The structural and functional unit of kidney is called Nephron. Nephron is an individual unit that produces urine. Its length is 3-5 cm. Each kidney has 10-12 lakh Nephrons. The tubules of the nephron in each kidney are collectively 36 km or 22.5 miles long. In both kidneys 50% of nephrons are always active and 50% of nephrons are always at rest.