Do digestive juices flow into the alimentary canal all the time? If it were so, it would mean terrible wastage of enzymes when there is no food in the alimentary canal. So, everything must be so timed that there is neither wastage, nor shortage.
1. When you see or smell good food or even think or talk about it, the mouth begins to "water" (salivation). This happens through stimulation by nerves coming from the brain. The secretion of thicker saliva is stimulated by chewing action.
2. On reaching the stomach, the presence of food stimulates the stomach lining to secrete gastric juice. Secondly, the mechanical stimulation of stomach wall produces a hormone, gastrin which again stimulates the secretion of gastric juice.
3. As the food enters duodenum, the duodenal epithelium secretes four hormones: Secretin, Pancreozymin, Cholecystokinin, and Enterogastrone.
- Secretinstimulates the flow of pancreatic juice, which is rich in bicarbonates (to neutralize acid).
- Pancreozymin helps in the flow of pancreatic enzymes.
- Cholecystokinin stimulates flow of bile from gall bladder.
- Enterogastrone stops secretion of gastric juice, because stomach becomes empty as food now passes from stomach to duodenum.
Several nerves (from sympathetic and parasympathetic nervous system) stimulate and control the gut to accelerate or slow down the movements of alimentary canal as termed peristalsis.