The colon and rectum are parts of the digestive system, which is also called the
gastrointestinal (GI) system (see illustration). The first part of the digestive system (the
stomach and small intestine) processes food for energy while the last part (the colon and
rectum) absorbs fluid to form solid waste (fecal matter or stool) that then passes from the
body. To understand colorectal cancer, it helps to know something about the normal structure
of the digestive system and how it works.