Small Intestine

The small intestine is made up of three parts:

  • Duodenum
  • Jejunum
  • Ileum

The first part of the small intestine is called the duodenum. The acidic chyme that is leaving the stomach would damage the lining, so to make it safe it is neutralised by an alkali (similar to how Tanno and Iguda escaped the stomach acid). The duodenum also receive enzymes from the pancreas that continue to digest the food chemicals, as well as the liver producing bile, which is important in helping fat digestion.

SOMSO® model from Adam,Rouilly

In this model of the digestive system you can see how the small intestine follows on from the stomach. It is over 6 metres long, but coiled up to fit into a much smaller space…

Image Credit: SOMSO® model from Adam,Rouilly

The food then moves through the jejunum and into the ileum, where digestion is completed by even more enzymes

The digested food molecules are absorbed in the small intestine. This means that they pass through the wall of the small intestine and into our bloodstream. Once there, the digested food molecules are carried around the body to where they are needed.

The villi (a single one is called a villus) stick out and provide a big surface area. They also contain blood capillaries to quickly carry away the absorbed food molecules

model of the jejunum wall
SOMSO® model from Adam,Rouilly

Efficient absorption across a surface happens when the surface is thin and its area is large. The inner wall of the small intestine is adapted so that substances pass across it quickly by having a thin wall which is only one cell thick, and many microscopic villi to give a really big surface area. This means all the useful digested food is absorbed into the blood.

Only small, soluble substances can pass across the wall of the small intestine. Large insoluble substances cannot pass through, which explains why digestion has to happen in the first place.

Learn about the entire digestive system from this video:


Investigating enzymes

WARNING: some people are allergic to biological washing powder – do NOT do this activity if you are allergic.

1) Take 3 glasses or clean empty jam jars and fill them with water.
2) Add 2 tablespoons of biological washing powder to one, 2 tablespoons of a different brand of biological washing powder to another, and leave the third as a control with just water.

person removing egg shell
Photo by Ron Lach on

3) Cut (with the help of an adult) the white of a hard-boiled egg into 3 lumps of about the same size or thickness, approximately 5mm thick.

4) Place 1 lump into each of the glasses / jars and leave somewhere warm for 3 or 4 days.Compare the appearance of the lumps.

5) Allow the experiment to run for longer if you want to see bigger differences.

Biological washing powder contains enzymes that are used to digest food stains on clothes, making them easier to clean. The egg white is made up of proteins so will be broken up by a washing powder with enzymes to break up proteins. Not all washing powders use the same enzyme and different brands may add different amounts of enzyme, so they don’t all work as well as one another.

You could extend this investigation by using a non-biological washing powder as well, just to check that it is the enzymes that affect the egg white and not some other ingredient.

Make your own small intestine model!

You could use an old sock and fill it with newspapers or old packaging or whatever you can find.

You can go to the previous PostcardsFromTheBody secret pages by clicking the places below.

Photo of PostcardFromThe Mouth