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Digestion is the way that an organism changes a substance into nutrients. This happens in the gastrointestinal system. Humans start digesting food in the mouth. Food is chewed by the teeth. Food is swallowed, which means it goes through the oesophagus. It goes into the stomach, where it is mixed with acid.
After we swallow food, it travels down a muscular tube to the stomach. There, it is mashed into a mixture like soup. The mixture passes into the small intestine, where tiny bits of food pass into the bloodstream. The food that is still left goes into the large intestine. Finally, waste products leave the body. Digestion usually takes about 18 hours. Food stays in the stomach for about three hours. If uncoiled, the small intestine would be about six meters (20 feet) long. Most digestive tracts are about as long as a bus.
Food slowly enters the small intestine from the stomach. This is where nutrients are taken into the blood. It then enters the large intestine. Water is taken away from it. The food that is left is called feces. The feces are stored in the rectum until it can leave the body through the anus.
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