Bloody stool is also called bloody stool. Blood in stool is blood discharged from anus, which can be mixed with feces and mucus, and the amount varies, and it can be bright red, dark red or tarry stool. There are many reasons for hematochezia, which can be symptoms of systemic diseases or local diseases of digestive tract, respiratory tract and oral cavity. Blood diseases, septicemia, vitamin deficiency, drug poisoning and other systemic diseases can cause hematochezia, accompanied by obvious primary symptoms; Children with nosebleeds or tooth bleeding can also cause bloody stool after swallowing, which is often a positive test for discharging black stool or occult blood in stool. But most hematochezia is caused by digestive tract diseases, such as peptic ulcer, enteritis, dysentery, intussusception, rectal polyp, anal fissure and so on. If bleeding occurs in the upper digestive tract or small intestine, the blood stays in the gastrointestinal tract for a long time, and it will turn black under the action of gastric acid and intestinal bacteria, which is a tar stool. Colon, rectum and anal bleeding are usually bright red.