Blood in the stool can be bright red, maroon in color, black and tarry, or occult (not visible to the naked eye). Causes of blood in the stool range from harmless, annoying conditions of the gastrointestinal tract such as hemorrhoids and anal tears (anal fissures) from straining against hard stools with constipation to serious conditions such as cancer. Blood in the stool should be evaluated by a health care professional.
Rectal bleeding (known medically as hematochezia) refers to passage of bright red blood from the anus, often mixed with stool and/or blood clots. Most rectal bleeding comes from the colon, rectum, or anus and can be associated with diarrhea. The color of the blood during rectal bleeding often depends on the location of the bleeding in the gastrointestinal tract. Generally, the closer the bleeding site is to the anus, the blood will be a brighter red. Thus, bleeding from the anus, rectum, and the sigmoid colon tend to be bright red, whereas bleeding from the transverse colon and the right colon (transverse and right colon are several feet away from the anus) tend to be dark red or maroon colored.
In some patients, bleeding can be black and "tarry" (sticky) and foul smelling. The black, smelly, and tarry stool is called melena. Melena occurs when the blood is in the colon long enough for the bacteria in the colon to break it down into chemicals (hematin) that are black. Therefore, melena usually signifies bleeding is from the upper gastrointestinal tract (for example, bleeding from ulcers in the stomach or the duodenum or from the small intestine) because the blood usually is in the gastrointestinal tract for a longer period of time before it exits the body. Sometimes melena may occur with bleeding from the right colon. On the other hand, blood from the sigmoid colon and the rectum 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 black.
Sometimes, bleeding from the gastrointestinal tract can be too slow to cause either rectal bleeding or melena. In these patients, bleeding is occult (not visible to the naked eye). The blood is found only by testing the stool for blood (fecal occult blood testing) in the laboratory. Occult bleeding has many of the same causes as rectal bleeding and may result in the same symptoms as rectal bleeding. It is often associated with anemia that is due to loss of iron along with the blood (iron deficiency anemia).
Causes of rectal bleeding include hemorrhoids, anal fissure, proctitis, Crohn's disease, diverticulosis, ulcerative colitis, rectal ulcer, rectal prolapse, ischemic colitis, angiodysplasia, rectal trauma, anal cancer, colon cancer, proctitis from radiation therapy, and others.
Kasper, D.L., et al., eds. Harrison's Principles of Internal Medicine, 19th Ed. United States: McGraw-Hill Education, 2015.
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