The kidney releases erythropoietin, a hormone that functions in the maturation of white blood cells
This is not the function of urinary system, because erythropoietin secreted by the kidney stimulates red blood cell production (erythropoiesis) not white blood cells. Erythropoietin also called hematopoietin is secreted as a response to hypoxia (Insufficient oxygen). Elevated levels of this glycoprotein are usually in disorders such as anemia, and hypoxemia (extremely low oxygen levels) due to chronic lung disease.