Hematin is a dark, pigment-like substance found in the stomach or upper digestive tract, signaling the presence of old, altered blood. This compound forms when blood from an internal bleed is exposed to the highly acidic environment of the stomach. Its presence confirms that bleeding has occurred in the upper gastrointestinal system, such as the esophagus, stomach, or the initial segment of the small intestine.
The Chemistry of Hematin Formation
The formation of hematin is a direct chemical reaction involving hemoglobin, the protein responsible for carrying oxygen in red blood cells. Hemoglobin contains an iron-containing molecule called heme, which normally gives blood its bright red color. When bleeding occurs into the stomach, this protein is immediately exposed to hydrochloric acid, the primary component of gastric acid. The acid acts as an oxidizing agent, chemically altering the structure of the heme molecule. Specifically, the iron atom within the heme group is converted from its ferrous state to its ferric state through an oxidation process, creating hematin, also known as acid hematin.
Recognizing Hematin in Visible Symptoms
The dark pigment of hematin is responsible for two distinct visible signs of upper gastrointestinal bleeding. The first is coffee-ground emesis, which describes vomit that has the appearance and texture of spent coffee grounds. This occurs when stomach acid partially digests the blood before it is expelled.
The second sign is melena, the passage of black, sticky, and tar-like stools. This appearance indicates that the altered blood has traveled through the entire digestive tract. For melena to occur, an estimated 100 to 200 milliliters of blood must have entered the upper GI tract. The presence of either coffee-ground emesis or melena strongly suggests the bleeding source is located above the ligament of Treitz, the anatomical boundary separating the upper and lower digestive tracts.
Common Sources of Upper Gastrointestinal Bleeding
The detection of hematin directs medical attention to underlying conditions causing upper GI bleeding. The most frequent cause is peptic ulcer disease, involving open sores on the lining of the stomach (gastric ulcers) or the first part of the small intestine (duodenal ulcers). These ulcers often result from infection with the bacterium Helicobacter pylori or from the regular use of non-steroidal anti-inflammatory drugs (NSAIDs). Another source is erosive gastritis, which is inflammation and superficial damage to the stomach lining, often associated with NSAID use, excessive alcohol consumption, or physiological stress. A less common but serious cause involves esophageal varices, which are enlarged, fragile veins in the lining of the lower esophagus that develop most often in individuals with severe liver disease.