Gastric juice
Secretion of gastric juice occurs in three phases. The first is the nervous phase. The presence of food in the buccal cavity and its swallowing trigger reflex nerve impulses which pass along the vagus nerve from the brain to the stomach. The sight, smell, taste and even the thought of food can trigger the same reflex. The gastric glands of the stomach are stimulated to secrete gastric juice. This takes place before the food has reached the stomach and therefbre prepares it to receive food. The nervous phase of gastrio secretion lasts for approximately one hour. The second phase is the gastric phase which takes place in the stomach. It involves both nervous and hormonal control. stretching of the stomach by the food it contains stimulates stretch receptors in the wall of the stomach. These send nerve impulses to Meissner's plexus in the submucosa, which in turn sends nerve impulses to the gastric,glands, stimulating the flow of gastric juice. Stretching of the stomach and the presence of food also stimulates special endocrine cells in the mucosa to secrete thehormone gastrin. This reaches the gastric glands by way of the bloodstream and stimulates them to produce gastric juice rich in hydrochloric acid for about four hours. The third phase is the intestinal phase which takes place in the small intestine. When acidified chyme enteres and makes contact with the walls of the duodenum, it triggers both nervous and hormonal responses. Receptors in the small intestine are stimulated by the presence of food, but the reflexes, which pass through the brain, inhibit secretion of gastric juice and slow the release of chyme from the stomach. This prevents too much food being released into the small intestine at once. In addition, the mucosa of the duodenum produces two hormones, cholecystokinin (CCK) and secretin. (CCK may also be known as pancreozymin, but use only one of these names! CCK is easier and more widely used.) The two hormones are taken in the bloodstream
to the stomach, pancreas and the liver. In the stomach secretin inhibits secretion of gastric juice and CCK inhibits stomach emptying.
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