The three major steps involved in the production of urine are : (a) Ultrafiltration : The liquid part of blood flows through the glomerulus under great pressure and filter out into the renal tubule. (b) Selective Reabsorption : Reabsorption the selected useful substances like water, glucose, mineral ions etc. from glomerular filtrate. (c) Tubular Secretion : The secretion of harmful substances like $\mathrm{K}^{+}$, drugs from the blood into the nephric filtrate through the walls of distal convoluted tubule.