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The human digestive system conveys food through what is essentially a long muscular tube which begins at the mouth and ends at the anus. Enzymes, which are special proteins secreted by the stomach and intestines, aid in breaking down food in a series of controlled steps so that food gets absorbed into the blood. There are three phases to the process.
This occurs before you actually eat your food. In this phase, the body prepares for digestion. The sight and smell of food stimulate the brain which releases acetylcoholine in the stomach. Stomach secretions increase in preparation to begin processing the food. The gastric secretions reach only about 40% since there is no food to buffer the acidic substances. This phase takes place in the minutes before you sit down to a meal.
The gastric phase takes three to four hours. Food enters through the mouth and passes down the oesophagus. As it approaches the stomach, a ringlike muscle called the lower esophageal sphincter opens as the food approaches, allowing the food to pass through to the stomach, and then closes behind. As food enters the stomach, the stomach is stretched and the food reduces the acidity inside the stomach. The musculature of the stomach is activated, and the stomach releases more gastric juices, including acetylcholine, histamine and hydrochloric acid (which breaks down the food and prepares it for absorption).
Partially-digested food enters and fills the duodenum. This triggers the first part of the intestinal phase. Intestinal gastrin is released. When this happens, muscular fibres are triggered and close the pyloric sphincter, which is a valvelike structure between the duodenum (the first segment of the small intestine) and the stomach. The pyloric sphincter prevents more food from entering the small intestine until the duodenum is emptied. Food move through the small intestine, by small muscular movements called peristalsis. Nutrients are absorbed through the walls of the small intestine and transported throughout the body. Depending on the type of food, it can remain in the small intestine for three to 10 hours. Undigested parts of food, including fibre and other waste products, are pushed on into the large intestine or colon where more water is absorbed from the waste materials. Waste is retained in the colon, which when it is full triggers the body to expel faeces by a bowel movement. Depending on how much food is consumed and the amount of fibre and undigested waste products in the food, waste may remain in the colon for hours or even days.
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