Human body is made up of countless millions of cells. Food is needed to built up new cells and replace the worn out cells. However, the food that we take must be changed into substances that can be carried in the blood to the places where they are needed. This process is called digestion. The first digestive process takes place in the mouth. The food we eat is broken up into small pieces by the action of teeth, mixed with saliva, a juice secreted by glands in the mouth. Saliva contains digestive juice which moisten the food, so it can be swallowed easily. From the mouth, food passes through the esophagus (the food passage) into the stomach. Here, the food is mixed with the juices secreted by the cells in the stomach for several hours. Then the food enters the small intestine. All the time the muscular walls of the intestine are squeezing, mixing and moving the food onwards. In a few hours, the food changes into acids. These are soon absorbed by the villi (microscopic branch projections from the intestine walls) and passed into the bloodstream.