Gastric secretion

The presentation aims to discuss on the mechanism, phases and regulation of gastric acid secreion

1. GASTRIC
SECRETION

2. OBJECTIVES
By the end of the class you should know:
Different cells lining gastric mucosa and their
secretions,
Composition of gastric juice,
Mechanism of gastric acid secretion,
Phases and regulation of gastric acid
secretion,
Experimental demonstrations of the role of
regulatory mechanisms and
Applied aspects

3. LINING OF GASTRIC MUCOSA
•Inner surface of stomach is highly folded into
rugae.
•Gastric mucosa has gastric pits in its folds.
Cells that line folds deeper in the mucosa, are
exocrine gastric glands.

4. The different cells that line the gastric pits are:
1.Surface foveolar cells- tall columnar mucin
secreting cells that line entire gastric mucosa as
well as gastric pits.
2.Mucous neck cells- secrete mucous and are
progenitor cells for the surface epithelium as
well as cells of the gastric glands.

5. 3.Glandular cells form the gastric glands-
these are of 3 types:
a.Main gastric glands- found in body and fundus
of stomach.
Cells of these glands are of 2 types
–Chief/peptic cells= secrete pepsinogen
–Parietal/oxyntic cells= secrete hydrochloric acid
b.Cardiac tubular glands- secrete soluble mucous
c.Antral glands
Contain 2 types of cells
–Mucus cells= secrete soluble mucus
–G cells= secrete Gastrin

8. COMPOSITION OF GASTRIC JUICE
Daily Secretion: 2.5 – 3 liters/day, isotonic
pH : 1 – 2 (Acidic)
Water : 99.45%
solids : 0.55%

9. Solids : 0.55% which include
A.Electrolytes-
Cations :Na
+
; K
+
; H
+
; Mg
2+
Anions :Cl
-
; HCO
3
-
; HPO4
2
;SO4
2-
B.Enzymes
–Pepsinogen
– Gelatinase – Liquifies gelatin
– Gastric Lipase
– Lysozymes – Bacteriocidal
– Urease – Hydrolyse urea to produce ammonia
– Carbonic Anhydrase

10. C.Mucus
– Soluble Mucus – Pyloric and cardiac
tubular glands
– Visible Mucus – Surface epithelium of
gastric mucosa
D.Intrinsic Factor – Absorption of
Vitamin B
12

11. MECHANISM OF HCl SECRETION
SECRETION OF H
+
1.Carbonic anhydrase helps in formation of H
2
CO
3
from
CO
2
& H
2
O within the parietal cell
2.H
2
CO
3
disassociates to form H
+
and HCO
3
-
3.H
+
is secreted into the lumen of canaliculi of parietal
cell in exchange for K
+
by H
+
-K
+
ATPase pump
4.HCO
3
-

produced are transported out through antiport
in exchange with Cl
-
at basolateral membrane into
blood

12. SECRETION OF CL
-
Because of high intracellular negativity, Cl
-
is
forced out through Cl
-
channels on apical
membrane.
Cause of high intracellular negativity:
1.Na
+
-K
+
pump located at basolateral membrane
pumps out 3Na
+
for 2K
+
2.K
+
pumped in diffuses out through K
+
channels
present on basolateral as well as apical
membranes.

13. HCl secreted by the parietal cells
has the following functions:
 Activates pepsinogen to pepsin.
 Necessary for optimum pepsin activity
 Bacteriocidal Effect
 Iron Absorption
 Stimulates bile and pancreatic juice
secretion

14. MECHANISM OF HCL SECRETION

15. REGULATION OF SECRETION OF
GASTRIC JUICE
Two regulatory mechanisms :
1.Nervous Mechanism
– Local autonomic reflexes
– Impulses from the CNS X Nerve
1.Humoral Mechanism
–Mediated by Gastrin & Histamine

16. PHASES OF GASTRIC JUICE
SECRETION
Neural and hormonal mechanisms regulate the
release of gastric juice.
Stimulatory and inhibitory events occur in 3
phases:
1.Cephalic Phase
2.Gastric Phase
3.Intestinal Phase

17. Cephalic
Phase
Gastric
Phase
Intestinal
Phase
Psychic
stimuli
Presence of food
in the stomach
Presence of
food in the
duodenum
Vagally
mediated
Local Reflex and
response to
gastrin
Reflex and
hormonal feed
back
30 – 50% of
total gastric
juice secretion
50-60% of total
gastric juice
secretion
Very less
secretion

18. 1.Cephalic (Reflex) Phase:
•Begins prior to food entry
•Prepares stomach to receive ingested
material.
•Increases production of gastric juices (saliva,
gastric secretions, and pancreatic
secretions)

19. Excitatory events include
1.Sight or thought of food.
2.Stimulation of taste or smell receptors
Inhibitory events include
1.Loss of appetite or depression
2.Decrease in stimulation of parasympathetic
division

21. 2. Gastric Phase:
•Begins with the arrival of food in the stomach.
•Once food enters the stomach
Neural, hormonal – secretion of gastrin
•Stomach wall stretches which triggers stretch
receptors to send a nerve impulse along the vagus
nerve to the medulla oblongata

22. Excitatory events include:
1.Stomach distension - activation of stretch receptors
(neural activation)
2.Release of gastrin to the blood
Inhibitory events include:
1.a pH lower than 2
2.Emotional upset that overrides the parasympathetic
division

24. 3. Intestinal Phase:
•Begins as partially digested food enters the
duodenum
•Stretch receptors and chemoreceptors in the
duodenum stimulate reflex pathways through the
medulla that initially lead to more gastric secretion,
gastric motility, and emptying of the stomach.
•But later release of hormones inhibits the rate of
gastric secretion.

25. 1.Distension of duodenum and/or
2.The presence of fatty, acidic, or hypertonic chyme,
and/or
3.Irritants in the duodenum initiates inhibition of local
reflexes and vagal nuclei that causes:
•Closure of the pyloric sphincter
•Release of inhibitory hormones i.e. secretin and
cholecystokinin that inhibit gastric secretion and
stomach emptying.

27. Agent Sources Mechanism of
Action
Gastrin G-cells Via gastrin
receptors
Histamine Enterochromaffi
n like cells
(ECL), food stuff
e.g. meat,
cabbage etc…
Binds to H2
receptors and
via Gs
Acetyl-
Choline
Parasympatheti
c Nerve Endings
Via M3
muscarinic
receptors
MECHANISM OF ACTION OF
REGULATORY AGENTS

29. Antral Gastrin
Secretin, GIP,
VIP, Glucagon,
Calcitonin
Vagal stimulation
Products of Digestion
Distension of stomach
Calcium, Epinephrine
Parietal Cell
Acid Secretion
Acid Entering
The Duodenum
Histamine
Acetyl Choline
PGE
2
Regulation of Secretion of HCl
(+)
(+)
(+)
(+)
(-)
(-)
(-)
(-)
(-)

30. EXPERIMENTAL DEMONSTRATIONS OF
THE ROLE OF REGULATORY MECHANISMS
1.Sham feeding
2.Pavlov’s pouch
3.Heindenhain’s pouch

31. SHAM FEEDINGSHAM FEEDING
•To demonstrate cephalic phase of feeding.
•Esophagus of dog is exposed and divided in the
middle of the neck. The two cut ends are brought to
the surface.
•When dog swallows, the food comes out through
the upper cut end and doesnot enter the stomach.

32. •Through lower cut end a tube is passed
through the stomach and gastric sections are
collected.
•Gastric secretion caused by sight, smell & taste
of food represents the cephalic phase.
Features:
1.Begins after a latency of 5-7 min
2.Volume reaches peak in 1 hour
3.Continues for 3 hours
4.Gastric juice is rich in pepsin

33. THE PAVLOV POUCH EXPERIMENTTHE PAVLOV POUCH EXPERIMENT
•To demonstrate that vagus is secretomotor nerve to
stomach.
•Under general anesthesia, a pouch of stomach with
intact nerve & blood supply is separated from body
of stomach .
•Outlet is made in smaller part which is separated
and brought to abdominal wall to provide drainage
of gastric secretion.

34. •Vagus nerve is exposed and divided in neck.
•Animal allowed to recover.
•After some days, peripheral cut end of Vagus
is stimulated in unanesthesized dog.
•Flow of gastric rich juice in HCl and pepsin
after a short latent period demonstrates
Vagus is secretomotor to stomach.

35. THE HEIDENHAN’S POUCH THE HEIDENHAN’S POUCH
EXPERIMENTEXPERIMENT
To demonstrate existence of some blood-borne
mechanism that regulates gastric secretion

36. APPLIED ASPECT= PEPTIC ULCER
Normal gastric mucosal barrier:
1.Lined with highly resistant mucous cells that
secrete a viscid and adherent mucus and
2.It has tight junctions between the adjacent
epithelial cells.
3.Bicarbonate secretion.
4.High mucosal blood flow.
5.Prostaglandins

37. •A peptic ulcer is an excoriated area of stomach
or intestinal mucosa caused principally by the
digestive action of gastric juice or upper small
intestinal secretions.
Cause:
1.Diminished ability of mucosal barrier to
protect against acid-pepsin complex due to
Helicobacter pylori bacterial infection
2.Excessive secretion of gastric acid

38. Treatment plans
1.Antacids- Aluminium/magnesium hydroxide
2.Atropine- blocks Muscarinic receptors of
Ach action
3.H-K-ATPase inhibiting drugs=Omeprazole.
4.Surgery, Vagotomy

39. QUESTIONS ASKED IN EXAMS
LONG ESSAY:
•Give the composition & functions of gastric
juice. Explain mechanism of secretion of gastric
juice

40. SHORT ESSAYS
•Mechanism of secretion of gastric acid/HCl secretion by
parietal cells
•Phases of gastric secretion
•Methods of study of gastric secretion
•Pavlov’s pouch- its importance in study of gastric physiology
•Sham feeding
•Peptic ulcer
•Digestive lipase

41. THANK YOU