The mouth, stomach and intestines

The gastrointestinal (GI) tract comprises a long tube with anatomical and functional specializations, beginning at the mouth and ending at the anus. The histology of the tract wall is relatively regular from the oesophagus to the large intestine and comprises mucosa, submucosa, muscularis and serosa layers. The tract functions primarily in digestion and absorption of nutrients, which begins in the mouth with the chewing and mixing of food with saliva. Small boluses of food are then swallowed during a complex reflex process and make their way through the oesophagus to the stomach, where they can be stored while further digestion takes place. Gastric contents are slowly emptied into the small intestine, where the majority of digestion and absorption occurs, before any undigested components are moved into the large intestine. The final absorption of water and electrolytes takes place here to produce a stool that only contains around 100–200 ml of water. The large intestine has a diverse bacterial population that contributes to digestion and can influence the health of an individual. Each component of the digestive tract has secretions that contribute to digestive function, as well as immunity and the excretion of waste.