Illustration of changes to the kidney in a case of chronic (long term) hypertension (high blood pressure). The kidney has shrunk and its surface is exhibiting widespread granulation. The kidneys filter waste products from the blood. Sustained high blood pressure damages the kidney's filtering units (nephrons), which become replaced with fibrous scar tissue, making them unable to function. Toxic substances build up in the blood, leading to kidney failure, which can be fatal.