GASTRIC ACID SECRETION – MECHANISM & PHASES
ACID (H+) PRODUCTION & SECRETION:
* H+ produced inside parietal cell via metabolic CO2 reaction with water.
* H+ secreted by proton pump (H+-K+-ATPase) into gastric lumen in exchange for K+ ion.
– Catalyzed by carbonic anhydrase.
1. CEPHALIC/APPETITE PHASE:
* Gastric secretion initiated before food entry into stomach.
* Sight/smell/even thought of food stimulate acid secretion.
* Mediated by Parasympathetic system via vagus.
* Accounts for 20% acid secretion.
2. GASTRIC PHASE:
* Starts when food contacts gastric mucosa.
2 Factors influencing acid secretion:
Hormonal stimulation:
* Due to gastrin.
Stomach wall stretch:
* Due to gastric distension.
* Activates vaso-vagal & local intra-gastric reflex.
* Accounts for 72-80% acid secretion.
3. INTESTINAL PHASE:
* Starts when food enters duodenum (upper part of small intestine).
* Reduced gastric juice secretion.
– Because of gastrin from duodenal mucosa.
Acid Inhibitors in duodenum:
* Presence of food – Major role.
* Acids.
* Fats.
* Protein digestion products.
* Increased duodenal osmolarity.
Mechanism:
* 2 ways –
Hormonal mechanism:
* By release of various local GI hormones inhibiting acid secretion & gastric motility –
* Secretin.
* Cholecystokinin (CCK).
* Vasoactive Intestinal Peptide (VIP)
* Gastric Inhibitor Polypeptide (GIP).
* Somatostatin.
Neural mechanism:
* By intrinsic neural reflex mechanism – (Entero-gastric reflex).
Exam Important
* H+ secreted by proton pump (H+-K+-ATPase) into gastric lumen in exchange for K+ ion.
* Sight/smell/even thought of food stimulate acid secretion in cephalic phase.
* Cephalic phase is mediated by Parasympathetic system activation via vagus.
* Cephalic phase accounts for 20% of acid secretion.
* Gastric phase is stimulated hormonally due to gastrin.
* Stretching of stomach wall during gastric phase is mainly due to gastric distension.
* Gastric phase accounts for 72-80% acid secretion.
* Local GI hormones released inhibiting gastric acid secretion & motility are secretin, CCK, VIP & Somatostatin.
* Presence of food plays a lmajor role in inhibiting duodenal acid secretion.
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