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Diet, drugs and health


Absorption and egestion

These are the processes that happen in the digestive system:

ingestion (eating) → digestion (breaking down) → absorptionegestion

We've already looked at how foods are broken down by enzymes in digestion. Now we will look at how the products of digestion are absorbed into the body.


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 in the bloodstream, 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. Study the slideshow to check your understanding of this.

Absorption into bloodstream

Diagram shows food in the small intestine, and the wall of the small intestine which divides it from the blood stream.

The inside wall of the small intestine needs to be thin, with a really big surface area. This allows absorption to happen quickly and efficiently. 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.

To get a big surface area, the inside wall of the small intestine is lined with tiny villi (one of them is called a villus). These stick out and give a big surface area. They also contain blood capillaries to carry away the absorbed food molecules.

Diagram of villli, showing the walls which are just 1 cell thick, and the network of capillaries, and the blood vessels


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.


Diet, drugs and health activity

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Gut Instinct

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