Blood in the urine can be a sign of injury or disease in the kidney, according to Mayo Clinic. However, it can also be a sign of cancer, genetic disorders, a kidney stone or a urinary tract infection.
The medical term for blood in the urine is hematuria, and it happens when the kidneys or other sections of the urinary tract permit blood cells to make their way into the urine, explains Mayo Clinic. People who suffer a blow to the kidney during an automobile wreck or while playing contact sports can develop hematuria, as can people who have glomerulonephritis, which results in inflammation of the filtering system inside the kidneys. This can happen on its own or as a part of a systemic condition such as diabetes.
Other potential causes of hematuria include inherited disorders such as sickle cell anemia, in which people have a hereditary hemoglobin defect in the red blood cells, and Alport syndrome, which alters the filtering membranes in the kidneys. Stones in the kidney or bladder can cause considerable pain as well as bleeding into the urine, notes Mayo Clinic. Urinary tract infections can allow blood into the urine, although in some people, the blood is only visible under a microscope.