Gastric Secretions Mine

8/2/2019 Gastric Secretions Mine

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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.

Gastric Secretions Mine

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