Urine formation occurs in three steps:

a. Ultrafiltration: Blood containing nitrogenous wastes flows with a great pressure through the glomerulus. The liquid portion of the blood gets filtered through glomerulus and collects in the Bowman’s capsule which is called glomerular filtrate. This process is called ultrafiltration.

b. Selective reabsorption: The glomerular filtrate contains a lot of useful substances like glucose, water, salts, etc. which gets selectively reabsorbed by different parts of the renal tubule. This process is called selective reabsorption.

c. Tubular secretion: Distal convoluted tubule part of nephron secretes some harmful substances like salts, antibiotics like penicillin etc. to the forming urine which is called tubular secretion.

The formed urine is then received by collecting ducts which pours into the pelvis of kidney.