These are the processes that happen in the digestive system:
ingestion (eating) → digestion (breaking down) → absorption → egestion (removal from 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 there, the digested food molecules are carried around the body to where they are needed.
Absorption across a surface happens quickly and efficiently if:
The inner wall of the small intestine has adaptation so that substances pass across it 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.
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.
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: