Digestion in the stomach

Main processes and motility

Upon entering the stomach cavity, food begins to swell and liquefy, while its components gradually dissolve. The entire ingested mass does not mix with the acidic gastric secretion instantly, which creates conditions for different types of digestion to occur in different layers of the food bolus.

In the central part of the food content, salivary enzymes (carbohydrases) actively work, continuing to break down carbohydrates. Since the stomach's own gastric juice lacks amylolytic enzymes, this hydrolysis proceeds only until the acidic environment penetrates deep into the bolus and inactivates the carbohydrases.

Simultaneously, in the parietal layer (right next to the mucosa), protein digestion begins via gastric juice enzymes, provided the acidic secretion is not neutralized by the food's own buffering properties. The width of the active digestion zone depends on the properties of the food and the volume of juice. As chemical processing and liquefaction progress, the contractions of the stomach wall shift the digested layer of chyme into the antral part, from which it is evacuated into the intestine in portions.

Gastric juice and secretory cells

Secretions from the mucosal glands play a leading role in digestion. The fundus (fundic region) produces juice rich in pepsins and hydrochloric acid. Conversely, juice from the pyloric region is poor in enzymes. The reaction of the secretion is acidic: after a meal, due to the buffering properties of food, the pH ranges from 1.8 to 4.0.

Specific glandular cells are responsible for producing the components of the juice:

• Chief glandulocytes (zymogenic cells) — synthesize inactive forms of enzymes (pepsinogens) and lipases.
• Parietal glandulocytes — secrete hydrochloric acid (HCl) and intrinsic factor (Castle's intrinsic factor).
• Mucocytes (accessory/mucus neck cells) — secrete mucoid substances (gastric mucus).

By chemical composition, the juice is divided into inorganic substances (hydrochloric acid, chlorides, sulfates, phosphates, bicarbonates, sodium, potassium, calcium, and magnesium cations) and organic substances (enzymes, mucin, amino acids, urea, and uric acid).

Enzymatic breakdown of nutrients

The main enzymatic process in the stomach is the initial hydrolysis of proteins into peptones and albumoses. Proteolytic enzymes are produced in an inactive form and require activation by hydrochloric acid.

Two groups of enzyme precursors are distinguished:

1. Group I pepsinogens (5 types). Produced in the fundus of the stomach, converted into pepsins. Their pH optimum is 1.5–2.0 (at this acidity, maximum hydrolysis rate is achieved).
2. Group II pepsinogens (2 types). Produced in the pyloric part, converted into gastricsin. The optimum action is in a less acidic environment — pH 3.2–3.5.

Clinical significance: the presence of enzymes with different pH optima ensures efficient protein breakdown in different layers of chyme with varying acidity.

Among additional enzymes, rennin (chymosin) stands out, which curdles milk in the presence of calcium ions. Fat digestion is limited: local lipase breaks down lipids into fatty acids and glycerol, but acts only on emulsified fats.

Functions of hydrochloric acid and the mucosal-bicarbonate barrier

Hydrochloric acid performs crucial tasks in gastric digestion:

• Triggers protein denaturation and swelling, facilitating their subsequent breakdown.
• Converts inactive pepsinogens into functional pepsins.
• Provides the acidic environment (pH optimum) necessary for the action of proteolytic enzymes.
• Exerts a pronounced antibacterial effect, sanitizing the chyme.
• Participates in the regulation of gastrointestinal motility.

To prevent aggressive acid and pepsins from damaging the stomach wall, a protective mechanism exists — the mucosal-bicarbonate barrier. It consists of gastric mucus (mucoids) and bicarbonate anions ($HCO_3^-$) produced along with it. This barrier covers the mucosa, protecting it from mechanical trauma and autodigestion.

Mucoids also include Castle's intrinsic factor (gastromucoprotein). It is critical for the intestinal absorption of vitamin $B_{12}$ (extrinsic factor). Their interaction forms the anti-anemic blood factor.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanism of pepsinogen activation by hydrochloric acid
• The influence of food buffering properties on the width of the parietal digestion zone
• Differences in secretory activity between the fundic and pyloric regions
• The chemistry of milk curdling by rennin and calcium ions
• Physiology of the mucosal-bicarbonate barrier formation

Other topics in the "Digestion" section

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