Rectal Bleeding (cont.)
Origin of rectal bleeding (where the blood comes from)
Most rectal bleeding comes from the colon, rectum, or anus. The colon is the
part of the gastrointestinal tract through which food passes after it has been
digested in the small intestine. The colon is primarily responsible for removing
water from the undigested food and storing it until it is eliminated from the
body as stool. The rectum is the last 15 cm of the colon. The anus (anal canal)
is the opening through which stool passes when it is being eliminated from the
body. Together, the colon, rectum, 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 esophagus, stomach, duodenum, and
small intestine are referred to as the upper 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 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 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 tend to be dark red or maroon.
In some patients bleeding from the right colon can be black, "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 duodenal 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 (upper or lower) can be too slow to cause either
rectal bleeding or melena. For example, slow bleeding from colon polyps or
cancers can cause small amounts of blood to mix and be lost within the stool.
Special tests (fecal occult blood tests) are needed to demonstrate the blood.
Occult gastrointestinal bleeding
Rectal bleeding is a type of gastrointestinal bleeding. Rectal bleeding needs
to be distinguished from another type of gastrointestinal bleeding, occult
gastrointestinal bleeding. Occult gastrointestinal bleeding refers to a slow
loss of blood into the upper or lower gastrointestinal tract that does not cause
changes in the color of the stool or result in visible bright red blood. 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). For more information, please see the colon cancer screening and
fecal occult blood test articles.
Next: What are the causes of rectal bleeding? »
- Colonoscopy - Learn about the colonscopy procedure, what it is, why it is performed, preparation, complications, alternatives and the after effects of the screening exam on MedicineNet.com
- Abdominal Pain - Learn about abdominal pain (pain in the stomach / abdomen) including causes, symptoms, how abdominal pain is diagnosed, and how abdominal pain is treated.
- Fecal Occult Blood Test - Read about the fecal occult blood test in which there are two types, chemical and immunologic. Fecal occult blood tests are used to screen for colon cancer and colon polyps.
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