The process of urine formation in kidneys include the following steps :
(i) Glomerular filtration - Urine formation begins when the blood is filtered by the glomerulus and enters the Bowman's capsule and the glomerular filtrate is formed. The afferent arteriole entering the glomerulus is wider than the efferent arteriole in diameter. This increases the blood pressure within the glomerulus helps in the filtration. Glomerular filtrate is formed of all blood constituents except corpuscles and plasma proteins.
(ii) Tubular reabsorption - When the glomerular filtrate comes to the proximal tubule then the essential nutrients are reabsorbed. Glucose, amino acids, water, ions like sodium, potassium, chloride, bicarbonate, magnesium, calcium are reabsorbed from the glomerular filtrate. When the filtrate finally comes to the collecting duct ADH (anti-diuretic hormone) acts and reabsorbs the excess water and decreases the urine volume.
(iii) Tubular secretion - When the glomerular filtrate is in the proximal tubule cretinine, uric acid are secreted in it. When the filtrate goes to the distal part urea, potasium ion and some hydrogen ions are secreted in it.