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Gastric secretions
Gastric secretion is a colorless, watery, acidic, digestive
fluid produced in the stomach . Physical properties;
It is a watery fluid, that has a pale yellow colour , pH is 1-3 ,the volume secreted per day is 2-3 L .
The stomach is famous for its secretion of acid, but acid is
only one of four major secretory products of the gastricepithelium, all of which are important either to the digestiveprocess or to control of gastric function.
Chemical composition; It is 97-99% water , it containsinorganic salts ,and organic components that include ,mucin,digestive enzymes , hormones .
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Digestive system
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The gastric secretion is produced by the epithelium cellsof the gasric glands .
Goblet cells or mucus cells:aremucous cells, which cover the entire
lumenal surface and extend down intothe glands as "mucous neck cells".These cells secrete a bicarbonate-richmucus that coats and lubricates thegastric surface, and serves an
important role in protecting theepithelium from hydrochloric acid andpepsin and other chemical insults.
.
Gastric Glands and secretions
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Gastric Mucus-Bicarbonate Barrier
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Gastric Glands and secretions Parietal cells: They secret HCl into the stomach
lumen where it establishes an extremely acidicenvironment. This acid is important for activation ofpepsinogen and inactivation of ingested microorganismssuch as bacteria. .
It also secrets the intrinsic factor, a glycoprotein secretedby parietal cells that is necessary for intestinal absorptionof vitamin B12.
Chief cells: It secrets pepsinogen(zymogen). Oncesecreted, pepsinogen is activated by stomach acid into the
active protease pepsin, which is largely responsible for thestomach's ability to initiate digestion of proteins. In younganimals, chief cells also secrete rennin a protease thathelps coagulate milk allowing it to be retained more thanbriefly in the stomach
The epithelium cells also produce important hormonesgastrin a peptide that is important in control of acid
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Gastric Glands and secretions
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Synthesize and
secrete the HCl acidresponsible for the
acidic pH in the
gastric lumen.
Synthesize and
secrete the proteaseprecursor known as
pepsinogen.
Produce alkaline
mucus that coversmucosa layer
Exocrine gland cells of gastric pits
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HCl Functions
Makes gastricjuice very acidic.
Denaturesingested
proteins (altertertiarystructure) sobecome moredigestible.
Activatespepsinogen topepsin. Pepsin is more
active at pH of2.0.
Insert fig. 18.9
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HCl Production
Parietal cellssecrete H+ intogastric lumenby primary
activetransport,through H+/ K+
ATPase pump.
Parietal cellsbasolateral
membranetakes in Cl-
against itselectrochemicalgradient, bycoupling its
transport withHC0 -.
Insert fig. 18.8
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Gastric secretion phases
Gastric acid secretion can be divided
into three phases:
Cephalic phase mediated by the CNS and
triggered by smelling, chewing or even the thought
of food. Mediated by the vagus and acounts for 10- 30% of the cid secreted).
Gastric phase triggered by the presence of food in
the stomach (both chemical and mechanical
sensing are involved). Accounts for 70-90% of the acid secretion
Intestinal phase. Presence of chyme, most
probably amino acids, in the intestine triggers
approximately 5% of the gastric acid secretion.
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Gastric and Peptic Ulcers
Peptic ulcers: Erosions of the mucous membranes of the stomach
or duodenum produced by action of HCl.
Zollinger-Ellison syndrome: Ulcers of the duodenum are produced by excessive
gastric acid secretions.
Helicobacter pylori:
Bacterium that resides in GI tract that may produceulcers.
Acute gastritis: Histamine released by tissue damage and
inflammation stimulate further acid secretion.
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HCl Production
Parietal cellssecrete H+ intogastric lumenby primary
activetransport,through H+/ K+
ATPase pump.
Parietal cellsbasolateral
membranetakes in Cl-
against itselectrochemicalgradient, bycoupling its
transport withHC0 -.
Insert fig. 18.8
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Hydrochloric Acid Production
1. CO2 and Cl- diffuse from the
blood into the stomach cell.2. CO2 combines with H2O
to form H2CO3.3. H2CO3 dissociates into
bicarbonate (HCO3-) and H+.
4. H+ combines with Cl- in ductof gastric gland to form HCl-.
5. An ATP pump is necessary to
pump the HCl- into the duct
since the concentration of
HCl-
isabout a million timesmore concentrated in the
duct than in the cytosol of the
cell.
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II.2 Composition and function of
gastric secretions1. HCl
converts pepsinogen to pepsin for chemical
digestion
provides optimal pH environment for pepsin
destroys some bacteria
stimulates the small intestinal mucosa to releasesecretinand CCK
promotes the absorption of Ca2+ and Fe2+ in
small intestine
C i i d f i f
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Composition and function of
gastric secretions
2. Pepsinogen(precursor of pepsin)
digestion of proteins
3. Mucus
forms a protective barrier: Mucus-bicarbonatebarrier
4. Intrinsic factor
combines with vitamin B12 to make it
absorbable
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Control of Gastric Acid Secretion
Gastric acid secretion is controlled by three
mechanisms:
Neurocrine (denoting an endocrine influence on or by thenerves).
Endocrine (gastrin)
Paracrine (histamine) in contrast to true endocrines thesehormones are not released into the bloodstream butinto the surrounding tissues and act in the immediatevicinity, e.g. intestinal mucosal hormones.
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Regulation of Gastric Secretion
Neural and hormonal mechanismsregulate the release of gastric juice
gastric secretion occurs in three phases
Cephalic (reflex) phase: prior to food entry Gastric phase: once food enters the stomach
Intestinal phase: as partially digested foodenters the duodenum
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Cephalic Phase
Stimulated by sight, smell, and taste offood.
Activation of vagus:
Stimulates chief cells to secretepepsinogen.
Directly stimulates G cells to secrete
gastrin. Directly stimulates ECL cells to secrete
histamine.
Indirectly stimulates parietal cells tosecrete HCl.
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Arrival of food in stomach stimulates the gastricphase.
Gastric secretion stimulated by: Distension.
Chemical nature of chyme (amino acids and shortpolypeptides).
Stimulates G cells to secrete gastrin.
Stimulates chief cells to secrete pepsinogen.
Stimulates ECL cells to secrete histamine.
Histamine stimulates secretin of HCl. Positive feedback effect.
As more HCl and pepsinogen are secreted, morepolypeptides and amino acids are released.
Intestinal phase: as partially digested food
Gastric Phase
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G
gastrin
circulation
HCl
vagusnerve
FOODFOOD
DistensionPeptides
2. Gastric Phase ofGastric Secretion
(approx 60% of total)
(initiated by gastric events)
G
gastrin
circulation
HCl
vagusnerve
1. Cephalic Phase ofGastric Secretion (approx.
30% of total)
(initiated by brain)
B. Functional Phases of Gastric Secretion
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Regulation of GastricSecretion
Secretion of HCl is also regulated by a negativefeedback effect: HCl secretion decreases if pH < 2.5.
At pH of 1.0, gastrin secretion ceases.
D cells stimulate secretion of somatostatin.
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Endogenous substances regulating
gastric secretion
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Regulation of Gastric Secretion
Neural and hormonal mechanismsregulate the release of gastric juice
Stimulatory and inhibitory events occur in
three phases Cephalic (reflex) phase: prior to food entry
Gastric phase: once food enters the stomach
Intestinal phase: as partially digested food
enters the duodenum
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G
gastrin
circulation
HCl
vagusnerve
FOODFOOD
DistensionPeptides
2. Gastric Phase ofGastric Secretion
(approx 60% of total)
(initiated by gastric events)
G
gastrin
circulation
HCl
vagusnerve
1. Cephalic Phase ofGastric Secretion (approx.
30% of total)
(initiated by brain)
B. Functional Phases of Gastric Secretion
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Peptic ulcers:Erosions of the mucous
membranes of the stomach or
duodenum produced by action ofHCl.
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Control of Gastric Acid Secretion
Gastric acid secretion is controlled by three
mechanisms:
Neurocrine (denoting an endocrine influence on or by thenerves).
Endocrine (gastrin)
Paracrine (histamine) in contrast to true endocrines thesehormones are not released into the bloodstream butinto the surrounding tissues and act in the immediatevicinity, e.g. intestinal mucosal hormones.