these glands contain specialized cells that secrete substances into the lumen of the stomach through gastric pits
34
what is the location and function of parietal cells?
HCL + intrinsic factor
body of the stomach ( oxyntic glands )
35
what is the locations and function of chief cells ?
pepsinogen ( inact )
body of the stomach ( oxyntic glands )
36
what is the location and function of G cells ?
gastrin
antrums ( antral / pyloric glands )
37
what is the function and location of mucous cells?
produce mucus and pepsinogen ( pep in little amount, the main source is chief cells )
antrums ( antral / pyloric glands )
38
what is the location and function of D cells?
produce somatostatin ---> to inhibit gastrin
found in the antrum mainly + the body
39
what is the function and location of enterchromaffin-like cells ? (ECL )
produce histamine ( which stimulates HCL )
oxyntic glands ( found in the body and fundus )
40
why is G cells found in the antrum but not upper part of the stomach ?
the antrum is proximity to the pylorus allow the gastrin to regulate chyme acidity before it enters the duodenum
41
describe the function antrum in relation to the PH regulation?
antrum play key role in detecting the PH of the stomach contents
if the acidity is too low ( high PH ) -----> gastrin secretion is increased to stimulate the HCL from parietal to increase the acidity
placing the G cells here ( in the antrum ) allow for better regulation of acid production based on chyme readiness for digestion in the duodenum
42
what cell is primary source of pepsinogen?
chief cells
43
why is the regulation of gastrin through somatostatin is necessary ?
excess gastrin would lead to excessive HCL production which can damage stomach lining and disrupt PH balance
44
describe the oxyntic glands? ( gastric glands )
found in the body and fundus ( 80% of the stomach )
empty their secretory products via ducts into the lumen of the stomach
45
what are pits?
opening of the gland ducts onto the gastric mucosa
46
what are cells that present deep in the gland?
mucous neck cells ( found in both antrum and body so oxyntic and pyloric gland )
parietal cells ---> Secrete intrinsic factor and HCL
chief cells ( aka peptic cells ) --------> secrete pepsinogen
47
why are mucous cells called mucous neck cells ?
cuz mostly you will find them in the neck of the gland
48
describe the parietal cells secretory mechanism?
have small spaces called CANALICULI where HCL will be collected and then will come out through the gastric PIT and into the stomach
both HCL and intrinsic factor
49
describe the chief cells location and secretory mechanism ?
located in the base of the gastric glands as it ascends it will encounter HCL from parietal cells ( which will activate pepsinogen for protein digestion )
50
which cell type in the oxyntic glands is stimulated by gastrin and what does it secrete?
ECL ---> stimulated by gastrin to release Histamine to increase HCL production
51
what regulates the D cells to produce somatostatin ?
regulated by acidity and neural input---> when too high , somatostatin which inhibit gastrin release --> Decrease HCL
52
describe the pyloric glands ( antral glands )?
glands of the antrum of the stomach ( Distal 20% of the stomach )
53
what gland cells are found in the antrum?
G cells ---> gastrin ( not into the pyloric ducts but to the circulation )
Mucous neck cells ( it is found in both body and antrum but mainly antrum )
secrete mucus, HCO3 , pepsinogen ( mucous cells )
54
what is the function of Mucus and HCO3?
protective neutralizing effect on the gastric mucosa
neutralization is important in the antrum because it the gateway to the duodenum which cannot handle acidity ( bicarbonate/HCO3 ) neutralize
55
describe the mechanism of action of gastrin ?
released by G cells and doesnt act locally in the stomach
it enters the blooodstream and circulate before reaching target cells
parietal cells in the gastric glands ( is the target )
this process take long time causing the gastrin to influence HCL secretion later in digestion
56
what is the purpose behind the delay of gastrin?
stomach needs to maintain optimal PH for enzyme activation ( like pepsinogen ) and food breakdown
57
in net what is the things that regulate the parietal cell?
Gastrin :
direct ---> gastrin act directly on parietal cells via bloodstream to increase HCL secretions
indirect ----> gastrin stimulates ECL ( body) cells --> histamine --> enhance HCL production as paracrine signal
Inhibition :
D cells ---> somatostatin --> inhibit gastrin when acid levels are high ---> maintaining balance
58
how do G cells differ from parietal cells in terms of their secretions?
G cells secrete gastrin into the bloodstream ( hormonal secretion )
while parietal cells secrete HCL and intrinsic factor directly into gastric lumen
59
what are 2 membranes / sides of the parietal cell?
canicular membrane / side --> facing canaliculus ( empty spaces )
basolateral membrane / side ---> facing interstitial fluid / blood
60
what is canaliculus?
small intracellular channel where HCL is secreted into the stomach lumen
61
what is the function of canaliculus membrane ?
responsible for the production and release of HCL
62
what are the things found in the canalicular membrane ?
Proton pump (H+/K+ atpase )
chloride ions
sodium enter from this side ( to maintain the function of the Na/K atpase on the basalateral side aka the interstitial fluid side )
potassium once it enter via H/K atpase will exist from this side as well
63
describe the action of proton pump ( H+/K + atpase )
actively secrete hydrogen ions H into the canaliculus in exchange for potassium ions K +
Take H out and take K inside ( potassium )
requires ATP because its moving K against its gradient ( potassium is higher inside than outside )
64
describe the chloride flow on the canaliculus membrane ?
the chloride ions ( CL ) will flow outside to the canaliculus
the Chloride once it leaves it will go to the H that was sent outside by the atpase pump and fuse with it to form HCL in the canaliculus
65
how are H generated within the parietal cell?
H2O enter the cell via osmosis
its broken down into H and HO
H is sent outside
OH stays
66
what happens to OH ?
combine with CO2 to form HCO3
OH + CO2 = HCO3
67
what enzyme catalyzes OH + CO2 = HCO3?
Carbonic anhydrase
68
what are the thing on the basolateral membrane ? ( the interstitial fluid side )
Na/K atpase
Cl / HCO3 exchanger
chloride enter here and head out via canaliculi membrane ( directly )
H2O pass via osmosis ( break into H and OH )
CO2 enter and leave from here ( enter and fuse with OH to form HCO3 )
69
describe the function of Na/K atpase ?
Pumps 3 Na outside ( Na enter the cell )
put 2 k into the cell ( the K will leave via the canacu side )
70
describe the CL/HCO3 exchanger mechanism ?
chloride enter and travel towards the canaliculi membrane and leave
bicarbonate HCO3 will move OUT in exchange with CL ( will move out to the blood )
71
what will happen due to HCO3 leaving to the blood?
it is alkaline so it will make the blood more alkaline forming an ALKALINE TIDE ---> occurs during active gastric acid secretion ( more HCL is formed and more HCO3 is sent out )
72
what is alkaline tide ?
temporary increase in blood alkalinity after a meal due to bicarbonate HCO3 released into the blood from parietal cells during the production of HCL
when the acid goes into the stomach the HCO3 will go the blood to keep things balanced
the result is slight rise in blood PH after eating
73
how do we balance the alkaline tidal?
the pancreas uses the bicarbonate to neutralize stomach acid when chyme enter the duodenum to restore PH
74
describe the characteristics of HCL stomach secretion?
acid solution contains about 160 mmol/L of hydrochloric acid
the PH of this acid is about 0.8 = is extremely acidic
the hydrogen ion concentration is about 3 million times that of the arterial blood
requires 1500 calories of energy per liter of gastric juices
75
how is gastric juice created?
since CL and H are secreted into the lumen ----> increase the solute concentration compared to fluid
osmolarity increase so the the water will start to move from the surrounding tissue to the lumen to balance it out ---> gastric juice
76
what is the composition of gastric juice ?
Water
HCL
Enzymes ( pepsinogen )
mucus
77
describe the function of leaky potassium channels on the canalicular membrane ?
they are leaky channels on the canalicular membrane
allow potassium to leak inside the cell to the canalicular lumen
this will help the H/K Atpase function properly because it needs to take a Potassium inside to let a hydrogen out
maintain the electrochemical gradient across the parietal cell membrane
78
what are 3 main primary stimuli of gastric secretion?
gastrin ---> stimulate ECL to release histamine
histamine
ACH ---> stimulated by the vagus nerve ----> directly stimulates the parietal cells and chief cells and mucus cells
79
what other stimuli stimulate the gastric secretions?
protein in food
gastrin releasing peptide ( GRP ) from vagus nerve
80
what are some inhibitory signals that inhibit the gastric secretions?
somatostatin ( SST ) secreted by D cells in response to high acidity
81
what is the receptor on the parietal cells respond to histamine?
H2 receptors
82
what distinguishes ACH from gastrin and histamine in its effect on gastric cells?
ACH stimulates all 3 cells --> parietal , chief , mucus cells
Gastrin and histamine only stimulate parietal cells
83
what are mechanisms of activating parietal cells?
Apical surface amplification
Receptor pathway
Synergistic effect
paracrine and endocrine actions
84
describe the amplification of apical surface area?
when the apical surface is increased ----> it increases the density of H/K atpase pump molecules --> enhanced acid secretion
85
describe the receptor pathway?
Gastrin ---> bind to CCK ---> Increase calcium
Ach ----> bind to M3 receptor --------> increase calcium
Histamine -----------> bind to H2 receptor ---> CAMP and it is not calcium pathway
86
describe the synergistic mechanism?
if all 3 work at the same time the effect is greater than the sum of their individual
to achieve maximum H/K atpase activity ----> all 3 should be there
87
describe the paracrine and endocrine actions?
histamine works locally ---> paracrine
because ECL and parietal cells are located next to each others
Gastrin works distally -----> endocrine
why? cuz it travels through bloodstream
somatostatin works locally ----> PARACRINE
because D cells and G cells are proximal to each others ( will inhibit nearby G cells )
88
what are the phases of gastric acid secretion?
cephalic phase
gastric phase
intestinal phase
89
what is the function of parasympathetic system on pepsin and acid?
it stimulates pepsin and acid production
90
describe cephalic phase ?
acid production will start before even food enter the stomach
30% of the HCL secretion occur in this phase
stimulated by smell, sight, thought , taste of food
91
from where does the signals originate for the cephalic phase?
cerebral cortex
appetite centers of amygdala
hypothalamus
92
describe the mechanism of cephalic phase?
neural via the vagus nerve ---> ACH ---> ACH directly activate parietal cells, chief cells , G cells
indirect stimulation --> gastrin released from G cells will stimulate parietal cells to produce HCL
93
describe the gastric phase ?
the food enter the stomach ----> distention of the antrum ------------> stimulate the release of gastrin
Food breakdown products ---> proteins , amino acids , small peptides directly stimulate G cells
60% of HCL secretion
94
describe the mechanism of gastric phase ?
Local enteric reflexes ---> distention of stomach --> neural reflex within the enteric nervous system ---> stimulate G cells and parietal cells
Gastrin mechanism :
ECL cells and parietal cells ---> Gastrin will come and stimulate histamine release --> enhance Parietal cells
Vagovagal reflex :
vagus nerve in response to the stomach distention -----> stimulate both parietal cells and G cells
so distention contributes in 2 ways :
1- local enteric reflex
2- vagovagal reflex
95
describe intestinal phase ?
10% of HCL secretion
when food reaches the duodenum
it is primarily an inhibitory phase responsible for regulating the secretion of gastrin + maintain the Slow gastric emptying
96
what are the components of the intestinal phase that take part in duodenal distention ?
Reverse enterogastric reflex
hormonal regulation
97
how does reverse enterogastric reflex work?
neural
stimuli : distention , acidity , irritation to mucosa , presence of fats or protein breakdown products
98
what is the function of secretin?
inhibit gastric acid secretion and motility
99
what is the function of cholecystokinin ( CCK )?
slow gastric emptying ,
stimulated by fats
since it slows down the emptying it targets the pump and the sphincter
100
what is the function of gastric inhibitory peptide GIP ?
reduces gastric secretions
101
what is the function of vasoactive intestinal peptide ? VIP
decrease gastric secretions
102
describe pepsinogen?
active proteolytic enzyme in a highly acidic medium ( optimum ph 1.8 to 3.5 )
above a ph of about 5 it almost has no activity and become innactive
it becomes inactive in the duodenum because the duodenum is alkaline
103
what stimulates Pepsinogen?
ACH from vagus nerve or enteric nervous plexus
Acid in the stomach to activate to form pepsin
104
what promotes gastric emptying ?
intense peristaltic contractions in the antrum
105
what oppose gastric emptying ?
resistance at the pylorus
106
which factors are promoting and which factors are inhibitory?
promoting factors ---> stomach
inhibiting factors ---> duodenum
107
what are the gastric factors that promotes emptying ?
increased food volume through local myenteric reflexes ---> stimulate pyloric pump and inhibit sphincter
Gastrin released from G cells of antral mucosa enhance pyloric pump activity
108
what are the duodenal factors that inhibit emptying ?
entergogastric reflexes inhibit pyloric pump and increase tone of sphincter
hormonal feedback from the duodenum : CCK , secretin, GIP
CCK is released by fat digestion products in the duodenum
Secretin is released when acid is in the duodenum
hyperosmotic solution in the duodeum and jejunum slow the rate of emptying
109
what are the mucosal barriers ?
pre-epithelial barrier
epithelial barrier
subepithelial barrier
prevent harsh acidic and enzymatic environment of the stomach from damaging its own tissue
110
what are the components of pre epithelial barrier?
thick layer of mucus --> mucus cells
bicarbonate HCO3 --> neutralize the acid near the stomach lining , near neutral at the surface and highly acidic in lumen
111
what are the components of the epithelial barrier?
epithelial cells with tight junctions ---> prevent backflow of HCL + enzymes
it is continually renewed with epithelial cells
112
what are the components of subepithelial barrier?
rich vascular network beneath the epithelium
113
what causes peptic ulcer?
when the protective layers are pierced or theres a decrease in the mucosal defense
or
hypersecretion of acid
114
what could decrease the effectiveness of mucosal barrier?
use of aspirin and NSAIDS
increase catecholamines due to stress
infection
Chemical irritants
115
what could cause hypersecretion of acid ?
chronic anxiety ---> increased vagal tone
zollinger ellison syndrome ( gastrin secreting tumor of the pancreas )
systemic mastocytosis ---> over activation of mast cells --> histamine
116
describe the characteristics of ulcer produced by H pylori?
H pyloric colonizes the gastric mucus ( OFTEN IN THE ANTRUM )
attaches to gastric epithelial cells
releases cytotoxins ( e.g, cagA toxin )
breakdown the mucous
also inhibit the somatostatin from D cells ( it wont be there to limit the activity of HCL secretions )
117
what is the effect of cigarette smoking on stomach?
reduce mucosal blood flow
inhibit mucosal prostaglandin production
acceleration of gastric emptying
inhibition of pancreatic bicarbonate secretion ( wont be there to neutralize )
118
what are the treatments for peptic ulcer?
Cimitidine ---> block H2 histamine receptors
Omeprazole ---> inhibit proton pump ( H/K atapase )
antibiotics for H pyloric infection
terminate the usage of NSAIDS
