The gastrointestinal (GI) tract is a tube of muscle that runs from the mouth to the anus. The upper GI tract starts at the mouth and includes the stomach and the oesophagus.

Digestion starts the moment food is eaten, due to the mechanical action of chewing. At the same time, salivary glands in the mouth release saliva, which contains an enzyme called salivary amylase. This helps to break down starch in a process known as chemical digestion.

After swallowing, food passes into the oesophagus and travels downwards, aided by waves of muscular contractions called peristalsis. At the end of the oesophagus is a ring of muscle called the lower oesophageal sphincter. This marks the opening to the stomach. In the stomach, food continues to be broken down due to the action of gastric juices, the main components of which are acid and pepsin. Mucus is produced to protect the lining of the stomach.

After one to two hours, the food that has been eaten has been turned into a thick liquid called chyme. The pyloric valve of the stomach opens and allows the chyme into the duodenum, where it mixes with more digestive enzymes, this time from the pancreas, and bile made by the liver. This mixture passes into the small intestine, from where most of the nutrients are absorbed into the body.