Kidney with illustration of the renal calices, pelvis, and ureter. The kidney produces primary urine, which is an ultrafiltrate of the blood. The tubuli secrete or resorb further substances, so that the actual urine (secondary urine) is produced. The glomerular filtration rate (GFR) is ca. 120ml blood which is filtrated per minute, 180l of primary urine is produced per day, but ony 1,8l secondary urine, which the kidney excretes.