The digestive system consists of the gut which forms a tube extending form the mouth at one end， throught the body， to anus at the other end. Food is ingested (it is taken in)， in ths mouth ，it chewed， mixed with saliva and swallowed. The food is now inside the wall of the gut but it is not yet insed the body itself. Before it can pass through the wall of the gut and into the blood， it must be digested. The food is mixed with digestive juives secreted by various glands as it is squeezed and pushed along by the miscular walls of the gut.
The digestive juices made by gland cells of the digestive system contain enzymes. The enzymes act on the large insoluble molecule of protein，starch and fats that are the main components of our food. The enzymes break down into smaller soluble molecules such protein is digested into amino acid， starch is digested into glucose and fats are digested to a mixtire of fatty acid and glycerol.
These small molecules are absobed through the small intestine wall into the blood and transported to the body’s cells. Some substance in food such as cellulose cannot be digested by the human gut. They pass out thourgh the anus， together with cells scraped from the lining ， enzymes and bacteria，and are egested as feaces