Mechanism Of Urine Formation In Human: Nitrogenous wastes are formed inside all the body cells. These wastes are transported to the kidneys by blood and are changed into urine by three processes- Ultrafiltration or Glomerular Filtration. Tubular Reabsorption or Selective Reabsorption. Tubular Secretion or Excretion or Augmentation. Glomerular Filtration: The first step in urine formation