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 gastric epithelium, all of which are
important either to the digestive process or to control of gastric function.
• Chemical composition; It is 97-99% water , it contains inorganic salts ,and
organic components that include ,mucin, digestive enzymes , hormones ….
Digestive system
Gastric Glands and secretions
• The gastric secretion is produced by the epithelium cells of the gasric
glands .
 Goblet cells or mucus cells:are mucous cells,
which cover the entire lumenal surface and
extend down into the glands as "mucous neck
cells". These cells secrete a bicarbonate-rich
mucus that coats and lubricates the gastric
surface, and serves an important role in
protecting the epithelium from hydrochloric
acid and pepsin and other chemical insults.
.
Gastric Mucus-Bicarbonate Barrier
4
Gastric Glands and secretions
• Parietal cells:
They secret HCl into the stomach lumen where it
establishes an extremely acidic environment. This acid is important for
activation of pepsinogen and inactivation of ingested microorganisms such
as bacteria. .
• It also secrets the intrinsic factor, a glycoprotein secreted by parietal cells
that is necessary for intestinal absorption of vitamin B12.
• Chief cells:
It secrets pepsinogen(zymogen). Once secreted,
pepsinogen is activated by stomach acid into the active protease pepsin,
which is largely responsible for the stomach's ability to initiate digestion of
proteins. In young animals, chief cells also secrete rennin a protease that
helps coagulate milk allowing it to be retained more than briefly in the
stomach
– The epithelium cells also produce important hormones gastrin a
peptide that is important in control of acid secretion and gastric
motility.
and somatostatin.
Gastric Glands and secretions
Exocrine gland cells of gastric pits
Produce alkaline
mucus that covers
mucosa layer
Synthesize and
secrete the protease
precursor known as
pepsinogen.
Synthesize and
secrete the HCl acid
responsible for the
acidic pH in the
gastric lumen.
8
HCl Functions
• Makes gastric juice
very acidic.
– Denatures ingested
proteins (alter
tertiary structure)
so become more
digestible.
• Activates
pepsinogen to
pepsin.
– Pepsin is more
active at pH of 2.0.
Insert fig. 18.9
HCl Production
• Parietal cells
secrete H+ into
gastric lumen by
primary active
transport, through
H+/ K+ ATPase
pump.
• Parietal cell’s
basolateral
membrane takes in
Cl- against its
electrochemical
gradient, by
coupling its
transport with
HC03-.
Insert fig. 18.8
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.
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 produce ulcers.
• Acute gastritis:
– Histamine released by tissue damage and inflammation stimulate
further acid secretion.
HCl Production
• Parietal cells
secrete H+ into
gastric lumen by
primary active
transport, through
H+/ K+ ATPase
pump.
• Parietal cell’s
basolateral
membrane takes in
Cl- against its
electrochemical
gradient, by
coupling its
transport with
HC03-.
Insert fig. 18.8
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 duct
of gastric gland to form HCl-.
5. An ATP pump is necessary to
pump the HCl- into the duct
since the concentration of
HCl- is about a million times
more concentrated in the
duct than in the cytosol of the
cell.
II.2 Composition and function
of gastric secretions
1. HCl
 converts pepsinogen to pepsin for chemical
digestion
 provides optimal pH environment for pepsin
 destroys some bacteria
 stimulates the small intestinal mucosa to release
secretinand CCK
 promotes the absorption of Ca2+ and Fe2+ in
small intestine
15
Composition and function of
gastric secretions
2. Pepsinogen (precursor of pepsin)
 digestion of proteins
3. Mucus
 forms a protective barrier: Mucus-bicarbonate
barrier
4. Intrinsic factor
 combines with vitamin B12 to make it
absorbable
16
Control of Gastric Acid Secretion
Gastric acid secretion is controlled by three
mechanisms:
•
•
•
Neurocrine (denoting an endocrine influence on or by the nerves).
Endocrine (gastrin)
Paracrine (histamine) in contrast to true endocrines these
hormones are not released into the bloodstream but into the
surrounding tissues and act in the immediate vicinity, e.g. intestinal
mucosal hormones.
17
Regulation of Gastric Secretion
• Neural and hormonal mechanisms regulate 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 food enters the
duodenum
Cephalic Phase
• Stimulated by sight, smell, and taste of food.
• Activation of vagus:
– Stimulates chief cells to secrete pepsinogen.
– Directly stimulates G cells to secrete gastrin.
– Directly stimulates ECL cells to secrete
histamine.
– Indirectly stimulates parietal cells to secrete HCl.
• Continues into the 1st 30 min. of a meal.
Gastric Phase
• Arrival of food in stomach stimulates the gastric phase.
• Gastric secretion stimulated by:
– Distension.
– Chemical nature of chyme (amino acids and short polypeptides).
• 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, more polypeptides and
amino acids are released.
– Intestinal phase: as partially digested food enters the
duodenum
B. Functional Phases of Gastric Secretion
2. Gastric Phase of
1. Cephalic Phase of
Gastric Secretion (approx.
30% of total)
Gastric Secretion
(approx 60% of total)
(initiated by brain)
(initiated by gastric events)
vagus
nerve
vagus
nerve
FOOD
HCl
HCl
Distension
Peptides
circulation
G
circulation
G
gastrin
gastrin
Regulation of Gastric Secretion
• Secretion of HCl is also regulated by a negative feedback effect:
– HCl secretion decreases if pH < 2.5.
– At pH of 1.0, gastrin secretion ceases.
• D cells stimulate secretion of somatostatin.
Endogenous substances regulating
gastric secretion
协同作用
23
Regulation of Gastric Secretion
• Neural and hormonal mechanisms regulate 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
B. Functional Phases of Gastric Secretion
2. Gastric Phase of
1. Cephalic Phase of
Gastric Secretion (approx.
30% of total)
Gastric Secretion
(approx 60% of total)
(initiated by brain)
(initiated by gastric events)
vagus
nerve
vagus
nerve
FOOD
HCl
HCl
Distension
Peptides
circulation
G
circulation
G
gastrin
gastrin
Peptic ulcers:
Erosions of the mucous
membranes of the stomach or
duodenum produced by action of
HCl.
Control of Gastric Acid Secretion
Gastric acid secretion is controlled by three
mechanisms:
•
•
•
Neurocrine (denoting an endocrine influence on or by the nerves).
Endocrine (gastrin)
Paracrine (histamine) in contrast to true endocrines these
hormones are not released into the bloodstream but into the
surrounding tissues and act in the immediate vicinity, e.g. intestinal
mucosal hormones.
27