Describe briefly mechanism of urine formation in human kidney.
Urine formation in kidney can be describe in 3 headings :

1. Ultrafiltration : It is the filtration which occurs under high blood pressure. The afferent arteriole supplies blood to glomerulus and efferent arteriole collects blood from glomerulus. The afferent arteriole has more breath than the capillaries of glomerulus. Therefore, the pressure in glomerulus is two and half times more than ordinary capillary bed. Thus smaller molecules of glucose, water, salts, ammonia etc. diffuse from glomerulus into Bowman's capusle. And this process is called ultrafiltration. 

2. Selective reabsorption : It is the process by which useful materials 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. For example, substances like glucose, amino acids, Na+, etc., in the filtrate are reabsorbed actively whereas the nitrogenous wastes are absorbed by passive transport. Reabsorption of water also occurs passively in the initial segments of the nephron

3. Selective secretion : The metabolic waste products like ammonia, creatine, H+ ions etc. present in the blood diffuse into distal convoluted tubule DCT. This process is called selective secretion. The tubular cells secrete substances like H+, K+ and ammonia into the filtrate. Tubular secretion helps in the maintenance of ionic and acid base balance of body fluids.