Rectal cancer may be of the adenocarcinoma type and usually arise from the epithelium (the layer of cells) which lines the large intestine.
The colon is part of the large bowel. The large bowel starts at the end of the small bowel (the ileum), at the caecum. The caecum has the appendix running off it. The start of the colon is the ascending colon and where this rises to meet the liver (the hepatic flexure) it becomes the transverse colon. The transverse colon goes across the upper abdomen until it becomes adjacent to the spleen (the splenic flexure) and at this point it becomes the descending colon. The large bowel at this point goes down the abdomen to the pelvis at which point it becomes the sigmoid colon (because it curves in an “S” shape, sigma being the Greek for “S”). The sigmoid colon terminates at the rectum, which acts as a storage pouch for faeces before it is evacuated through the anus.