What are causes of blood in the stool (rectal bleeding)?

Color of blood in the stool

Blood in the stool primarily comes from the upper gastrointestinal tract (stomach and duodenum although occasionally the esophagus) or the lower gastrointestinal tract (colon, rectum, and anus). Although the small intestine also may be the source of blood in the stool, compared to the upper and lower parts of the gastrointestinal tract, the small intestine is infrequently the source. Most people think of blood in the stool as meaning that the stool contains red blood, but this is not always true. As discussed previously, the bloody stool may be maroon colored or black.

The colon is the part of the gastrointestinal tract through which undigested food passes after the digestible part of the food has been digested and absorbed in the small intestine. The colon is primarily responsible for removing water from the undigested food, and storing the undigested food until it is eliminated from the body as stool. The rectum is the last 15 cm (6 inches) of the colon. The anal canal, approximately an inch in length, connects the rectum with the anus opening through which stool passes when it is eliminated from the body. Together, the colon, rectum, anal canal, and anus form a long (several feet in length), muscular tube that also is known as the large intestine, large bowel, or the lower gastrointestinal tract.

The colon can be divided further into three regions; the right colon, the transverse colon, and the left colon. The right colon, also known as the ascending colon, is the part of the colon into which undigested food from the small intestine is first deposited. It is furthest from the rectum, anal canal, and anus. The transverse colon forms a bridge between the right and the left colon. The left colon is made up of the descending colon and the sigmoid colon. The sigmoid colon connects the descending colon to the rectum.

The color of blood in the stool often depends primarily on the location of the bleeding in the gastrointestinal tract. Generally, the closer the bleeding site is to the anus, the brighter red the blood will be. Thus, bleeding from the anus, rectum, and the sigmoid colon tends to be bright red, whereas bleeding from the transverse colon and the right colon tends to be dark red or maroon-colored. With bleeding from the upper GI tract and depending on how long the blood remains in the stomach and small intestine, the color in the stool will change from bright red, to maroon, to black. Blood in the stool that is red or maroon is most commonly is referred to as rectal bleeding.

Bleeding that occurs from the stomach and duodenum frequently is black, "tarry" (sticky), and foul smelling. The black, smelly and tarry stool is called melena. Melena mostly occurs when the bleeding is in the stomach where the blood is exposed to acid or is in the small intestine for a prolonged amount of time; however, melena also may occur with bleeding from the colon if the transit of the blood through the colon is slow, and there is enough time for the intestinal bacteria to break the blood down into chemicals (hematin) that are black.

Blood from the sigmoid colon, rectum and anus usually does not stay in the colon long enough for the bacteria to turn it black. Rarely, massive bleeding from the right colon, from the small intestine, or from ulcers of the stomach or duodenum can cause rapid transit of the blood through the gastrointestinal tract and result in bright red rectal bleeding. In these situations, the blood is moving through the colon so rapidly that there is not enough time for the bacteria to turn the blood darker or black.

Occult gastrointestinal bleeding

Another "type" of blood in the stool is occult blood. Occult gastrointestinal bleeding refers to a slow loss of blood into the upper or lower gastrointestinal tract that does not change the color of the stool or result in the presence of visible bright red blood. Blood in the stool is detected only by testing the stool for blood (fecal occult blood testing) in the laboratory. Occult blood in the stool has many of the same causes as rectal bleeding, and may be associated with the same symptoms as more active bleeding. For example, slow bleeding from ulcers, colon polyps, or cancers can cause small amounts of blood to mix and be lost within the stool. Chronic occult bleeding often is associated with anemia due to the loss of iron along with the blood (iron deficiency anemia).