Blood in the stool refers to the discharge of blood from the anus or blood in the stool, which may be bright red, dark red or tarry in color. Blood may be mixed with stool, attached to the surface of the stool, or dripping fresh blood after the stool. The color of blood in the stool and its relationship to the stool depends on the site of bleeding, the amount of bleeding and the time the blood stays in the intestine. Upper gastrointestinal bleeding may appear as black stool or dark red blood. When the bleeding in the small intestine is large and fast, the blood stool may be dark red, bright red or purple clots; when the bleeding in the small intestine is small and the blood stays in the intestine for a long time, it may appear as tarry stool. When bleeding from the colon and rectum, the blood tends to be fresher because of the shorter residence time in the intestine. In the case of bleeding from the descending colon and above, blood is often mixed with stool, and in the case of bleeding from the sigmoid colon or rectum, fresh blood may be attached to the surface of the formed stool. Fresh blood dripping after stool is common in anorectal diseases, such as hemorrhoids, anal fissures, and rectal polyps. In addition, bleeding from hemorrhoids is more common in adults, bleeding from polyps is more common in children and youth, bleeding from anal fissures is more common in young women, and bleeding from malignant tumors is more common in middle-aged and elderly people. The causes of blood in the stool are systemic diseases and digestive tract diseases. 1, systemic diseases (1) blood system diseases: such as primary or secondary thrombocytopenic purpura, leukemia, aplastic anemia, hemophilia and diffuse intravascular coagulation due to various causes, etc. (2) Vitamin deficiency: vitamin K, vitamin C deficiency, etc. (3) Acute infectious diseases: epidemic hemorrhagic fever, fulminant hepatitis, typhoid, paratyphoid, typhus, paratyphoid, bacterial dysentery, leptospirosis, neonatal sepsis, etc. (4) Parasitic diseases: hookworm, schistosomiasis, scrub typhus and amoebic dysentery, etc. (5) Food allergy: such as milk protein allergy. (6) Poisoning: plant poisoning, chemical poisoning, uremic poisoning, etc. (7) Connective tissue diseases: systemic lupus erythematosus, dermatomyositis and polyarteritis nodosa, etc. (8) Adverse drug reactions: The most common are painkillers, such as aspirin, indomethacin (anti-inflammatory pain), piroxicam (inflammatory pain Xicam), ibuprofen, etc. 2, upper gastrointestinal tract diseases (1) esophageal diseases: esophageal varices rupture bleeding, esophagitis, esophageal cancer, etc. (2) Gastroduodenal and biliary tract diseases: gastroduodenal ulcer, acute gastritis, biliary tract bleeding, tumor bleeding, etc. 3. Lower gastrointestinal diseases (1) Anal diseases: hemorrhoids, anal fissure, anal fistula, etc. (2) Rectal diseases: rectal inflammation (ulcerative proctitis, radiation proctitis, etc.), rectal tumors (cancer, carcinoid tumors, papillary adenomas, polyps, etc.), rectal injuries (foreign bodies, stab wounds, hard fecal abrasions, injuries caused by instruments and biopsies, etc.). (3) Colon diseases: infections and parasites (bacterial dysentery, amoebic enteropathy, schistosomiasis, etc.), inflammation (ulcerative colitis, Crohn’s disease, radiation colitis, colonic diverticulitis and diverticular ulcer, etc.), tumors (colon cancer, malignant lymphoma, smooth muscle tumor, fibrosarcoma, etc.). (4) Small bowel diseases: infections (typhoid and paratyphoid, etc.), inflammatory diseases (acute hemorrhagic necrotizing enterocolitis, Crohn’s disease, diverticulitis and diverticular ulcer, etc.), tumors (malignant lymphoma, carcinoma, smooth muscle sarcoma, hemangioma, etc.).