An ostomy is an opening in the abdominal wall to which a portion of the gastrointestinal tract is attached. Its purpose is to provide a pathway for digested material to leave the body when the normal pathway is blocked due to a variety of situations. It can involve a part of the small intestine, i.e. an ileostomy, or a part of the large intestine, i.e. a colostomy. Thus, an overview of the anatomy of the gastrointestinal tract is a logical place to begin.