(a) Urea is produced as a waste by decomposition of unused proteins in the liver, body must get rid of these waste materials because their accumulation in the body is poisonous and harmful for us. Kidneys are the organs which remove the poisonous substance urea, other salts and excess water from the blood and excrete them in the form of a yellow liquid called urine.
(b) During filtration, the substances like glucose, amino acids, salts, water and urea etc., present in the blood pass into Bowman's capsule and then enter the tubule of nephron. When the filtrate containing useful substances as well as the waste substances passes through the tubule, the useful substances like glucose, amino acids, most salts and water etc., are reabsorbed into blood through blood capillaries surrounding the tubule. Only the waste substance like urea, some unwanted salts and excess water remain behind in the tubule. Therefore, reabsorption in nephrons of kidneys is important.