The Stomach: A Chemical Factory for Digestion
Think of the stomach as a muscular mixing bag. Food arrives from the oesophagus as a chewed-up lump, and the stomach's job is to turn that lump into a semi-liquid paste called chyme. It does this in two ways: by churning mechanically, and by drenching the food with a powerful chemical cocktail.
That cocktail is gastric juice — a mix of hydrochloric acid (HCl), enzymes, and mucus. The acid kills bacteria, the enzyme pepsin starts breaking proteins into smaller pieces, and the mucus protects the stomach from digesting itself. The cells that make these secretions are not scattered randomly; they are organised into gastric glands tucked into the stomach wall.
The Three Regions of the Stomach
The stomach is not a uniform bag. It has three distinct zones, each with a slightly different lining and function:
| Region | Location | Key Feature |
|---|
| Cardiac region | Narrow ring near the oesophagus entrance | Mostly mucus-secreting cells; protects the oesophagus from acid splash |
| Fundic region | The dome-shaped upper part and the main body | The main factory — contains the most abundant and diverse gastric glands |
| Pyloric region | The lower funnel leading to the small intestine | Produces mucus and the hormone gastrin, which controls acid secretion |
The fundic region is the largest and most important for digestion. When textbooks talk about "gastric glands" in general, they usually mean the fundic glands.
The Gastric Glands: Four Cell Types
Deep in the stomach wall, the lining invaginates into narrow pits called gastric pits. At the bottom of each pit, the epithelium dives deeper to form a gastric gland. Each gland contains four specialised cell types, each secreting a different substance:
1. Mucus neck cells — The Shield
These cells sit near the top of the gland, in the "neck" region. They secrete mucus — a thick, slimy glycoprotein that coats the stomach lining. This mucus barrier is alkaline (pH ~7) and physically separates the delicate epithelial cells from the acidic gastric juice (pH ~2). Without it, the stomach would digest itself.
2. Chief cells — The Protein Chopper
Found mainly in the lower part of the gland. They secrete pepsinogen, an inactive precursor (zymogen) of the enzyme pepsin. Pepsinogen is harmless until it meets acid.
3. Parietal cells — The Acid Pump
Also called oxyntic cells. These are large, triangular cells scattered among the chief cells. They secrete hydrochloric acid (HCl) into the stomach lumen. HCl does three things:
- Kills most swallowed bacteria
- Denatures (unfolds) proteins, making them easier to cut
- Converts pepsinogen into active pepsin
Pepsinogen + HCl → Pepsin. This is the key activation step. Pepsin then activates more pepsinogen in a positive feedback loop.
4. Enteroendocrine cells — The Hormone Messengers …