The accessory organs aid in digestion by adding various chemicals and enzymes to the food as it travels through the digestive tract. Digestion is the process of altering the chemical and physical composition of food for absorption and then use by the body cells. This process depends upon endocrine and exocrine secretions as well as the controlled movement of ingested food materials along the digestive tract. Chemical reduction of ingested nutrients into particles, then into molecules small enough to cross the lining of the gut and reach the internal environment of the body. Ingestion occurs when materials enter the digestive tract by the mouth.
Mechanical processing begins with the chewing and mastication by the teeth and tongue.
It is continued all along the digestive tract by the involuntary muscle contractions of the smooth muscle lining the stomach walls and intestines. Chemical digestion is the breakdown of food into small organic fragments suitable for absorption by the digestive epithelium. It starts in the mouth where saliva containing certain enzymes begin to break down carbohydrates and continues in the stomach and intestines as other enzymes and acid is released as the food progresses through the digestive tract. Absorption takes place mainly in the small intestines, though important nutrients and water are absorbed by the large intestine. Compaction is the job of the large intestines.