When the esophagus, stomach, duodenum, upper jejunum, pancreas and biliary tract bleed, the iron of hemoglobin in red blood cells combines with sulfide to form iron sulfide under the action of intestinal bacteria, thus making the feces appear black. This kind of black stool is black, shiny and looks like asphalt, so it is also called cypress oil stool. Acid stool is the manifestation of upper gastrointestinal bleeding.
However, it must be noted that discharging tarry black stool is only a typical manifestation of upper gastrointestinal bleeding. The color change of stool during upper gastrointestinal bleeding is not uniform tar-like, but varies widely. For example, when the amount of bleeding is small, the change of stool color can't be found by naked eyes, and it can only be found through special inspection (such as fecal occult blood test); When the amount of bleeding reaches more than 50 ml, the color of stool turns black with naked eyes. However, in addition to the above-mentioned tarry stool, the stool color can also turn brown, reddish brown or even bright red when the upper digestive tract bleeds.
The change of stool texture and color during upper gastrointestinal bleeding depends entirely on the location of upper gastrointestinal bleeding, the amount of bleeding, the bleeding speed and the time of blood staying in the intestine. If the amount of bleeding is large, the speed is fast, and the retention time in the intestine is short, the stool texture will become very soft or even close to liquid, and the color can be further changed from black to reddish brown or even bright red. Only when the amount of bleeding in the upper digestive tract is small, the bleeding speed is slow and the bleeding stays in the intestine for a long time can it show a typical tarry black stool.
The cause of melena is not just upper gastrointestinal bleeding.
As mentioned above, black stool is the manifestation of upper gastrointestinal bleeding. According to statistics, about half of patients with upper gastrointestinal bleeding are due to ulcer bleeding, and about three quarters are due to duodenal ulcer bleeding.
Besides ulcer, acute and chronic gastritis, liver cirrhosis complicated with esophageal or gastric varices rupture, gastric cancer and stress ulcer are also common causes of upper gastrointestinal bleeding. Therefore, when melena occurs, besides considering whether it is ulcer bleeding, we should also be alert to the existence of these diseases.
In addition, respiratory bleeding, such as nasopharyngeal bleeding, can also cause melena if swallowed into the digestive tract; When the lower digestive tract (i.e. lower jejunum, ileum, colon, rectum and anus) bleeds, dark red stool or new bloody stool usually appears. However, when the amount of bleeding is small, the intestinal tract is not stimulated much, the intestinal peristalsis is slow, and the blood stays in the intestinal tract for a long time, hemoglobin iron will turn into black iron sulfide, thus discharging black stool.
Know the "fake black stool" that has nothing to do with bleeding.
Sometimes, there is no bleeding in the upper digestive tract, lower digestive tract or respiratory tract, but the color of stool changes from yellow, light brown or dark brown to black. This kind of black stool that has nothing to do with bleeding is often confusing, so it should be "fake".
The cause of "false black stool" is mainly related to eating certain foods and drugs. For example, after eating pig's blood, sheep's blood or chicken's, goose's, duck's blood, or a large amount of beef and pig's liver, the iron in animal's blood can also form black iron sulfide in human intestine, making the stool black. For example, every time a couple eats pig blood, it is a good proof that they excrete black stool the next day. Even if the blood volume of animals eating food is relatively small, which is not enough to cause the color change of stool that can be observed by naked eyes, the positive result of fecal occult blood test can appear. Therefore, before the clinical examination of occult blood test, we must limit our diet, and generally we should be vegetarian for three days to exclude false positive results. In addition, oral active carbon powder, blood-enriching iron agent, bismuth agent for treating stomach diseases and some traditional Chinese medicines can also cause melena. Other foods, only the color of itself or its metabolites is black, can make the stool black. For example, eating a lot of bayberry jam and mulberry jam may make the stool black.
Therefore, when you find that the stool turns black, you must carefully distinguish it. You can't see the wind and rain, and you can't be careless. Black stool caused by bleeding generally has a history of primary disease (such as stomach pain, ulcer, liver cirrhosis, etc.). ) and the symptoms of the corresponding diseases. The appearance of black stool is black and shiny. With the increase of bleeding volume, the body will have symptoms of blood loss such as dizziness, weakness, thirst, palpitation, increased heart rate, pale face and even fainting. "False black stool" often has a special eating history, that is, it occurs after taking drugs that can cause black stool or eating animal blood products. Black stool is dark in color and rarely glows except by eating animal blood. Moreover, after stopping using this kind of food (medicine), the "black stool" disappears within three days without any other discomfort symptoms.
Be careful that "fake black stool" covers up "real bleeding"
Although "fake black stool" doesn't really have bleeding, if you are used to discharging black stool after eating pig blood, you won't care about the discomfort of hungry stomach caused by stressful life and irregular diet, but only consider the symptoms such as fatigue and anemia caused by bleeding from the perspective of nutrition. They didn't realize that the discharged black stool, in addition to eating pig blood, may also have ulcer bleeding. Until the doctors came to the conclusion that "duodenal ulcer, ulcer bleeding, severe anemia and hemorrhagic shock", they still thought that "pig blood obviously caused black stool".
This "fake black stool" covers up the "real bleeding" and is easy to happen to patients who have taken some stomach medicines. For example, it is normal for patients with gastric diseases to take bismuth tripotassium citrate, which can protect gastric mucosa and inhibit Helicobacter pylori in the stomach. It is a typical "fake black stool" that the stool is gray-black. The occult blood test of this kind of black stool is negative and can be distinguished from true bleeding. However, if the vigilance against bleeding is relaxed because of "fake black stool", so that the patient is rushed to the hospital because of excessive bleeding and serious bleeding symptoms and signs, it will often delay the best treatment opportunity and cause additional losses to the patient.
Therefore, if you are not completely sure whether the black stool is true or not, it is better to go to the hospital for examination.