Digestion includes mechanical digestion and chemical digestion, mechanical digestion is through the contraction of the muscles of the digestive tube wall, the food is ground, so that the food and digestive fluid are fully mixed, and the digested food components and the digestive tube wall close contact for absorption, so that the indigestible food residue discharged from the end of the digestive tract. Chemical digestion is the process by which digestive juices secreted by digestive glands break down food into small molecules that can be absorbed.

Food in the digestive tract in the decomposition process is referred to as “digest”, the basic function of the digestive system is the digestion and absorption of food, for the material and energy needed for the body, the nutrients in food in addition to vitamins, and inorganic salt water can be direct absorption use, substances such as protein, fat and sugar were not directly absorbed by the body to use, It needs to be broken down into simple small molecules in the digestive tube before it can be absorbed and used. In the small intestine, food is chemically digested by pancreatic fluid, bile and small intestinal fluid, as well as mechanically digested by the small intestine. Various nutrients are gradually broken down into simple absorbable small molecules which are absorbed in the small intestine.