Hematochezia (blood in the stool): blood coming out of the anus, or coming down with the stool, or pure blood. It is generally believed that black stool may occur when the amount of gastrointestinal bleeding is 50 ml or more. Because the iron in hemoglobin combines with sulfide in the intestinal canal to produce iron sulfide, the stool is tarry black. Those who vomit blood mostly have black stools. There are many causes of blood in the stool, and almost all GI bleeding can cause blood in the stool, but the common causes are: ① upper GI bleeding (see vomiting blood). ② small intestine bleeding, such as intestinal tuberculosis, restrictive enteritis, acute hemorrhagic necrotizing enteritis, small intestine tumor, intestinal entrapment, etc. ③ Colon bleeding, such as dysentery, ulcerative colitis, restrictive enteritis, colon cancer, etc. ④ Rectal bleeding, such as rectal cancer, rectal damage, hemorrhoids, anal fissure, etc. ⑤ Other diseases, such as various blood diseases, epidemic hemorrhagic fever, typhoid and paratyphoid fever, hookworm disease, vitamin deficiency, etc. Upper gastrointestinal bleeding and small intestinal bleeding are mostly dark red or black stools, which are often shiny because of a thin layer of mucus covering the surface. But if the bleeding rate is faster, bleeding volume, intestinal peristalsis increases, blood stool can be dark red or bright red. When the colon and rectum bleed, the blood stays in the intestine for a shorter period of time, so the blood stool is often bright red or brighter red. When the right hemicolectomy bleeds, blood is often mixed with feces, and “jam-like” blood stools can be passed. In rectal and sigmoid bleeding, blood often adheres to the surface of the stool. In acute massive blood in stool, the patient may present with acute hemorrhagic anemia and acute peripheral circulatory insufficiency. In addition to history and physical examination, endoscopy of the digestive system is the first choice for the diagnosis of blood in the stool. If necessary, barium x-ray, nuclear and angiography may also be used to aid in the diagnosis.