The intestines - Higher tier
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.
Absorption into bloodstream
The inside wall of the small intestine is thin, with a large surface area. This allows absorption to happen quickly and efficiently. To get a big surface area, the inside wall of the small intestine is lined with tiny villi. These stick out and give a big surface area. They also contain blood capillaries to carry away the absorbed food molecules.
The villi have a rich blood supply. The blood supply has a lower concentration of food molecules and so diffusiondiffusion: The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration occurs quickly.
Diagram of villli, showing the walls which are just 1 cell thick, and the network of capillaries, and the blood vessels.
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The Digestive System
- As food goes through the body, it goes through the following parts of the body:
- The Mouth, Oesophagus, Stomach, Duodenum, Ileum, Colon, Rectum and the Anus.
- Peristalsis is the rippling movements of the muscles to help the food travel along different parts of the alimentary canal.
- Goblet cells protect the living cells of the alimentary canal from hard rough food and enzymes. They make mucus. The mucus covers the inside of the alimentary canal with a slimy layer over which undigested food can easily glide. Also enzymes cannot get at living cells and harm them.
- Sphincter muscles are what control the outlet and inlet of food from the stomach to prevent blockages. They are rings of muscle, which can contract to squeeze the tube in and out of the stomach closed so that food stays inside the stomach while it is churned up with enzymes and hydrochloric acid.
- Sphincter muscles are found at both ends of the stomach; where the oesophagus enters the stomach and where the duodenum leaves the stomach. There is also the anal sphincter at the end of the alimentary canal, which opens to let faeces pass out of the rectum.
- Features which makes it the ileum good at absorbing digested food are:
- It is the longest part of the alimentary canal (5 metres long), so food takes a long time to pass through it so there is plenty of time for the food to be absorbed.
- It is one of the narrowest parts of the alimentary canal so food is always quite near the walls making it easier for food molecules to pass through the walls to the blood.
- The inner surface of the ileum is thrown into folds. These increase the surface area and the larger the surface area the faster the food is absorbed. On the folds are thousands of finger like projections called villi. These also increase the surface area and even have their own little projections called microvilli.
- There is a good blood supply to the ileum. Inside each villus is a blood capillary. The blood capillary absorbs amino acids and glucose. The blood supply is able to transport absorbed food to the rest of the body.
- Each villus also contains a lacteal. This absorbs fatty acids and glycerol and is a branch of the lymphatic system
- The walls of the ileum are thin and moist to aid absorption.