When the volume of the filtrate is compared with that of the urine that is formed per day, which is 180 litres and 1.5. Litres respectively, most of the filtrate needs to be reabsorbed by the renal tubules, the phenomenon is referred to as reabsorption. Substances such as amino acids, Na+, glucose are actively reabsorbed in the filtrate hence, are not found in urine.