Secretions of the stomach Flashcards
Functions of the stomach
- Secretions: different cell types
- Motility: peristalsis
- Digestion: carbohydrate, protein and fat
- Absorption
Ingested food (now
bolus) enter the
stomach through the
lower oesophageal
sphincter.
Anatomy of the stomach
- cardia - near gastroesophageal sphincter
- oxyntic mucosa - fundus and body ( corpus)
- pyloric - antrum and pyloric sphincter
The Oesophagus
Mucous cells - mucous is secreted
Neck cell - bicarbonate is secreted
Parietal Cell - acid and intrinsic factors are secreted
EC like cell - histamine is secreted
chief cell - pepsinogen and lipase
D cell - somatostatin
G cell - gastrin is secreted
Cells found in the oesophagus
in the corpus - Predominantly
Parietal and Chief
cells (though does
also contain ECL
and D cells)
Antrum and Pylorus -
*No parietal cells
*Contains G-cells and D cells
Acid secretion by parietal cells
- Cl- transported from the cytoplasm of the parietal
cells into the lumen - Na+ transported from the canaliculus into the
cytoplasm of the parietal cell - Water dissociates into H+ and OH- in the cytoplasm of
the cell - The H+ are secreted into the canaliculus in exchange
for K+ - CO2 combines with OH- to form bicarbonate ions
(HCO3-) using carbonic anhydrase. - HCO3- diffuse out of the cell cytoplasm into the
extracellular fluid in exchange for Cl- (alkaline tide)
What regulates the acid secretion ?
neural ( vagus nerve ) - indirect -
-Act on G cell by releasing gastrin
releasing peptide → secrete gastrin →
act on CCKB receptor of parietal cell → ↑ HCl secretion
- Act on ECL → secrete histamine → act on H2 receptor of parietal cell → ↑ HCl secretion
- direct - Ach act on parietal cells → ↑ HCl secretion
Paracrine - histamine - Act on H2 receptor of parietal cell → ↑
HCl secretion
Inhibitors of acid secretion
Enterogastrones: are hormones secreted by the duodenum
that inhibits peristalsis and secretions in the stomach. The
hormone is stimulated by fat.
Examples: Somatostatin, CCK, Prostaglandins and Secretin
- Secretin: (released by S cells in duodenum) stimulated
by fat and acid in duodenum can inhibit acid secretion
by inhibiting Antral gastrin release and Somatostatin
release. - CCK is produced by I-cells of the duodenum and
jejenum in response to fat. It directly reduces parietalcell acid secretion
Pharmacological inhibitors of acid secretion
- Proton pump inhibitors
(Omeprazole): Direct action on the proton pump itself - H2 receptor antagonists
(cimetidine & ranitidine)- old hat.
The H2 antagonists are competitive
antagonists of histamine at the
parietal cell H2
receptor
Phases of gastric secretion
-Basal phase - Follows a circadian rhythm. Acid secretion is low (am) and high (pm)
-Cephalic phase
(30% of acid secretion) -
Initiated by smell, sight, taste, thought, and swallowing
of food. Mediated by vagus nerve → acid secretion
-Gastric phase
(50-60% of acid
secretion) -
Stimulated by distention of stomach
(mechanoreceptors) and partially digested proteins
(gastrin release)
-Intestinal phase
(5-10% of acid secretion) -
Stimulated by digested peptides in proximal part of the
small intestine (gastrin release from duodenal G cells)
Pepsin - gastric secretion
- Pepsin initiate protein digestion
- Pepsin is obtained from pepsinogen (secreted
by chief cells) - Acetylcholine is the major trigger for
pepsinogen secretion - Interlinked with acid secretion because of
similar trigger (Ach) and resulting peptides
trigger antral G-cell gastrin secretion
Mucus - gastric secretion
Thick mucus, which contains trapped bicarbonates (HCO3-), is
secreted by mucus cells.
- The mucus layer formed protects the stomach lumen from its harsh
(acidic; pH < 1) contents - Prostaglandins are important in maintaining the mucosal diffusion
barrier. They in part do this by inhibiting acid secretion, stimulating
HCO3- secretion and mucus secretion - Importance of Prostaglandins exemplified by the effects of NSAIDS on
gastric mucosa
Intrinsic factor - gastric secretions
- Intrinsic factor (IF) is produced by the parietal cells.
- Necessary for the absorption of vitamin B12 in terminal ileum.
- In the stomach IF binds to vit. B12 in food and protects it from
degradation by acid. - Damaged parietal cells → no IF produced → no vit. B12 → effect on red
blood cells synthesis → Pernicious (megaloblastic) anaemia - Pernicious anaemia is an autoimmune atrophic gastritis, in which autoantibodies are directed against parietal cells and results in their
destruction
Peptic Ulcer
- The leading cause of peptic ulcer is
Helicobacter pylori. - H. pylori is an acid loving, Gram
negative, bacteria that colonises the
stomach of about 40% of humans. - H. pylori causes acid production and
barrier damage
Mode of action and treatment
In the antrum, H. pylori infection causes G-cells to hypersecrete
gastrin and decrease antral D-cell somotastatin release → ↑ acid
→ hypergastrinaemia → duodenal cancer
In the corpus, H. pylori infection leads to reduced acid secretion
and Hypochlorhydria → gastric ulcers.
* Urease produced by HP converts
* Urea → NH3 + CO2+ H2O → NH4+ + HCO3- → neutralises the
stomach acid
* Treatment-Triple therapy consisting of the antibiotics
amoxicillin and clarithromycin, and a proton pump inhibitor
