Mucus and mucosal protection in the gut
- Mechanism
- Exam priority: high
A slippery, alkaline gel and a fast-healing lining let the gut hold acid and enzymes without digesting itself.
Also called: Gastric mucosal barrier, Brunner glands, Aggression vs defence
See it in 3D: Stomach · Duodenum · Transverse colon
What it is
The gut lining is protected by a mucus gel, bicarbonate trapped under it, tight epithelial junctions, rapid repair and a rich blood supply. Prostaglandins keep these defences running. An ulcer forms when aggressors (acid, pepsin, bile, NSAIDs, H. pylori) outrun the defences, so most ulcer physiology is a balance sheet.
Key facts
- Mucus is mostly water held by large, heavily glycosylated mucin proteins; it lubricates, resists digestion by enzymes and buffers small amounts of acid or base.
- Goblet cells make mucus along the whole intestine; surface mucous cells and neck cells make it in the stomach.
- Mucus secretion rises with direct contact, mechanical irritation and chemical irritation of the lining, so it is a local, on-demand defence.
- In the small intestine, crypt cells secrete water and Cl⁻ (via CFTR) that dilute and wash away irritants; enteric reflexes triggered by distension and contact drive it.
- The colon secretes mostly mucus with some bicarbonate; its inner mucus layer is dense and nearly free of bacteria, while the outer layer hosts the microbiota.
- After small injuries, neighbouring cells slide over the gap within minutes (restitution), well before new cells divide.
How it works, step by step
- Surface mucous cells release mucin and HCO₃⁻; the gel slows mixing, so HCO₃⁻ stays trapped against the epithelium.
- H⁺ that diffuses into the gel is neutralized before it reaches the cells, keeping a steep pH gradient across a fraction of a millimetre.
- Prostaglandins made by COX-1 keep mucus and HCO₃⁻ output high and dilate mucosal arterioles.
- Good blood flow carries away any H⁺ that enters the tissue and delivers HCO₃⁻ (the parietal cell's alkaline tide helps here).
- Chyme entering the duodenum stimulates Brunner glands (contact, vagal input, secretin), adding alkaline mucus as pancreatic HCO₃⁻ arrives.
- If defences fall (NSAID, ischemia, H. pylori) or aggressors rise (acid, pepsin, bile reflux), H⁺ back-diffuses, cells die and an erosion can deepen into an ulcer.
Aggressors and defences of the gastroduodenal lining
| Side | Factor | How it acts |
|---|---|---|
| Aggressor | Acid and pepsin | Digest and denature exposed tissue; pepsin needs low pH to work |
| Aggressor | NSAIDs, aspirin | Block COX-1 prostaglandins and injure cells directly |
| Aggressor | H. pylori | Inflames the mucosa, weakens mucus, alters gastrin and acid output |
| Aggressor | Bile reflux, alcohol, smoking | Dissolve membrane lipids, break the barrier, slow healing |
| Defence | Mucus–bicarbonate layer | Keeps a near-neutral pH at the cell surface |
| Defence | Prostaglandins | More mucus, HCO₃⁻ and blood flow; less acid |
| Defence | Blood flow and restitution | Clear back-diffused H⁺; reseal small defects fast |
Clinical notes
- NSAID ulcers are often painless and gastric; a prostaglandin analogue (misoprostol) replaces the missing defence rather than lowering acid alone.
- Sucralfate polymerizes in acid and sticks to the ulcer base; bismuth coats the mucosa and is toxic to H. pylori; PPIs act on the aggressor side.
- Critically ill patients (shock, burns, head injury, mechanical ventilation) develop stress-related mucosal damage because ischemia removes the blood-flow defence.
- In cystic fibrosis, thick, poorly hydrated intestinal mucus can block the ileum at birth (meconium ileus).
Test yourself
Think of the answer, then tap the question to see it.
Why can gastric lumen pH be near 1 while the surface epithelial cells sit at a pH close to 7?
The unstirred mucus gel traps bicarbonate secreted by surface cells against the epithelium; H⁺ diffusing in is neutralized inside the gel before it reaches the cells.
How do NSAIDs cause gastric ulcers even when acid secretion is normal?
They block COX-1 prostaglandin synthesis, which lowers mucus, bicarbonate and mucosal blood flow and removes the brake on acid, so normal acid overwhelms weakened defences.
What do Brunner glands secrete, and what inhibits them?
Alkaline mucus into the proximal duodenum, driven by contact, vagal input and secretin; sympathetic stimulation inhibits them, which may help explain ulcers of the duodenal bulb under stress.
Why does mucosal ischemia in shock produce stress ulcers?
Low blood flow cannot remove back-diffusing H⁺ or deliver bicarbonate, and the cells lose energy for repair, so acid injures the mucosa within hours to days.
Which is the primary mechanism of misoprostol on the gastric mucosa?
It is a prostaglandin E₁ analogue that restores the prostaglandin defence: more mucus and bicarbonate, more blood flow, and some reduction of parietal cell acid output.
Connections
See it in 3D3
The structures of Durgical's 3D anatomy where it happens.
In Biochemistry1
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Peptic Ulcer Disease (StatPearls (NCBI Bookshelf), 2025); Helicobacter Pylori (StatPearls (NCBI Bookshelf), 2025); Upper Gastrointestinal Bleeding (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.