Absorption and egestion

These are the processes that happen in the digestive system:

ingestion (eating) → digestion (breaking down) → absorptionegestion (removal from the body)

Absorption

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.

Only small, soluble substances can pass across the wall of the small intestine. Large insoluble substances cannot pass through. The slideshow shows how this happens:

Food in the small intestine, and the wall of the small intestine which divides it from the blood stream

Food molecules in the small intestine are too large to pass across its wall and into the bloodstream

Adaptations for absorption

Absorption across a surface happens quickly and efficiently if:

  • the surface is thin
  • its area is large

The inner wall of the small intestine has adaptation so that substances pass across it quickly and efficiently:

  • it has a thin wall, just one cell thick
  • it has many tiny villi to give a really big surface area

If the small intestine had a thick wall and a small surface area, a lot of digested food might pass out of the body before it had a chance to be absorbed.

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

Shows that the walls of the villi are just one cell thick, and the network of capillaries, and the blood vesselsThe structure of villi

Egestion

Excess water is absorbed back into the body in the large intestine. What is left then is undigested food. This is stored in the rectum, the lower part of the large intestine, until we are ready to go to the toilet. It then comes out of the rectum through the anus as faeces. This process is called egestion. Take care not to confuse egestion with excretion.

The digestive system contains many bacteria and about half of the dry weight of faeces consists of bacteria. Bacteria in the digestive system are important. For example, they:

  • can digest some substances that humans cannot digest, such as certain carbohydrates
  • reduce the chance of harmful bacteria multiplying and causing disease
  • produce some vitamins that humans need, such as vitamins B and K

The process of how we digest our food from start to finish