Urine formation includes three processes: 

(i) Ultrafiltration, (ii) Selective reabsorption, (iii) Selective Secretion. 

(i) Ultrafiltration: It is filtration which occurs under high blood pressure. The afferent arteriole supplies blood to glomerulus and efferent arteriole collects blood from glomerulus. Therefore, the blood pressure in glomerulus becomes 75 mm of Hg. The smaller molecules of glucose, water, salts, ammonia etc. diffuse from glomerulus to Bowman's capsule and this process is called ultrafiltration. About 1.5-2.5% of water and solutes are removed from the blood plasma that flows through glomerulus. The quantity of glomerular filtrate formed each minutes in all nephrons of both kidneys is called the glomerular filtration rate. In a normal person, the glomerular filtration rate is about 125ml. per minute. 

(ii) Selective reabsorption: It is the process by which useful material are absorbed by the blood from glomerular filtrate. The tubular epithelial cells in different segments of nephron perform this either by active or passive mechanisms. Substances like glucose, amino acids, Na⁺ , etc., in the filtrate are reabsorbed actively where as the nitrogenous wastes are absorbed by passive transport. In collecting tubules some water is absorbed. 

(iii) Selective secretion: The metabolic waste products like ammonia, hippuric acid, creatinine, H⁺ ions etc., present in the blood diffuses into DCT. This process is called selective secretion. It changes the complete composition of glomerular filtrate which is now called urine.