Digestion is the breakdown of food into smaller particles or individual nutrients. It is accomplished through six basic processes, with the help of several body fluids—particularly digestive juices that are made up of compounds such as saliva, mucus, enzymes , hydrochloric acid, bicarbonate, and bile .

The six processes of digestion involve:

(1) the movement of food and liquids;

(2) the lubrication of food with bodily secretions;

(3) the mechanical breakdown of carbohydrates , fats, and proteins ;

(4) the reabsorption of nutrients—especially water;

(5) the production of nutrients such as vitamin K and biotin by friendly bacteria; and

(6) the excretion of waste products.

Digestion begins in the mouth with the action of salivary amylase. The food material then progresses past the esophagus and into the stomach. A bolus (soft mass) of chewed food moves by muscular wave actions, called peristalsis, from the mouth to the pharynx, and then past the epiglottis that covers the larynx. The epiglottis closes off the air passage so that one doesn't choke. The cardiac sphincter prevents reflux of stomach contents into the esophagus.

Food mixtures leaving the stomach are called chyme, and this empties into the small intestine after about two to four hours in the stomach. The small intestine is where most digestion takes place. A pyloric sphincter controls the rate of flow of chyme from the stomach into the small intestine.

Most digestion occurs in the upper portion of the small intestine, called the duodenum. Below the duodenum is the jejunum, and then there is the last segment, called the ileum. About 5 percent of undigested food products are broken down in the ileum. This is why some people can have a small part of their intestine removed and still seem to digest most foods with little problem.

Digestion of food that enters the small intestine is usually complete after three to ten hours. Once digestion is essentially finished, waste products leave the ileum with the help of fiber, and these