Melena pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Pathophysiology
Melena, is stool with blood, that has been altered by the gut flora, and appears black/"tarry".
Melena vs. Hematochezia
Bleeding originating from the lower GI tract (such as the sigmoid colon and rectum) is generally associated with the passage of bright red blood, or hematochezia, particularly when brisk. Blood acts as a cathartic agent in the intestine, promoting its prompt passage. Only blood that originates from a high source (such as the small intestine), or bleeding from a lower source that occurs slowly enough to allow for oxidation, is associated with melena. For this reason, melena is often associated with hemorrhage in the stomach or duodenum (upper gastrointestinal tract), for example by a peptic ulcer. A rough estimate is that it takes about 14 hours for blood to be broken down within the intestinal lumen; therefore if transit time is less than 14 hours the patient will have hematochezia and if greater than 14 hours the patient will exhibit melena. One often-stated rule of thumb is that melena only occurs if the source of bleeding is above the ligament of Treitz.