Nephrons filter blood and produce urine. Each kidney contains about one million nephrons. In each nephron is a tiny tube that drains the fluid that becomes urine. Urine drains from the collecting ducts of many thousands of nephrons into a cuplike structure called a calyx. Each kidney has several calices, all of which drain into a single central chamber called the renal pelvis. Urine drains from the renal pelvis of each kidney into a ureter