Urine formation takes place in three stages.These are ultrafiltration.selective reabsorption and tubular secrection.(I)Ultrafiltration:(1)Outer partietal layer and inner visceral layer of Bowman's capsule enclose a space called urinary space.The inner visceral layer contains podocytes which are specialized cells.Here ultrafiltration takes place.(2)Due to semipermeable glomerular endothelial membrane.Low miolecule weight substance along with water other substances are pilled ou of the blood and enter into urinary space.This filtrate is blood plasma except protiens and blood corpuscles.This is a physical process.(II)Selective reabsorption:(1)In this process 99% of the filtrate is reabsorbed.(2There are two process involved in this step:Passive transport or osm osis and active transport.Active transport uses ATP Molecules since it is against the concentration gradient.(3)When the filtrate passes through the renal tubule there is exchange of substances between the surrounding blood and renal tubular fluid.Useful substances are reabsorbed while nitrogenous waste products are sent further.(III)Tubular secretion:(1)Some abnormal and unwanted substances which have passed into pertitubular capillaries are secreted back in DCT,During this process.(2)Nitrogenous substances like creatinine and ions like potassium and hydrogen are also taken back in the lumen of uriniferous tubule through the peritubular capillaries.(3)If any toxic and abnormal constituent is present in the body it is secreted back into urine ,in this stage.
