First Phase: Complex Reflex (Cephalic) Phase
This stage, also known as the cerebral phase, acts as the trigger. Its primary mechanism relies on neural impulses, and juice secretion begins even before the bolus is swallowed.
- Conditioned reflex pathway: A response to the sight, smell, thoughts of food, sounds, and the surrounding environment.
- Unconditioned reflex pathway: Triggered when food stimulates the taste and pharyngeal receptors in the oral cavity.
The existence of this phase was demonstrated in classical experiments using sham feeding (in animals with an esophagostomy). The food dropped out without reaching the stomach, yet 5–10 minutes later, profuse secretion of 'appetitive juice' began. This juice features extremely high enzymatic activity because it contains enzymes accumulated by the glands during the resting period.
Stimulatory signals are transmitted to the stomach via the vagus nerve (n. vagus). Humorally, the first phase is supported by the hormone gastrin, which is secreted in the pyloric region. Transection of the vagus nerve or removal of the pylorus abolishes the complex reflex secretion.
Second Phase: Gastric Phase
The gastric phase is triggered when food makes direct contact with the gastric mucosa. Its defining mechanism is a neurohumoral response to distention and chemical irritation.
Mechanism of the gastric phase:
- Food mechanically stretches the pyloric region.
- This activates G-cells, which release gastrin into the bloodstream.
- Gastrin acts on the gastric glands, stimulating their secretory activity.
- Concurrently, histamine is released into the blood, driving parietal cells to secrete hydrochloric acid (HCl).
In addition to gastrin, secretion is stimulated by acetylcholine, meat and vegetable extractives, protein breakdown products, and bombesin. Secretin and cholecystokinin (CCK) play a dual role, exerting mixed effects: while they suppress hydrochloric acid synthesis, they enhance the release of pepsinogens. Inhibitors of the second phase include fat breakdown products, vasoactive intestinal peptide (VIP), serotonin, gastrone, and enterogastrone.
Third Phase: Intestinal Phase
The final stage of gastric digestion is regulated by signals originating from the duodenum and upper jejunum. Both neural pathways (activated by intestinal mechano- and chemoreceptors) and humoral factors are engaged.
When chyme enters the duodenum, local regulators are produced in response to extractives and nutrients:
- Enterogastrin — maintains and enhances gastric secretion.
- Enterogastrone — inhibits gastric secretion.
Stimulation at this stage is primarily driven by protein hydrolysis products. However, inhibitory influences progressively mount. A strict hierarchy governs substances that suppress secretion: fat hydrolysis products exert the most potent inhibitory effect, followed by polypeptides, amino acids, and starch breakdown products. Furthermore, hydrogen ions sharply block acid output when the luminal pH drops below 3.
Dependence of Secretion on Food Type
Analysis of gastric juice shows that the dynamics and intensity of the secretory phases vary depending on the macronutrient composition of the ingested meal.
| Food Type | Stimulatory Intensity | Phase Characteristics |
|---|---|---|
| Meat (Proteins) | Strong stimulant | Triggers a sharp rise in secretion, with all three phases fully and robustly expressed. |
| Bread (Carbohydrates) | Moderate stimulant | Secretion lasts up to 10 hours. The gastric phase is entirely absent due to a lack of chemical stimulants (extractives) in bread. |
| Milk (Fats) | Fats act as strong inhibitors | The first phase is weakly expressed. The secretory peak is significantly delayed, occurring only by the 3rd hour (during the gastric phase). |