Blood in urine may be caused by physiological factors, acute injury, urinary tract infection, urinary stones, urinary tract tumors, systemic diseases and other factors. 1. Physiological factors: for example, in the menstrual cycle, menstrual blood into the urine, will lead to blood in the urine, after the menstrual cycle may return to normal. 2. Acute injuries: for example, external blows, resulting in localized damage to the urethra, which can lead to blood in the urine, which may also be accompanied by swelling, pain and other symptoms. 3. Urinary tract infection: for example, cystitis or urethritis, which may be related to poor hygiene, holding urine for a long time and other factors, due to infection, resulting in urethral mucous membrane breakage and bleeding, which will cause hematuria. 4. Urinary stones: for example, bladder stones, when urinating or exercising, the stones move and cause damage to the bladder, which will cause blood in the urine, accompanied by difficulty in urination, frequent urination and other symptoms. 5. Tumor of urinary system: for example, bladder tumor, after the tumor gradually increases, the local tissue necrosis and bleeding will cause hematuria, usually painless hematuria. 6. Systemic diseases: the situation of hematuria for a long time may also be caused by systemic diseases, such as systemic lupus erythematosus, leukemia, purpura, and so on. Blood in the urine, ruled out physiological factors, should be timely medical examination and treatment, to avoid aggravation of the condition.