An unconditioned nervous reflex causes the production of saliva when food is placed in the mouth. The control of gastric juice release occurs in three phases:-

1. Nervous stimulation of the glands by the autonomic nervous system when food is placed in the mouth and when the swallowing reflex occurs.
2. When the food reaches the stomach it causes distension (stretching) of the stomach which induces further secretion of the gastric juice.
3. The chemical effect of food in the stomach stimulates the release of the hormone gastrin which travels in the bloodstream to the gastric glands and stimulates the production of juices. The presence of fatty acids in the food stimulates the release of endogastrone which reduces the production of hydrochloric acid and slows down stomach peristalsis.

When acidic chyme enters the small intestine it triggers the duodenal mucosa to produce intestinal juice and the hormones cholecystokinin-pancreozymin and secretin. The intestinal juices help to neutralise the acid in the food while the hormones stimulate activity of the pancreas, liver and gall bladder. The pancreozymin component causes the pancreas to produce enzymes, the cholecystokinin component causes contraction of the smooth muscle in the wall of the gall bladder and therefore the release of bile juice into the gut. Secretin causes the pancreas to produce sodium hydrogen carbonate solution and the liver cells to produce bile.