The large intestine is a hollow tube that makes up the last 6 feet of the digestive tract. It is often referred to as the large bowel or colon (which is technically just one part of the large intestine). The large intestine consists of the cecum (a pouch-like structure at beginning of the large intestine), colon, rectum and anus. The colon and rectum are next to other organs, including the spleen, liver, pancreas, and reproductive and urinary organs. Each of these organs can be affected if colorectal cancer spreads beyond the large intestine.
The colon begins at the cecum, where it joins the end of the small intestine (ileum). The colon changes to rectal tissue in its last 6 inches. Because there is not a clear border between the colon and rectum, colon and rectal cancers are grouped together as colorectal cancer.
The colon is divided into 4 parts:
- ascending colon – begins at the cecum, where it joins the end of the small intestine, and travels upward along the right side of the body to the transverse colon
- transverse colon – connects the ascending colon to the descending colon and lies across the upper abdomen
- descending colon – connects the transverse colon and the sigmoid colon and lies along the left side of the body
- sigmoid colon – connects the descending colon and the rectum
The colon and rectum are made up of a number of different tissues organized into layers:
- inner lining (epithelium), lamina propria (connective tissue) and thin layer of muscle (muscularis mucosa)
- connective tissue, glands, blood vessels, lymphatic vessels and nerves
- muscularis propria (muscular layer)
- serosa (serous layer)
- outer lining of the colon but not the rectum
Mesentery is a fold of tissue that connects organs to the body wall. Part of the colon is connected to the abdominal wall by mesentery. The upper two-thirds of the rectum is also covered by mesentery called the mesorectum. The mesentery is made up of fatty connective tissue that contains the blood vessels, nerves, lymph nodes and lymphatic vessels that supply the colon. When surgeons remove part of the colon or rectum to treat cancer in these areas, they will remove the mesentery next to these organs as well. The lymph nodes within the removed mesentery will be examined to see if they contain cancer.
The main functions of the colon and rectum are to absorb water and nutrients from what we eat and to move food waste out of our body.
- The colon receives partially digested food, in a liquid form, from the small intestine.
- Bacteria (bowel flora) in the colon break down some materials into smaller parts.
- The epithelium absorbs water and nutrients. It forms the remaining waste into semi-solid material (feces or stool).
- The epithelium also produces mucus at the end of the digestive tract, which makes it easier for stool to pass through the colon and rectum.
- Sections of the colon tighten and relax (peristalsis) to move the stool to the rectum.
- The rectum is a holding area for the stool. When it is full, it signals the brain to move the bowels and push the stool from the body through the anus.