<P> In mammals, preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach . Mechanical and chemical digestion begin in the mouth where food is chewed, and mixed with saliva to begin enzymatic processing of starches . The stomach continues to break food down mechanically and chemically through churning and mixing with both acids and enzymes . Absorption occurs in the stomach and gastrointestinal tract, and the process finishes with defecation . </P> <P> The human gastrointestinal tract is around 9 meters long . Food digestion physiology varies between individuals and upon other factors such as the characteristics of the food and size of the meal, and the process of digestion normally takes between 24 and 72 hours . </P> <P> Digestion begins in the mouth with the secretion of saliva and its digestive enzymes . Food is formed into a bolus by the mechanical mastication and swallowed into the esophagus from where it enters the stomach through the action of peristalsis . Gastric juice contains hydrochloric acid and pepsin which would damage the walls of the stomach and mucus is secreted for protection . In the stomach further release of enzymes break down the food further and this is combined with the churning action of the stomach . The partially digested food enters the duodenum as a thick semi-liquid chyme . In the small intestine, the larger part of digestion takes place and this is helped by the secretions of bile, pancreatic juice and intestinal juice . The intestinal walls are lined with villi, and their epithelial cells is covered with numerous microvilli to improve the absorption of nutrients by increasing the surface area of the intestine . </P> <P> In the large intestine the passage of food is slower to enable fermentation by the gut flora to take place . Here water is absorbed and waste material stored as feces to be removed by defecation via the anal canal and anus . </P>

Where does absorption occur in the digestive tract and what structures are needed for absorption