Melena (Gk. melas = black) is a medical term for a black tarry stool due to blood in the stool, usually originating from the upper gastrointestinal tract (the stomach, esophagus, mouth, nose). It is often thought that blood in the stool will appear bright red and fresh but this is known as hematochezia. When there is bleeding in the upper gastrointestinal tract, the blood degrades by the time it reaches the rectum to be passed out in feces.
Red to Black Blood in the Stool
Bleeding in the upper gut may occur for various reasons. Inflammation, ulcers and even tears may occur for various reasons. Less frequently, some vessels become enlarged and tortuous and are easily rupture (varices). When blood leaks into the upper gut, it has to go through the various digestive processes like food. Digestive enzymes, especially those enzymes that breakdown protein, air in the gut and the action of bowel bacteria in the lower gut systematically degrades blood. By the time it reaches the lower bowel, it has already degraded substantially.
With melena, the stool has a black tarry appearance due to large volumes of degraded blood (dark old blood). If the bleeding was from the lower gut, then blood may be able to retain its red fluid consistency. However, if a person is constipated then even bleeding in the lower gut can appear dark and degraded. This is because it is retained in the system for longer than normal and the air in the gut plus colon bacteria break it down.
On the other hand, bleeding in the upper gut could appears as bright red and fresh in the stool if a person has diarrhea. This is because the movement of food is so rapid through the gut in diarrhea that the blood does not have time to degrade. The bright red and fresh blood in the stool is known as hematochezia and one of the most common causes i rectal bleeding due to hemorrhoids.
It is important to differ between melena and black bowel movement from reasons, other than bleeding.