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The colon and rectum
The intestine is part of the digestive system. It is made up of the small intestine and the large intestine. The colon and rectum are parts of the large intestine. The colon is a U-shaped tube made of muscle, found below the stomach. The rectum is a shorter tube connected to the colon. Together, the colon and rectum are about 2 metres (6.5 feet) long. They are surrounded by other organs including the spleen, liver, pancreas, bladder and reproductive organs.
Parts of the large intestine
The large intestine is made up of the cecum, colon, rectum and anus. The colon and rectum are held in the abdomen by folds of tissue called mesenteries.
The cecum is a pouch-like passage that connects the colon to the ileum (the last part of the small intestine). If cancer develops in the cecum, it is treated like colon cancer.
The colon is the longest part of the large intestine. It receives almost completely digested food from the cecum, absorbs water and nutrients, and passes waste (stool or feces) to the rectum. The colon is divided into 4 parts:
- The ascending colon is the start of the colon. It is on the right side of the abdomen. It continues upward to a bend in the colon called the hepatic flexure.
- The transverse colon follows the ascending colon and hepatic flexure. It lies across the upper part of the abdomen. It ends with a bend in the colon called the splenic flexure.
- The descending colon follows the transverse colon and splenic flexure. It is on the left side of the abdomen.
- The sigmoid colon is the last part of the colon and connects to the rectum.
The proximal colon is the ascending colon and the transverse colon together. The distal colon is the descending colon and the sigmoid colon together.
The rectum is the lower part of the large intestine that connects to the sigmoid colon. It is about 15 cm (6 in) long. It receives waste from the colon and stores it until it passes out of the body through the anus.
The anus is the opening at the lower end of the rectum through which stool is passed from the body. Cancer in the anal canal or anus is treated differently from colorectal cancer. Find out more about anal cancer.
Mesentery is made of fatty connective tissue that contains blood vessels, nerves, lymph nodes and lymph vessels. The mesocolon is a mesentery that attaches the colon to the wall of the abdomen. The rectum is surrounded by a mesentery called the mesorectum.
When part of the colon or rectum is removed to treat cancer, nearby mesentery is also removed. The lymph nodes within the mesentery are examined to see if they contain cancer cells.
Layers of the colon and rectum
The colon and rectum are made up of the following layers of tissues.
The mucosa is the inner lining of the colon and rectum. It is made up of:
- a thin layer of epithelial cells (called the epithelium)
- a layer of connective tissue (called the lamina propria)
- a thin layer of muscle (called the muscularis mucosa)
The submucosa is a layer of connective tissue that surrounds the mucosa. It contains mucous glands, blood vessels, lymph vessels and nerves.
The muscularis propria lies outside the submucosa. It is a thick layer of muscle. It has an inner ring of circular muscle fibres and an outer ring of long muscle fibres that surround the wall of the colon and rectum.
The serosa is the outer layer of the colon. It is not found on most of the rectum.
The colon and rectum:
- absorb water and some nutrients from what we eat and drink
- form and store stool
- move waste out of the body
Partly broken down or digested food moves from the small intestine into the colon. Sections of the colon tighten and relax to move the food through the colon and rectum. This movement is called peristalsis.
In the colon, bacteria break down food into smaller pieces. The inner layer of the mucosa (called the epithelium) absorbs, or takes up, water and some nutrients. The liquid waste remaining in the colon is formed into semi-solid stool.
The mucosa also makes mucus that helps stool move easily through the colon and rectum. As stool moves through the colon, more water is absorbed from it and it becomes more solid.
Stool leaves the colon and moves into the rectum. The rectum is a holding area for the stool. When the rectum is full, it pushes the stool out of the body through the anus.
The group of organs that work together to take in food and liquid, break them down, absorb nutrients and pass waste from the body.
The digestive system includes the organs of the gastrointestinal (GI) tract, or digestive tract. The organs of the GI tract are the mouth, pharynx (throat), esophagus, stomach, small intestine and large intestine. The digestive system also includes other organs of digestion, which are the teeth, tongue, salivary glands, liver, pancreas and gallbladder.
A specialized type of tissue that surrounds and supports various organs, structures and other tissues in the body.
Connective tissue includes blood, bone, cartilage and fat.
A specialized cell that makes up the epithelium (a layer of cells that makes up the surface of the skin, and lines cavities, glands and passages in the body). Some epithelial cells make mucus, hormones or other secretions.
The 4 types of epithelial cells are squamous cells, columnar cells, cuboidal cells and transitional cells.
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