Food enters the human digestive system through the mouth, where it is broken down by the chewing action of the teeth and enzymes in saliva (amylase, lysozyme, lipase). The tongue moves the food mass (bolus) into the throat and the food is swallowed via the esophagus into the stomach. Food is “churned” by the smooth muscles in the stomach wall, and mixed with pepsin (an aid to protein digestion), HCl (hydrochloric acid), and other gastric juices to form a liquefied material called chyme. The pyloric sphincter, at the end of the stomach, opens to allow chyme to move into the duodenum, the first part of the small intestine.

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The small intestine is the most important organ for chemical digestion of food and the absorption of nutrients. It has three parts –the duodenum, the jejunum and the ileum — and is usually around 7 yards long. Many enzymes are produced in the small intestine, including enzymes that help digest carbohydrates and proteins. Also, the small intestine produces hormones that regulate the process of digestion. For example, GIP (gastric inhibitory peptide), secretin, and cholecystokinin (CCK) are hormones that affect gastric secretions (GIP and secretin inhibit secretions, while CCK increases them). GIP also inhibits churning and increases insulin production, while CCK slows the emptying of the stomach. These substances, along with the nerves that activate or inhibit the parts of the digestive system,

The duodenum also receives secretions from the pancreas and the liver. Carbohydrates are digested via pancreatic amylase, and proteins are digested by pancreatic trypsin and chymotrypsin. The pancreas also produces lipases, which help to digest lipids (fats). These enzymes are regulated by the hormone CCK. Secretin regulates the pancreatic production of bicarbonate, which neutralizes the acid from the stomach. Although the liver has many other purposes as well, it is important in digestion, particularly chemical digestion of lipids via bile. Secretin and CCK regulate the production of bile.

After passing through the small intestine, the remaining parts of the food move into the large intestine. This is where water is removed and indigestible waste is eliminated. The parts of the large intestine include the cecum, ascending, transverse, and descending colon, sigmoid colon, and the rectum. From the rectum, waste is eliminated through the anus.

There are many factors affecting the time required for food to pass completely through the digestive tract, including diet, exercise, medications, stress, and illnesses. For example, if a person eats too quickly and does not properly chew food, this leaves more work for the stomach. It can take up to five hours for the stomach to empty after a meal, especially if it includes a large proportion of protein and fat. In the small intestine, there are two processes which physically move the food: segmentation contractions, which further mix the chyme so that it is easier to break down, and peristalsis, which pushes the food along the small intestine. Carbohydrates are absorbed (sugars, then complex carbohydrates), followed by protein and fats. The trip through the large intestine takes the longest, generally one to two days, but this can be affected by anything that irritates the intestinal walls (producing diarrhea) and lack of exercise and fiber, as well as certain drugs (which can cause constipation).

Foods that can aid digestion include anything with a lot of fiber (such as beans, whole grains, and many fruits and vegetables), which help keep food moving and prevent constipation; foods with beneficial bacteria (such as yogurt or buttermilk), which make them easier to digest and can also repopulate the intestines with good bacteria; and ginger, which helps move food into the small intestine more quickly.