The small intestine

After the stomach, the partly digested food moves into the small intestine. Don't be fooled by the name: it is about 6 metres long, almost twice the height of your classroom, and it is the longest part of the alimentary canal. It fits inside you because it is coiled up.

The small intestine gets digestive juices from three sources: its own inner lining, and two organs linked to the canal, the liver and the pancreas.

The liver makes bile, which is mildly basic. Remember neutralisation from the acids-and-bases chapter: bile neutralises the acid in the food coming from the stomach. It also breaks fats into tiny droplets, so they are easier to digest. Bile is stored in a small bag near the liver until it is needed.

The pancreas makes pancreatic juice, which is also basic and helps neutralise the acid. It also breaks down carbohydrates, proteins and fats.

The juice made by the wall of the small intestine itself breaks fats, proteins and partly digested carbohydrates down further, into their simplest forms.

Now the digested food passes from the small intestine into the blood in the tiny blood vessels in its walls. This is called absorption of nutrients.

How does so much get absorbed? The inside lining is thin and covered with thousands of finger-like projections. They give a much bigger surface area — more surface touching the food — so absorption is quick and efficient. The blood then carries the nutrients all over the body to give energy, help growth and repair, and keep the body working.