The human body uses various kinds of food for energy and growth. To be used, however, food must be changed into a form that can be carried through the bloodstream. The body's process of extracting useful nutrients from food is called digestion. The of humans and other higher animals is the group of organs that changes food--carbohydrates, fats, and proteins--into soluble products that can be used by the body. Both mechanical action and chemical action are necessary to change food into products that are usable by the body. Human digestio ....