In the human digestive system, the process of digestion has many stages, the first of which starts in the mouth (oral cavity). Digestion involves the breakdown of food into smaller and smaller components which can be absorbed and assimilated into the body.he organs known as the accessory digestive glands are the liver, gall bladder and pancreas. Other components include the mouth, teeth and epiglottis. The largest structure of the digestive system is the gastrointestinal tract (GI tract).