Gastric Secretion Phases: Cephalic, Gastric, and Intestinal

Gastric acid secretion occurs in three integrated phases:

1. Cephalic Phase (30–50% of total)

• Trigger: Thought, sight, smell, taste of food.
• Pathway: Vagus nerve (CN X) → parietal cells (direct) + G cells → gastrin release.
• Mediator: Acetylcholine (M3 receptors), GRP (gastrin-releasing peptide).

2. Gastric Phase (50–60% of total)

• Trigger: Distension, peptides/amino acids, gastric pH >3.
• Pathway: Local enteric reflexes + vagovagal reflexes → G cells → gastrin → parietal cells (CCK2 receptors).
• Mediators: Gastrin (primary), acetylcholine, histamine (ECL cells).

3. Intestinal Phase (5–10% of total)

• Trigger: Chyme in duodenum (peptides, fats, low pH).
• Pathway: Enterogastric reflex (inhibitory) + hormones.
• Mediators: Stimulatory: CCK, secretin (minor). Inhibitory: CCK, secretin, GIP, somatostatin, enteroglucagon → suppress gastrin, parietal cells.

Regulatory Integration

Negative feedback: luminal pH <3 → somatostatin (D cells) → inhibits G and parietal cells. Neural/humoral coordination ensures acid matches digestive need.

Understanding phases guides ulcer therapy (PPIs, H2RAs, antacids) and surgical planning.