A nephrologist (neh-frah-leh-jist) is a doctor with expertise in the care of kidneys. Your kidneys are part of your urinary system. They're bean-shaped organs that filter your blood and remove waste, excess water and electrolytes, which leave your body as urine (pee). Nephrology is the subspecialty of internal medicine that focuses on the diagnosis and treatment of diseases of the kidney. Because the kidney performs so many critical functions, nephrologists maintain expertise in primary kidney disorders, but also the management of the systemic consequences of kidney dysfunction.