Kidney hypertension, light micrograph. Section through a kidney blood vessel (round structure) with hypertrophy (enlargement) of its walls as a result of high blood pressure (hypertension). 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. As more nephrons are damaged, toxic substances start to build up in the blood, eventually leading to kidney failure, which can be fatal.