Blood in urine may be caused by menstrual blood mixing, kidney stones, leukemia, urinary tract infection, urinary system tumor and other factors. 1. Mixed menstrual blood: During menstruation, menstrual blood may be mixed in the urine, resulting in blood in the urine. 2. Kidney stones: due to the stimulation of the mucous membrane tissue of the urinary system by the stones, it may lead to the symptoms of hematuria, which may be accompanied by renal colic, nausea, vomiting and other symptoms. 3. Leukemia: patients with leukemia may have bleeding symptoms, and bleeding in the urinary system or reproductive system may cause blood to enter the urine, resulting in symptoms of blood in urine. 4. Urinary tract infection: when the urinary system is infected, it will lead to congestion of the mucous membrane and increased exudation of inflammation, resulting in the symptom of blood in the urine, and the common diseases are cystitis, glomerulonephritis, and so on. 5. Tumor of urinary system: when there is tumor in urinary system, the tumor invades the vascular tissue, which will lead to bleeding of mucous membrane tissue of urinary system and the symptom of blood in urine, and the common diseases are bladder cancer. There are other reasons for blood in urine in old age, patients need to consult the doctor in time for examination and early treatment.