Digestion

About this course

A meal takes 24 to 72 hours to pass through a tube about 7.6 metres long. Each part of the tract moves food along, breaks it into smaller pieces, or both. Mechanical digestion is physical: chewing, churning and squeezing. Chemical digestion uses juices that contain enzymes, acid and bile to split large molecules into small ones. This course is general information about how the body works and is not medical or diet advice.

The six units follow the food: a map of the digestive system, mouth, teeth and swallowing, the stomach, liver, pancreas and gallbladder, the small and large intestines, and finally molecules, absorption and how we found out. The first set of 20 baby teeth begins to appear at about 6 months. The 28 permanent teeth other than wisdom teeth usually emerge between about ages 6 and 13. Wisdom teeth usually emerge later; the full adult set can include up to 32 teeth. The stomach does not digest itself because of a mucosal barrier: mucus rich in bicarbonate, tight junctions between lining cells, and stem cells that renew the whole surface lining every 3 to 6 days. Protein enzymes from the pancreas are made in an inactive form, and in the small intestine enteropeptidase switches trypsinogen into trypsin, which switches on the others. Segmentation mixes the chyme about 12 times a minute in the duodenum and about 8 in the ileum. The small intestine has 200 square metres of absorbing surface.

The last unit is a story of discovery. In 1982, stress and lifestyle were considered the major causes of peptic ulcers. The pathologist Robin Warren in Perth, Australia, had noticed small curved bacteria in the lower stomach in about half of the patients whose biopsies he examined, always close to signs of inflammation. The 2005 Nobel Prize recognised the discovery of Helicobacter pylori and its role in gastritis and peptic ulcer disease.