HCl Secretion and Mechanism (MBBS 1st Year Physiology Notes)

1) Site of secretion

• Hydrochloric acid (HCl) is secreted by parietal (oxyntic) cells of gastric glands, mainly in fundus and body of stomach.

2) Functions of HCl

• Converts pepsinogen to pepsin
• Provides acidic pH for protein digestion
• Bactericidal action (kills ingested microbes)
• Helps absorption of iron and indirectly supports calcium, magnesium, vitamin B12 handling

3) Mechanism of HCl secretion by parietal cell

Stepwise mechanism

1. Inside parietal cell:
   [ CO_2 + H_2O \xrightarrow[\text{carbonic anhydrase}]{} H_2CO_3 \rightarrow H^+ + HCO_3^- ]
2. H⁺ is pumped into canaliculus/lumen by H⁺/K⁺ ATPase (K⁺ enters cell).
3. Cl⁻ enters cell from blood side via Cl⁻/HCO₃⁻ exchange and then moves to lumen through chloride channels.
4. In lumen: H⁺ + Cl⁻ = HCl
5. HCO₃⁻ goes to blood (post-meal alkaline tide in gastric venous blood).

4) Regulation of HCl secretion

Major stimulants (3)

• Histamine from ECL cells -> H2 receptor (↑cAMP)
• Acetylcholine (vagus) -> M3 receptor (↑Ca²⁺)
• Gastrin from G cells -> CCK-B (G) receptor (↑Ca²⁺), also stimulates ECL histamine release

Inhibitors

• Somatostatin (D cells)
• Prostaglandins (PGE2)
• Low antral pH (negative feedback via somatostatin)
• Intestinal enterogastrones (e.g., secretin in acidic chyme state)

5) Phases of gastric acid secretion

1. Cephalic phase (~30%)
   Sight/smell/taste/thought of food -> vagal stimulation
2. Gastric phase (~50-60%)
   Food in stomach, distension, peptides -> vagovagal + gastrin
3. Intestinal phase (~10%)
   Brief stimulation, then mainly inhibition as chyme enters duodenum

6) Clinical correlation (high-yield)

• PPIs block H⁺/K⁺ ATPase -> most potent acid suppression
• H2 blockers inhibit histamine-mediated acid secretion
• Vagal overactivity/gastrinoma -> hyperacidity
• Atrophic gastritis/parietal cell loss -> hypochlorhydria/achlorhydria

Quick diagram memory line

• Bailey & Love’s Short Practice of Surgery, 28th ed., p.1171 (parietal cell pathways and receptor-level regulation)
• Harrison’s Principles of Internal Medicine, 21st ed., p.8896 (physiology and phases of gastric secretion)