In humans, digestion begins in the oral cavity where food is chewed (mastication) with the teeth. The process stimulates exocrine glands in the mouth to release digestive enzymes such as salivary amylase, which aid in the breakdown of food, particularly carbohydrates. Chewing (mechanical) also causes the release of saliva, which helps condense food into a bolus that can be easily passed through the esophagus. The esophagus is about 20 centimeters long. The food is pushed down by a movement called peristalsis, which is the wave-like contraction of smooth muscle tissue, characteristic of the digestive system. The uvula is a very sensitive organ that hangs from the roof of the mouth. Its main job is to close the nasopharynx, which prevents the food from entering the trachea. When swallowed, the food enters the pharynx, which makes special adaptations to prevent choking or aspiration when food is swallowed.
The food enters the stomach upon passage through the cardiac sphincter. In the stomach, food is churned and thoroughly mixed with a digestive fluid, composed chiefly of hydrochloric acid, and other digestive enzymes to further decompose it chemically for a few hours. As the acidic level changes in the stomach and later parts of the digestive tract, more enzymes are activated or deactivated to extract and process various nutrients.
After being processed in the stomach, food is passed to the small intestine via peristalsis. This is where most of the digestive process occurs. It passes through the pyloric sphincter and enters the first 10 inches of the small intestine, the duodenum, where it is further mixed with 3 different liquids, which are bile (which helps aid in fat digestion, otherwise known as emulsification), pancreatic juice (made by the pancreas), and intestinal juice. They also add several enzymes: maltase, lactase and sucrase, to process sugars. Trypsin and chymotrypsin are other enzymes added in the small intestine. (Bile also contains pigments that are by-products of red blood cell destruction in the liver; these bile pigments are eliminated from the body with the feces.) Most nutrient absorption takes place in the small intestine. The nutrients pass through the small intestine's wall, which contains small, finger-like structures called villi. The blood, which has absorbed nutrients, is carried away from the small intestine via the hepatic portal vein and goes to the liver for filtering, removal of toxins, and nutrient processing.
After going through the small intestine, the food then goes to the large intestine. The large intestine has 3 parts: the cecum (or pouch that forms the T-junction with the small intestine), the colon, and the rectum. In the large intestine, water is reabsorbed, and the foods that cannot go through the villi, such as fiber, can be stored in large intestine. Fiber helps to keep the food moving through the G.I. tract. The food that cannot be broken down is called feces. Feces is stored in the rectum until it is expelled through the anus.