Under normal conditions, the stool is yellow in color because it contains fecal bile. When the body has a problem, the stool will be bloody when defecating. This problem is caused by two main factors: 1) diseases of the digestive tract itself, and 2) diseases of other systems besides the digestive tract. The digestive tract, including the mouth, esophagus, stomach, duodenum, small intestine, large intestine, and anus. Lesions in one of these structures have the potential to cause blood in the stool. If there is a lot of bleeding from the mouth and swallowing into the digestive tract, it can cause blood in the stool and make the stool dark black. Ruptured and bleeding esophageal and fundic varices, gastric ulcers and inflammation can lead to blood in the stool. Parasitic infections of the small and large intestines, polyps, cancer, and intestinal stasis can also cause blood in the stool. Anal fissures, hemorrhoids, dry stools, and abrasions can cause fresh blood in the stool. Other diseases outside the digestive tract, such as blood disorders: coagulation factor deficiency, thrombocytopenia, etc., acute infectious diseases, vitamin deficiency, poisoning, or toxic effects of drugs, can cause blood in the stool. When blood in the stool occurs, the cause of blood in the stool should be clarified by combining lifestyle habits and relevant tests. Effective treatment measures should be taken for digestive tract lesions or other systemic lesions. Early and effective treatment should be carried out to prevent deterioration of the condition.